Dissertations / Theses on the topic 'Galactic Black Holes'

Galactic nuclei represent one of the most fascinating and dynamically richest regions of our Universe. They are often found to host at least one supermassive black hole (MBH) at their centre; in addition, observations suggest that MBHs frequently coexist with massive and extremely dense nuclear star clusters, making galactic nuclei ideal laboratories for the study of a broad range of exotic dynamical phenomena. This thesis aims at providing new insights on the interplay between MBHs and their host environments by means of advanced numerical techniques. In particular, my work is relevant in the landscape of gravitational waves (GWs), as it explores the dynamical evolution of stellar compact objects and MBHs: these objects are expected to be promising GW sources detectable by present and future interferometers, as the forthcoming space-borne LISA observatory. In this framework, Bortolas et al. (2017) investigates the impact of natal kicks on the distribution of compact objects in the Milky Way Galactic Centre (GC). My results show that supernova (SN) kicks typically either unbind neutron stars from the MBH, or set them on very eccentric orbits. In contrast, stellar black holes are not significantly affected by the kick: this, combined with mass segregation, would suggest a cusp of stellar relics to inhabit the GC innermost region, as supported by the recent detection of a cusp of accreting X-ray binaries near the MBH. In addition, this thesis is the first to provide evidence that SN kicks may trigger extreme mass ratio inspirals (EMRIs), i.e. GW driven decays of stellar mass compact objects onto MBHs. In Bortolas & Mapelli (2019) I show that SN kicks effectively funnel infant black holes and neutron stars on low angular momentum orbits, promoting their GW decay onto the MBH. By applying this argument to the young stars in the GC, I predict up to 0.01% of SN kicks to induce an EMRI, meaning that LISA will detect up to a few SN-driven EMRIs from Milky-Way like galaxies every year. A further relevant GW source for the LISA observatory is constituted by the coalescence of MBH binaries (BHBs). BHBs are expected to form in large numbers along the cosmic history, being a natural outcome of galaxy collisions. Their coupling in gas-poor galaxies can be described as a three-step process: a dynamical friction dominated phase, a migration phase induced by slingshot ejections of stars, and a GW driven inspiral leading to rapid coalescence. It has been pointed out that the slingshot-driven pairing may be ineffective if too few stars are scattered in the BHB vicinity, and the shrinking may come to a halt at roughly pc separation. However, there is circumstantial evidence that MBH pairs are rare and BHBs are likely to merge: this motivated a series of works aimed to solve the 'final pc problem'. This thesis contributes to the forge of possible solutions in multiple ways. In Bortolas et al. (2018a), I explore the infall of a young massive star cluster onto a BHB. I show that a cluster approaching the BHB along a non-zero angular momentum orbit fails to enhance the BHB shrinking; in contrast, the same cluster free-falling onto the BHB considerably contributes to the BHB pairing, as the BHB separation shrinks by more than 10%. This suggests that several cluster infalls may effectively bring the BHB close to the regime at which GWs lead to a prompt coalescence. A more general solution to the final pc problem is currently believed to reside in the non-sphericity (triaxiality) of the host galaxy. If the host galaxy is triaxial (e.g. as a result of a merger), large scale gravitational torques ensure that stars are continually scattered in the BHB vicinity. This assumption was initially validated via direct summation N-body simulations. However, the reliability of such simulations has been questioned due to the modest achievable number of particles (~1M). In fact, resolution limits enhance the amplitude of the BHB random walk, artificially boosting the BHB shrinking rate. In Bortolas et al. (2016), I numerically explore the significance of such spurious effect: I show that Brownian motion does not affect the evolution of BHBs in simulations including 1M particles or more, providing more reliability to the conclusion that BHBs effectively find their way to coalescence in non-spherical systems. Finally, in Bortolas et al. (2018b) I explore the interplay between the BHB dynamics and the shape of its host system. My study suggests that no strong connection exists between the galaxy morphology and the BHB shrinking rate, which seems to depend only on the inner density slope of the host galaxy. Such result is particularly relevant for GW science, as the time needed for a BHB to reach its GW-emission stage can be assumed to scale only with the central density of the nucleus. In conclusion, this thesis adds several pieces of information to our knowledge of GW sources in galactic nuclei, in preparation for the future of GW observations.

In this thesis we investigated the structures of the central regions of galaxies. This was done by characterising some aspects of the central massive objects (CMOs) found to live in the galactic nuclei, such as studying the possible formation scenarios, stellar properties and scaling relations using a variety of methods. In Chapter 1 we highlighted the importance of understanding the physical properties of CMOs in galaxies by exploring their possible connection with the host galaxies. We reviewed the previous works on the properties, formations scenarios, and scaling relations of supermassive black holes (SBHs), nuclear star clusters (NSCs), and nuclear stellar discs (NSDs) which reside in galactic nuclei. SBHs are believed to be the cause of the past or present activity of the galaxies. The masses of the SBHs range between 10^6 and 10^10 M_⊙ and can be measured with several methods. SBHs are found to correlate with several properties of their host galaxy and their formation is still unclear. NSCs are commonly found in the centres of both elliptical and disc galaxies. They are very massive (M_NSC∼ 10^5 − 10^8 M_⊙), very compact (r_e ∼ 5 pc), and very bright (−14 < M_I < −10). They can have multiple stellar populations, possessing both an old spheroidal component and a younger elongated disc or ring component. The mass of NSCs tightly correlates with the total mass of the galaxy, but several other correlations have been proposed to link the properties of the NSCs with those of the host galaxy. A combined scenario where star formation occurs in the centre of galaxies after dissipation processes and mass accretes via the mergers of globular clusters seems the more plausible way to form NSCs. NSDs are small (h ∼ 10 − 50 pc) and bright (μ_0,V ∼ 16 − 19 mag arcsec^(−2^) discs. They never dominate the light distribution of the galactic nuclei, and locally contribute at most half the galaxy surface brightness. They are fragile systems and do not survive a major merger. Their stellar population has been studied in details in only a few objects, showing a variety of phenomena. NSDs follow the same relation between the central face-on surface-brightness and the scalelength as the main discs of lenticular and spiral galaxies and embedded discs of early-type galaxies. No other relation was found with the properties of the host galaxy. The external capture or the secular infall of gas into the centre where it accumulates, dissipates and forms stars are the most studied scenarios to form NSDs. SBHs, NSCs, and NSDs have been observed to coexist in some galaxies, rising the question whether they are incarnations of the same object and share a common formation scenario. Then in Chapter 2 we investigated the formation and properties of NSDs by looking for their presence in a set of N−body simulations studying the dissipationless merging of multiple star clusters in galactic nuclei. A few tens of star clusters with sizes and masses comparable to those of globular clusters observed in the Milky Way are accreted onto a pre-existing nuclear stellar component: either a massive super star cluster or a rapidly rotating, compact disc with a scalelength of a few parsecs, mimicking the variety of observed nuclear structures. Images and kinematic maps of the simulation time-steps were then built and analysed as if they were real and at the distance of the Virgo cluster. We used the Scorza-Bender method to search for the presence of disc structures via photometric decomposition. In one case the merger remnant had all the observed photometric and kinematic properties of NSDs observed in real galaxies. This shows that current observations are consistent with most of the NSD mass being assembled from the migration and accretion of star clusters into the galactic centre. In the other simulation instead, we detected an elongated structure from the unsharp masked image, that does not develop the photometric or kinematic signature of an NSD. Thus, in the context of searches for a disc structure, the Scorza-Bender method is a robust and necessary tool. In Chapter 3 we investigated the structure and properties of the stellar population of the nuclear regions of the interacting SB0 galaxy NGC 1023 through a detailed analysis of archival Hubble Space Telescope (HST) imaging and ground-based integral-field spectroscopy. The stars of the nuclear disc are remarkably younger and more metal-rich with respect to the host bulge. These findings support a scenario in which the nuclear disc is the end result of star formation in gas piled up in the galaxy centre. The gas can be of either internal or external origin, i.e. from either the main disc of NGC 1023 or the nearby interacting satellite NGC 1023A. The dissipationless formation from already formed stars through the migration and accretion of star clusters into the galactic centre is rejected. In Chapter 4 we presented a dynamical analysis aimed at constraining the mass of the CMOs in the lenticular galaxy NGC 383 at a distance of 63.4 Mpc. The central stellar velocity dispersion is consistent with a putative SBHwith a mass of 5.8 x 10^8 M_⊙. We presented archival HST imaging and spectroscopic observations obtained with the Wide Field and Planetary Camera 2 mounting the F814W filter and the Space Telescope Imaging Spectrograph using the G570M grism, respectively. The data provide detailed information on the structure and mass profile of the stellar component, the dust optical depth, and the spatial distribution and kinematics of the ionised gas within the innermost region of the galaxy. Dynamical models, which account for the observed stellar mass profile and include the contribution of a NSC and a central SBH, were constructed to reproduce the kinematics derived from the [N II]λ6583 emission line along three slit positions crossing the nucleus and parallel to the galaxy major axis. A secure SBH detection with a mass of 8.5 (+1.8 -1.3) x 10^8 M_⊙ was obtained when a single CMO is considered. If we account for the presence of the NSC, then the masses of the SBH and NSC were 6.0 (+1.8 -1.2) x 10^8 M_⊙ and M_NSC = 8.9 (+5.0 - 3.9) x10^7 M_⊙, respectively. Both are consistent with the scaling relations linking the mass of CMOs with the properties of their host galaxy. These measurements prove that SBHs can coexist with NSCs and represent an important step forward in the characterisation of CMOs. The main conclusions of this thesis can be summarised as follows: 1) NSDs can form via accretion events, but a certain amount of gas is necessary; 2) the young stellar population of the NSD of NGC 1023 suggests a formation via gas dissipation; 3) a SBH and a NSD coexist in NGC 383 and follow different scaling relations with the host galaxy. For the first time we were able to disentangle simultaneously the mass of both the CMOs using dynamical modelling.
La tesi è dedicata allo studio della struttura delle regioni centrali delle galassie. Attraverso metodi differenti si è proceduto a caratterizzare alcuni aspetti degli oggetti centrali massicci (CMO) che risiedono nei nuclei galattici investigando, ad esempio, i loro possibili scenari di formazione, le proprietà delle loro popolazioni stellari e le relazioni che li legano all’intera galassia. Nel primo capitolo si è evidenziata l’importanza di capire le proprietà fisiche dei CMO riassumendo i risultati degli studi precedenti in cui si discutono gli scenari di formazione, le relazioni di scala e le proprietà dei buchi neri supermassicci (SBH), degli ammassi stellari nucleari (NSC) e dei dischi stellari nucleari (NSD) che costituiscono gli oggetti massicci centrali conosciuti fino ad oggi. La massa dei SBH è compresa nell’intervallo tra 10^6 e 10^(10) masse solari e si può misurare in diversi modi. Si pensa che essi risiedano in tutte le galassie, alle quali sono legate mediante una serie di relazioni. Per questo motivo è plausibile che abbiano formazione ed evoluzione comuni. Tuttavia gli scenari proposti rimangono incerti. I NSC sono, invece, oggetti massicci (M_NSC∼ 10^5 −10^8 masse solari), molto compatti (r_e ∼ 5 pc) e molto brillanti (−14 < M_I < −10). Essi possono essere costituiti da popolazioni stellari multiple, ossia possono avere una componente vecchia di forma sferoidale ed una componente giovane a forma di disco o ad anello. La loro massa e la loro luminosità sono correlate a varie proprietà della galassia che li ospita. Lo scenario di formazione piu' plausibile sembra essere quello in cui il gas migra verso il centro della galassia, dove ha luogo la formazione stellare. Accanto a questi processi dissipativi, la loro massa può aumentare tramite eventi di fusione che coinvolgono ammassi globulari. I NSD, invece, sono dischi stellari piccoli (h ∼ 10 − 50 pc) e luminosi (μ_0,V ∼ 16 − 19 mag arcsec^(-2)). Essi contribuiscono al massimo alla metà della luminosità del nucleo e sono sistemi fragili che non sopravvivono ad eventi di fusione galattica. La popolazione stellare, di cui sono costituiti, è stata studiata in dettaglio solo per pochi oggetti e gli esiti hanno mostrato risultati non omogenei. La loro brillanza superficiale centrale è legata al raggio di scala, come mostrano anche i dischi galattici delle spirali o delle galassie lenticolari, o i dischi immersi nelle galassie ellittiche. I NSD sembrano essersi formati dal gas che si è accumulato nel nucleo dalla galassia e che ha cominciato a formare stelle. Tale gas può avere avuto origine interna alla galassia, provenendo da regioni periferiche, oppure può essere stato catturato dall’esterno a seguito di eventi di fusione. SBH, NSC e NSD possono risiedere nello stesso nucleo galattico, che porta a chiedersi se essi siano manifestazioni di uno stesso oggetto e condividano lo stesso scenario di formazione. Nel secondo capitolo della tesi si sono studiate la formazione e le proprietà dei NSD analizzando una serie di simulazioni che studia eventi di fusione non dissipativi di ammassi stellari nei nuclei galattici. Un ammasso stellare massiccio e un disco compatto nucleare vengono fatti accrescere dalla fusione con una decina di ammassi stellari, i quali hanno dimensioni e masse comparabili a quelle degli ammassi globulari osservati nella Via Lattea. In questo modo, le simulazioni riescono a riprodurre strutture che si osservano nei nuclei galattici. Sono, poi, state analizzate le immagini e le mappe cinematiche ricavate come se fossero realmente state osservate alla distanza dell’Ammasso della Vergine e come se avessero caratteristiche simili al nucleo di NGC 4244. Mediante il metodo di Scorza & Bender, si è proceduto alla decomposizione fotometrica delle immagini per ottenere i parametri strutturali degli eventuali NSD. Le simulazioni riguardanti l’accrescimento di ammassi globulari in un disco nucleare pre-esistente hanno mostrato la presenza di un NSD le cui proprietà cinematiche e fotometriche sono in accordo con quelle misurate per i NSD di galassie reali. Ciò è indice del fatto che i NSD possano essere frutto di eventi di fusione che coinvolgono ammassi stellari nei nuclei galattici. Le simulazioni realizzate a partire dalla struttura sferoidale, invece, mostrano un nucleo allungato che presenta caratteristiche dissimili da quelle dei NSD. Ciò evidenzia l’importanza di utilizzare il metodo di Scorza & Bender per verificare la presenza di NSD. Nel terzo capitolo della tesi si è proceduto ad analizzare la struttura e le proprietè della popolazione stellare del nucleo galattico di NGC 1023, una galassia interagente di tipo SB0. A tale scopo è stata condotta un’analisi fotometrica accurata delle immagini d’archivio ottenute con l’Hubble Space Telescope (HST) nonché uno studio spettroscopico dettagliato mediante spettri a campo integrale ottenuti con telescopi da terra. Le stelle del NSD sono significativamente piu' giovani e piu' metalliche rispetto a quelle dello sferoide. Ciò supporta uno scenario in cui il NSD è il risultato della formazione stellare avvenuta con il gas che è stato trasportato nel centro della galassia. Tale gas può avere avuto origine interna o esterna: può, infatti, provenire dal disco galattico di NGC 1023 o dalla galassia satellite NGC 1023A. Le caratteristiche del NSD di NGC 1023 escludono, dunque, un’origine non dissipativa, quale può essere la fusione di ammassi stellari nel nucleo. Il quarto capitolo della tesi è dedicato all’analisi dinamica della galassia di tipo lenticolare NGC 383, che dista 63.4 Mpc, allo scopo di misurare le masse dei CMO che sono presenti nel suo nucleo. La dispersione di velocità centrale di NGC 383 è consistente con una massa del SBH di 5.8 x10^8 masse solari. L’analisi è stata condotta utilizzando le immagini d’archivio ottenute con la camera Wide Field and Planetary Camera 2 e grazie agli spettri ottenuti con lo spettrografo Space Telescope Imaging Spectrograph di HST. Tali dati hanno fornito informazioni dettagliate sulla struttura, il profilo di massa della componente stellare, la profondità ottica delle regioni dominate dalla polvere e la distribuzione spaziale e cinematica del gas ionizzato presente nelle regioni centrali della galassia. Per riprodurre il profilo cinematico che è stato derivato dalla riga di emissione [N II]λ6583 lungo tre aperture parallele all’asse maggiore della galassia e di cui una passante per il centro, sono stati costruiti modelli dinamici che hanno incluso i profili di massa della componente stellare, del NSC e del SBH. Quando si è considerato un unico CMO, si è ottenuta una massa del SBH pari a 8.5 (+1.8 -1.3) x10^8 masse solari. Quando, invece, si è tenuto conto anche della presenza del NSC, si sono ricavate masse pari a 6.0 (+1.8 −1.2) x10^8 masse solari e 8.9 (+5.0 −3.9) x10^7 masse solari per il SBH e il NSC, rispettivamente. In entrambi i casi le masse dei CMO ottenute sono in accordo con le relazioni di scala che le legano ad alcune proprietà dell’intera galassia. Questi risultati costituiscono un importante passo in avanti nel contesto della caratterizzazione dei CMO e provano che i SBH possono coesistere con i NSC in alcune galassie. Le principali conclusioni della tesi possono, quindi, essere riassunte in tre punti: 1) I NSD possono formarsi anche attraverso eventi di fusione, ma una certa quantità di gas è comunque necessaria; 2) NGC 1023 ospita un NSD che ha una popolazione giovane e metallica, frutto di una formazione dovuta a processi dissipativi; 3) un SBH e un NSC coesistono nel nucleo di NGC 383 e seguono relazioni diverse se confrontate con le proprietà della galassia che li ospita. Per la prima volta le masse di questi oggetti sono state misurate in maniera simultanea mediante l’utilizzo di modelli dinamici.

This PhD thesis includes theoretical and observational studies of gamma-ray emission from radio-quiet accreting black holes. The theoretical motivation for the search of gamma-ray emission from such sources concerns the considerable hadronic production of gamma-rays predicted by models of hot flows, which most likely power these sources at low luminosities. I thoroughly investigated this model prediction and I found that the luminosity at either hundreds of MeV or in the GeV range, depending on proton distribution, can reach ~10-5 LEdd for the X-ray luminosities between ~10-4LEdd and 10-3LEdd. These levels of gamma-ray luminosities can be probed in some Seyfert galaxies. Comparing the model predictions with Fermi/LAT upper limits for NGC 4258, NGC 7213 and NGC 4151 I found interesting constraints on the acceleration efficiency of protons, plasma magnetization and black hole spins. I found an interesting hint for a gamma-ray signal in the LAT data from NGC 4151, which is only slightly below the formal detection threshold of 5 sigma. I also found hints for the correlation between the X-ray and gamma-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. I have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low and high level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. I found a 5 sigma difference between spectral parameters fitted to these datasets, and a similar significance of the reversal of the gamma-ray signal in significance maps for low and high gamma-ray energies. This X/gamma-ray correlation indicates that the gamma-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. I compared NGC 4945 with other starburst galaxies detected by LAT and I note similarities between those with active nuclei, e.g. unlikely high efficiencies of gamma-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the gamma-ray emission. Finally, I took into account AGILE observations of Cyg X-1. I found that it has not reached yet a sensitivity level needed for a detection of this source.
This PhD thesis includes theoretical and observational studies of gamma-ray emission from radio-quiet accreting black holes. The theoretical motivation for the search of gamma-ray emission from such sources concerns the considerable hadronic production of gamma-rays predicted by models of hot flows, which most likely power these sources at low luminosities. I thoroughly investigated this model prediction and I found that the luminosity at either hundreds of MeV or in the GeV range, depending on proton distribution, can reach ~10-5 LEdd for the X-ray luminosities between ~10-4LEdd and 10-3LEdd. These levels of gamma-ray luminosities can be probed in some Seyfert galaxies. Comparing the model predictions with Fermi/LAT upper limits for NGC 4258, NGC 7213 and NGC 4151 I found interesting constraints on the acceleration efficiency of protons, plasma magnetization and black hole spins. I found an interesting hint for a gamma-ray signal in the LAT data from NGC 4151, which is only slightly below the formal detection threshold of 5 sigma. I also found hints for the correlation between the X-ray and gamma-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. I have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low and high level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. I found a 5 sigma difference between spectral parameters fitted to these datasets, and a similar significance of the reversal of the gamma-ray signal in significance maps for low and high gamma-ray energies. This X/gamma-ray correlation indicates that the gamma-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. I compared NGC 4945 with other starburst galaxies detected by LAT and I note similarities between those with active nuclei, e.g. unlikely high efficiencies of gamma-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the gamma-ray emission. Finally, I took into account AGILE observations of Cyg X-1. I found that it has not reached yet a sensitivity level needed for a detection of this source.

Doctor en Ciencias, Mención Astronomía
Here I present an in-depth study of the central region of active galactic nuclei oriented to improve current mass estimation methods of distant super massive black holes and to infer the physical properties of the gas in their vicinity. In the first chapter I briefly introduce the basic concepts and present the most relevant discoveries and problematics associated to the central topic of this thesis. Then, in the second chapter, I present new calibrations of the so called single epoch black hole mass estimation method. This method uses emission lines from the broad line region such as the Halpha, Hbeta and MgII low ionization lines, and the CIV high ionization line. The novelty of this work is the usage of simultaneous observations of these emission lines that prevents from possible variability effects. The latter was possible thanks to the observations of 39 quasars a $z\sim1.55$ using the X-Shooter spectrograph of the VLT telescope whose wide spectral coverage allows simultaneous mapping of the aforementioned emission lines. In addition to presenting new calibrations, the results of this study indicate that low ionization lines provide more accurate mass estimations than CIV as it was suggested by previous studies. In the third chapter, I examine the possibility of improving current \CIV -based mass estimates of super-massive black holes by testing the performance of some methods proposed in the literature, including a method proposed in this thesis. All these methods are based on correlations found using small samples of less than 100 objects. In order to quantify the statistical robustness of these methods, in this work I use the Sloan Digital Sky Survey quasar database out of which I extracted a sample of nearly 30000 objects. The results suggest that the methods studied here have a very limited effect on the improvement of \CIV-based mass estimations. Finally, in the fourth chapter, I study the effect of gas distribution of the broad line region on mass estimations. This is possible thanks to the comparison between masses obtained from the single epoch method and those obtained from the fitting the accretion disc spectral energy distribution to standard accretion disk models. The results indicate a strong dependence of the ratio between both mass estimates with the observed width of the broad emission lines.

This thesis presents theoretical results on the tidal disruption of stars by supermassive black holes (SMBHs). The multiwavelength ares produced by tidal disruption events (TDEs) have supernova-like luminosities, and associated relativistic jets can be visible to cosmological distances. TDEs probe the demography of quiescent SMBHs, and are natural laboratories for jet launching mechanisms and super-Eddington accretion. The first chapter broadly surveys TDE physics. The second and third chapters estimate the TDE rate following gravitational wave (GW) recoil of a SMBH (after a SMBH binary merger). Immediately after GW recoil, the TDE rate increases, sometimes to \(~10^{-1}\) TDEs per year. This "burst" of TDE flares can provide an electromagnetic counterpart to low frequency GW signals, localizing sources and measuring cosmological parameters. Millions of years later, recoiled SMBHs wandering through their host galaxies will produce spatially offset TDEs at a rate which is likely detectable with the LSST. In the fourth chapter, we show that standard estimates for \(\Delta\epsilon\), the energy spread of TDE debris, are wrong, sometimes by orders of magnitude. Correcting this error reduces the observability of many TDEs. We introduce a new analytic model for tidal disruption, calculate \(\Delta\epsilon\)'s dependence on stellar spin, estimate general relativistic corrections to \(\Delta\epsilon\), and quantify the GW signal generated from tidal compression. The fifth chapter presents hydrodynamical simulations of TDE debris circularization, focusing on eccentric, rather than parabolic, orbits. General relativistic precession drives debris circularization, in contrast to past simulations using smaller black holes. In the sixth chapter, we show that TDE light curves can constrain or measure SMBH spins, as Lense-Thirring torques produce quasiperiodic variability in disk emission. Precession of a relativistic jet could also measure SMBH spin, and we apply our model to the relativistic Swift 1644+57 TDE. The seventh chapter considers the disruption of neutron stars (NSs) by stellar mass black holes (BHs) or other NSs. Jet precession in associated short-hard gamma ray bursts is uniquely possible for NS-BH (not NS-NS) mergers. We quantify typical precession amplitudes and periods, and calculate their time evolution. If disk viscosities are relatively low, electromagnetic observations alone could distinguish NS-BH from NS-NS mergers.
Astronomy

This third paper in the series modeling QSOs and AGN as clusters of accreting black holes studies the accretion flow within an externally supplied cluster. Significant radiation will be emitted by the cluster core, but the black holes in the outer halo, where the flow is considered spherically symmetric, will not contribute much to the overall luminosity of the source because of their large velocities relative to the infalling gas, and therefore their small accretion radii. As a result the scenario discussed in Paper I will refer to the cluster cores, rather than to entire clusters. This will steepen the high frequency region of the spectrum unless inverse Compton scattering is effective. In many cases accretion flow in the central part of the cluster will be optically thick to electron scattering resulting in a spectrum featuring optically thick radiative component in addition to power -law regimes. The fitting of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is worked out as an example.

This thesis is a theoretical study of supermassive black holes (SMBHs) in merging galaxies. We consider the dynamics that govern inspiralling SMBH pairs and gravitational-wave (GW) recoiling SMBHs, as well as the fueling of active galactic nuclei (AGN) during galaxy mergers. In particular, we focus on the observable signatures that could distinguish dual or recoiling AGN from those in isolated galaxies, and we explore the implications of these events for the coordinated evolution of SMBHs and galaxies. In the second and third chapters, semi-analytical models for GW-recoiling SMBHs are developed. The second chapter illustrates that bound recoiling SMBHs may have long wandering timescales and that recoil events can self-regulate SMBH growth. In the third chapter, we study the evolution of recoiling SMBHs in evolving, gaseous merger remnants. We find that the presence of gas greatly influences recoiling SMBH trajectories and may partially suppress even large recoil kicks in some cases. We also show that kinematically- and spatially-offset AGN can have substantial lifetimes for a wide range in kick speeds. Finally, this chapter illustrates that GW recoil influences the observed SMBH-galaxy relations as well as central star formation in the merger remnant. In the fourth chapter we turn our attention to inspiralling SMBH pairs with kiloparsec-scale separations. We use a novel approach to model the narrow-line emission from these SMBH pairs, in order to understand their relationship to observations of double-peaked narrow-line AGN. Our results indicate that double-peaked narrow-line AGN often arise from gas kinematics rather than from dual SMBH motion, but that the latter are a generic, short-lived phase of SMBH inspiral in gaseous mergers. We identify several diagnostics that could aid in distinguishing the true AGN pairs in the double-peaked sample. Finally, the fifth chapter examines a particular galaxy that exhibits signatures of both a recoiling AGN and an AGN pair. Applying methods developed throughout this thesis, we design models for both scenarios that are well-matched to the available data. Currently, neither possibility can be excluded for this object, but our models constrain the most relevant parameters for etermining its nature and for the design of future observations.
Astronomy

We investigate the possibility that present-day galaxies and their dark matter halos contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars in the Universe. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. The relation between SMBH and galactic bulge mass in our model displays the same slope as that found in observations. Many MBHs will not reach the centre of the host halo, however, but continue to orbit within it. In doing so MBHs may remain associated with remnants of the satellite halo systems of which they were previously a part. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with galactic disks, we follow the hierarchical merging of MBH systems and their subsequent dynamical evolution inside the respective host halos. In this context two types of dynamical processes are examined in more detail. We predict the mass and abundance of MBHs in present-day galactic halos and also estimate the MBH mass accretion rates considering two different accretion scenarios. On this basis we determine the bolometric, optical and X-ray luminosity functions for the accreting MBHs using thin disk and advection dominated accretion flow models. Our predicted MBH X-ray emissions are then compared with observations of ultra-luminous X-ray sources in galaxies. We find that the slope and normalisation of the predicted X-ray luminosity function are consistent with the observations. We also estimate the rate of gravitational wave events received from MBH mergers across all redshifts. At the end of their lives the first stars may explode in supernovae that are associated with gamma ray bursts (GRBs). Provided these are in principle detectable we have estimated the expected rate of events observed.

One of the exciting discoveries from the recent X-ray spectroscopic studies of active galactic nuclei (AGNs) is the so called "relativistically-broadened iron fluorescent emission line" often detected in the hard X-ray spectra. It is generally believed to originate from the inner part of the accretion disk surrounding a supermassive black hole (BH) at the center. Although we have begun to obtain some physical insight regarding such emission lines supported by theoretical models (e.g., disk-corona model), exactly how and where the observed fluorescence may take place is still disputable. Here, an X-ray data with XMM-Newton Observatory of a typical narrow-line Seyfert 1 galaxy, NGC 4051, is analyzed based on a partial covering model to consistently explain the observed time-resolved temporal/spectral variations. This model implies that the intrinsic emission varies significantly in the presence of the covering cloud. We often detect a hard X-ray continuum originating from a hot region close to the central engines of AGNs. As a promising X-ray source candidate, relativistic hydrodynamic (HD) shocks are investigated systematically and then extended to the magnetohydrodynamic (MHD) shocks, given the widely accepted suggestion that the presence of the magnetic fields could play an important role in the accreting flows. I show that both HD and MHD shocks can form in the vicinity of the BH, perhaps responsible for creating such a high temperature region where hard X-rays are produced. Particularly in the MHD shocked plasma, the hydro/magneto-dominated states are found. Considering the effect of such magnetic fields in the accretion disk, I calculate nonstandard iron fluorescent line profiles in the presence of spiral density waves and find multiple sharp sub-peak structures in extremely skewed line profiles, which will be detectable with upcoming X-ray satellites such as Astro-E2 XRS for testing the model. This dissertation is the result of my own work and also includes some work done in collaboration. Parts of this dissertation have been either already published in or submitted to the Astrophysical Journal and presented at conferences, while some are still in progress.

Black hole jets and accretion discs are the most extreme objects in modern astrophysics whilst dark energy is undoubtedly the most mysterious. This thesis focuses on understanding these three topics. The majority of this thesis is dedicated to investigating the structure and properties of black hole jets by modelling their emission. I develop an inhomogeneous jet model with a magnetically dominated parabolic accelerating base, transitioning to a slowly decelerating conical jet, with a geometry set by radio observations of M87. This model is able to reproduce the simultaneous multiwavelength spectra of all 38 Fermi blazars with redshifts in unprecendented detail across all wavelengths. I constrain the synchrotron bright region of the jet to occur outside the BLR and dusty torus for FSRQs using the optically thick to thin synchrotron break. At these large distances their inverse-Compton emission originates from scattering CMB photons. I find an approximately linear relation between the jet power and the transition region radius where the jet first comes into equipartition, transitions from parabolic to conical and stops accelerating. The decreasing magnetic field strength and increasing bulk Lorentz factor with jet power are the physical reasons behind the blazar sequence. I calculate the conditions for instability in a thin accretion disc with an α parameter which depends on the magnetic Prandtl number, as suggested by MHD simulations. The global behaviour of the instability induces cyclic flaring in the inner regions of the disc, for parameters appropriate for X-ray binary systems, thereby offering a potential solution to a long standing problem. Finally, I calculate the effect of an interacting quintessence model of dark energy on cosmological observables. I find that a scalar-tensor type interaction in the dark sector results in an observable increase in the matter power spectrum and integrated Sachs-Wolfe effect at horizon scales.

It is now believed that most, if not all, galaxies contain a supermassive black hole (SMBH) and that these play a crucial role in their host galaxies' evolution. Whilst accreting material, a SMBH (known as an active galactic nucleus, AGN, during this growth phase) releases energy which may have the effect of quenching star formation and constraining the growth of the galaxy. It is believed that AGNs can be divided into two broad fundamental categories, each with its own feedback mechanism. The radiative-mode of feedback occurs in gas-rich galaxies when substantial star formation is occurring and their young AGNs are growing rapidly through efficient accretion of cold gas. A fraction of the energy released by an AGN is transferred into the surrounding gas, creating a thermal "energy-driven" wind or pressure "momentum-driven" wind. Gas and dust may be expelled from the galaxy, so halting star formation but also cutting off the fuel supply to the AGN itself. The jet-mode occurs thereafter. The SMBH has now attained a large mass, but is accreting at a comparatively low level as gas slowly cools and falls back into the galaxy. The accretion process generates two-sided jets that generate shock fronts, so heating the gas surrounding the galaxy and partially offsetting the radiative cooling. This restricts the inflow of gas into the galaxy, so slowing the growth of the galaxy and SMBH. There are several convincing theoretical arguments to support the existence of these feedback mechanisms, although observational evidence has been hard to obtain. A new radio telescope - the Low Frequency Array (LOFAR) - recently started operations. LOFAR is especially suitable for investigating AGN feedback. It has been designed to allow exploration of low radio frequencies, between 10 and 240 MHz, which are particularly relevant for research into AGN activity. Also, with its large field-of-view and multi-beam capability, LOFAR is ideal for conducting extensive radio surveys. A project to image deeply the ELAIS-N1 field was started in May 2013. This thesis uses a number of surveys at different wavelengths, but particularly the low-frequency radio observations of the ELAIS-N1 field, to improve our knowledge of jet-mode AGN feedback and hence of the interplay between the complicated processes involved in galaxy formation and evolution. The more important pieces of research within the thesis are as follows: - A sample of 576 AGNs in the nearby universe was assembled and used to find a relationship between radio luminosity, X-ray luminosity and black hole mass. Moreover, the relationship is valid over at least 15 orders of magnitude in X-ray luminosity, strongly suggesting that the process responsible for the launching of radio jets is scale-invariant. - The established "Likelihood Ratio" technique was refined to incorporate colour information in order to optimally match the radio sources in the ELAIS-N1 field with their host galaxies. - The resulting catalogue was used to investigate ways in which radio sources can be matched automatically with their host galaxies (and so avoiding laborious visual examination of each source). The conclusions have helped the design of a pipeline for an extensive wide-area survey currently being conducted by the LOFAR telescope. - The catalogue was also used to investigate the evolution of jet-mode AGNs. This involved: deriving source counts; obtaining redshifts for each object; classifying the radio sources into the different populations of radiative-mode AGNs, jet-mode AGNs and star-forming galaxies; and using the above preparatory work in order to derive a luminosity function for jet-mode AGNs. - Key conclusions are that (1) feedback from jet-mode AGNs peaks at around a redshift of 0.75, (2) the space density of jet-mode AGNs declines steadily with redshift and (3) the typical luminosity of a jet-mode AGN increases steadily with redshift.

Des trous noirs supermassifs de plusieurs centaines de millions de masses solaires résident au centre de la plupart des galaxies massives. Dans 90% des cas, ces trous noirs sont dans état quiescent, très peu lumineux. Cependant, dans les 10% restant, des processus extrêmement violents sont observés, avec la libération d'énorme quantités d'énergie no- tamment en UV, X et gamma. On observe aussi parfois des jets puissants de matière pouvant s'étendre sur plusieurs centaines de kpc. Le coeur de ces galaxies sont appelés Noyaux Actifs de Galaxie (NAG). Ce sont parmis les objets les plus lumineux de l'univers. L'accrétion de la matière environnante sur le trou noir supermassif central est unanimement reconnue comme la source d'énergie la plus plausible pour expliquer la puissance phénoménale observée. L'énergie gravitationelle serait ainsi en partie libérée dans un disque d'accrétion, sous forme de rayonnement thermique piquant dans l'optique/UV, et en partie rayonnée en X/gamma par une couronne de plasma chaud présente dans l'environnement proche du trou noir.De nombreux phénomènes sont néanmoins encore très mal connus et beaucoup de ques- tions n'ont toujours pas de réponses satisfaisantes: quelles sont la dynamique et la structure des flots d'accrétion et d'éjection dans les NAG? Quels sont les processus radiatifs produisant le rayonnement UV/X? Quelle est l'origine des différentes composantes spectrales présentes dans ces domaines d'énergie? Cette thèse a pour objectif d'apporter de nouvelles contraintes observationnelles pour meux répondre à ces questions. Son originalité réside dans le développement et l'utilisation de modèles réalistes de Comptonisation thermique permettant d'une part de mieux contraindre les propriétés physiques et géométriques des régions d'émission UV/X et d'autre part de mieux comprendre l'origine des différentes composantes spectrales observées. Nous nous sommes notamment intéressés, au cours de cette thèse, à l'excès d'émission X-mou (<2 keV), présent dans un grand nombre de NAG, et dont l'origine est toujours inconnue.Ces travaux s'articulent autour de deux axes principaux. Le premier est l'étude spectrale détaillée de longues campagnes d'observation multi-longueur d'ondes de trois galaxies de Seyfert (NGC 5548, NGC 7213 et NGC 4593). La qualité des données ont ainsi permis de révéler les paramètres physiques (notamment la température et la profondeur optique) et géométriques de la couronne thermique à l'origine du continuum X. Le second axe porte sur l'analyse de données d'archives (en provenance du satellite XMM-Newton) d'un échantillon important de galaxies de Seyfert. Cela a permis d'apporter, cette fois ci, des contraintes plus générales sur les processus d'émission haute énergie observés dans ces objets. Ces deux approches ont notamment montré que l'exces d'émission X-mou pouvait provenir des couches supérieures chaudes du disque d'accrétion, suggérant un chauffage plus efficace en surface plutôt que dans les régions internes
Supermassive black holes of several hundred million solar masses lie at the centre of most massive galaxies. In 90% of cases, these black holes are in quiescent, very low luminous states. Nevertheless, in the remaining 10%, extremely violent processes are seen, with the liberation of huge amounts of energy especially in the UV, X-ray and gamma-ray bands. We also sometimes observe powerful jets, extending up to several hundred kpc scales. The cores of these galaxies are called Active Galactic Nuclei (AGNs). These are among the most luminous objects in the Universe. The accretion of surrounding matter onto the central supermassive black hole is generally considered as the most likely energy source to explain the extraordinary observed luminosity. The gravitational energy would be partly liberated into an accretion disc as thermal radiation peaking in the optical/UV band, and partly radiated in the X-ray/gamma-ray band by a corona of hot plasma lying in the environment close to the black hole.However, several phenomena are still poorly understood and a number of questions lacks satisfactory answers: what are the dynamics and the structure of the accretion and ejection flows in AGNs? What are the radiative processes producing the UV/X-ray radiation? What is the origin of the different spectral components present in those energy bands? The goal of this thesis is to derive new observational constraints to better answer to these questions. Its originality resides in the development and application of realistic models of thermal Comptonization, allowing on the one hand to better constrain the physical and geometrical properties of the UV and X-ray-emitting regions, and on the other hand to better understand the origin of the different observed spectral components. In particular, we studied the excess of the soft (<2 keV) X-ray emission, seen in a great number of AGNs, and whose origin is still unknown.This work is structured along two main branches. One is the detailed spectral analysis of long, multiwavelength observational campaigns on three Seyfert galaxies (NGC 5548, NGC 7213 and NGC 4593). The quality of the data permitted to reveal the geometrical and physical parameters (in particular the temperature and optical depth) of the thermal corona producing the X-ray continuum. The second branch is based on the analysis of archival data (from the XMM-newton satellite) of a large sample of Seyfert galaxies. This allowed us to derive more general constraints on the high-energy emission processes observed in these objects. These two approaches have shown, in particular, that the soft X-ray emission excess may arise in the warm upper layers of the accretion disc, suggesting a more effective heating of the surface rather than the inner regions

The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured active galactic nuclei (AGNs) at high accretion luminosities, L-X(2-10 keV) greater than or similar to 10(44) erg s(-1), and out to redshift z approximate to 1.5. The sample covers an area of about 14 deg(2) and provides constraints on the space density of powerful AGNs over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, N-H approximate to 10(24) cm(-2). The fraction of obscured Compton-thin (N-H = 10(22) - 10(24) cm(-2)) AGNs is estimated to be approximate to 0.35 for luminosities L-X(2-10 keV) > 10(44) erg s(-1), independent of redshift. For less luminous sources, the fraction of obscured Compton-thin AGNs increases from 0.45 +/- 0.10 at z = 0.25 to 0.75 +/- 0.05 at z = 1.25. Studies that select AGNs in the infrared via template fits to the observed spectral energy distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGNs (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGNs as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGNs is primarily a function of redshift. This fraction increases from about 20-30 per cent at z = 0.25 to about 50-70 per cent at z = 1.5.

Les trous noirs supermassifs (SMBH) se trouvent au coeur de presque toutes les galaxies massives dans l’Univers. La plupart sont en sommeil, mais lorsqu’il y a assez de gaz à proximité, ils entrent dans une phase active et forment ce qu’on appelle un noyau actif de galaxie (AGN). Ils ont alors un effet profond sur l'évolution de la galaxie hôte et jouent un rôle important sur leur environnement. La mesure fiable de la masse des SMBH est donc une tâche importante dans l'astronomie moderne. À cette fin, Afanasiev et Popovic (2015) ont récemment proposé une méthode qui utilise la rotation de l'angle de position du plan de polarisation sur le profil en fréquence des raies d'émission larges afin de tracer le mouvement Keplerien et de déterminer la masse du SMBH. Le but de la thèse est d'explorer théoriquement les possibilités de cette méthode. Pour ce faire, nous avons dans une première partie effectué de nombreuses simulations de transfert radiatif pour la modélisation de la diffusion équatoriale dans AGN à l'aide du code STOKES. Nous avons inclus les mouvements complexes présents dans le système sous forme d’accrétion et d’éjection, et nous avons également comparé nos résultats aux observations. Notre travail est important car nous avons démontré dans quelles circonstances cette méthode peut être utilisée pour mesurer la masse du trou noir de manière indépendante. La deuxième partie de ce travail consiste à prédire la polarisation des raies larges au cas où les AGN seraient alimentés par des trous noirs binaires supermassifs (SMBBH). Nous avons traité quatre cas différents avec des binaires de trous noirs séparés de moins d’un parsec, et modélisé à nouveau la diffusion équatoriale. Nous avons obtenu une signature de polarisation unique sur les raies larges. Nous avons montré que la spectropolarimétrie pouvait constituer un outil puissant et une première étape pour la recherche de SMBBH dans les futurs levés spectropolarimétriques systématiques
Supermassive black holes (SMBHs) reside in the heart of nearly every massive galaxy in the Universe. Most of them lie dormant, but when the nearby gas is abundant, it will enter an active phase and form an active galactic nucleus (AGN). When in their active phase, SMBHs have a profound effect on the host galaxy evolution and play an important role in shaping their environment. Reliable SMBH mass measuring is therefore an important task in modern astronomy. For that purpose, a method has been recently proposed by Afanasiev & Popovic (2015) that uses the rotation of the polarization plane position angle across the broad emission line profile in order to trace the Keplerian motion and determine the SMBH mass. The goal of the thesis is to theoretically explore the possibilities of this method. In order to do that, we performed numerous radiative transfer simulations for modeling equatorial scattering in AGNs using the code STOKES. We included complex motions present in the system in the form of inflows and outflows, and we also compared our results with observations. We have demonstrated under which circumstances this method can be used to measure the SMBH mass in a new independent way. The second part of this work involves predicting the broad lines polarization when AGNs are powered by supermassive binary black holes (SMBBHs). We treated four different cases with sub-pc SMBBHs, and again modeled equatorial scattering. We obtained a unique polarization signature across the broad lines. We have shown that spectropolarimetry could be a powerful tool and a first step for searching SMBBHs in the future systematic spectropolarimetric surveys

All massive nearby galaxies, including our own, host supermassive black holes. Active galactic nuclei (AGN) are seen when such black holes accrete, and when they produce powerful jets of synchrotron-emitting plasma, they are termed radio-loud AGN. The close correlation between black hole mass and galaxy bulge mass in elliptical galaxies indicates that AGN feedback may be the key to the regulation of galaxy formation. It is thus necessary to fully understand the structure of AGN, the way that they are fuelled, and their duty cycle, in order to study the feedback processes and get a clear picture of galaxy formation. In this thesis, independent methods are developed to constrain the accretion disk and radio jet angles to the line of sight. H IX emission from a sub-sample of high-redshift quasars is measured from near-infrared spectroscopy and modelled as sums of different components, including the characteristic double-peaked profile which results from a thin, rotating accretion disk. Comparing the models using Bayesian evidence, almost all quasars were found to have infrared spectra consistent with the presence of a disk. The jet inclination angles of the same set of quasars were constrained by fitting a model, including the effect of Doppler boosting and the receding torus model for dust obscuration, to the radio \ spectral energy distribution. The fitted disk and jet angles correlate strongly, and are consistent with a model in which the radio jets are launched orthogonally to the plane of the accretion disk, as expected if the jet is powered by energy drawn from the spin of the black hole. Both disk and jet angles correlate with the observed linear source size, which is a projection effect; when deprojected using the fitted angles, the distribution of source sizes agrees with a scenario in which the sources expand into the surrounding medium at a constant rate up to ~ 1 Mpc and then shut off, probably as the nuclei become quiescent. The accretion disk angle was found to correlate weakly with the low-frequency radio luminosity, which provides direct, albeit tenuous, evidence for the receding torus model.

In this thesis I use X-ray observations to study the cores and extended structures of radio-loud AGN, to determine their structure, accretion properties and the impact they have on their surroundings. I use new Chandra data and archival XMM-Newton observations ofMarkarian 6 to look for evidence of emission from shocked gas around the external radio bubbles, using spatially resolved regions in Chandra and spectral analysis of the XMM data. The results show that the bubbles in Mrk 6 are indeed driving a shock into the halo of the host galaxy, with a Mach number of 3.9. I also find that the spectrum of the AGN has a variable absorbing column, which changes from 8 × 1021 atoms cm−2 to 3 × 1023 atoms cm−2 on short timescales (2-6 years). This is probably caused by a clump of gas close to the central AGN, passing in front of us at the moment of the observation. Using new and archival Chandra observations of the Circinus galaxy, I match them to pre-existing radio, infrared and optical data to study the kpc-scale emission. As for Mrk 6, I find that the radio bubbles in Circinus are driving a shock into the interstellar medium of the host galaxy, with Mach numbers M 2.7–3.6 and M 2.8–5.3 for the W and E shells respectively. Comparing the results with those we previously obtained for Centaurus A, NGC 3801 and Mrk 6, I show that the total energy in the lobes (thermal+kinetic) scales approximately with the radio power of the parent AGN. The spatial coincidence between the X-ray and edge-brightened radio emission in Circinus resembles the morphology of some SNR shocks, a parallel that has been expected for AGN, but has never been observed before. I investigate what underlying mechanisms both types of systems may have in common, arguing that, in Circinus, the edge-brightening in the shells may be accounted for by a B field enhancement caused by shock compression, but do not preclude some local particle acceleration. I also carry out a systematic study of the X-ray emission from the cores in the 0.02 < z < 0.7 2Jy sample, using Chandra and XMM-Newton observations. I combine the results with the mid-IR, optical emission line and radio luminosities, and compare them with those of the 3CRR sources, to show that the low-excitation objects in our sample show all the signs of radiatively inefficient accretion. I study the effect of the jet-related emission on the various luminosities, confirming that it is the main source of soft X-ray emission for our sources. I also find strong correlations between the accretion-related luminosities, and identify several sources whose optical classification is incompatible with their accretion properties. I derive the bolometric and jet kinetic luminosities for the sample and find a difference in the total Eddington rate between the low and high-excitation populations, with the former peaking at 1 per cent and the latter at 20 per cent Eddington. There is, however, an overlap between the two, indicating that a simple Eddington switch may not be possible. The apparent independence of jet kinetic power and radiative luminosity in the highexcitation population in our plots allows us to test the hypothesis in which jet production and radiatively efficient accretion are in fact independent processes that can coexist in high-excitation objects.

For the past several decades, mass estimates for supermassive black holes hosted by active galactic nuclei (AGN) have been made with the reverberation mapping (RM) technique. This methodology has produced consistent results and has been used to establish several relations that link the characteristics of the host galaxy to the mass of the central black hole. Despite this success, there are less than 50 AGN with black hole masses derived from RM. This low number is generally attributed to the difficulties in coordinating large blocks of telescope time for making simultaneous photometric and spectroscopic observations. Spectroscopic observations also generally require several months of nightly observations with moderate to large size telescopes as the signal-to-noise ratio is too low for smaller telescopes. We have made photometric observations of NGC 5548 in four filters (a custom-made Hα10 filter, the Strömgren y filter, the Johnson/Cousins V filter and the Johnson/Cousins R filter) in order to evaluate a photometric methodology for determining the lag time between the variations observed in the continuum and the Hα emission from the broad-line region (BLR) gas. This time delay represents the mean light travel time to the BLR and is therefore a measurement of the mean BLR radius. Multiple JAVELIN analyses of the three continuum light curves (y, V, and R), relative to the light curve from the Hα10 filter yields a value for τ = 3.3 ± 0.1 days. Adopting a value of f = 5.5, along with a single-epoch spectroscopic measurement from Park et of Δv = 4354±25 km/s, enables us to estimate a black hole mass of M_BH = 67.2±2.2x10^6 M_sun.

Dans l’Univers, on observe des galaxies lointaines ne formant plus d’étoiles, mais les astrophysiciens n’ont pas encore identifié avec certitude les phénomènes physiques à l’origine de leur “mort”. Pour apporter des éléments de réponse, je me suis penchée sur l’étude de phénomènes qui pourraient y jouer un rôle : les processus de rétroaction des étoiles et des trous noirs supermassifs actifs, la formation stellaire, et les vents galactiques. Le Chapitre 1 présente toutes les notions nécessaires à la compréhension du problème : les caractéristiques des galaxies typiques de l’Univers proche et lointain ; les vents galactiques ; la mort des galaxies; les trous noirs supermassifs actifs (noyaux actifs de galaxies, AGN) et les étoiles ; et leur rétroaction. Dans le Chapitre 2, je présente les techniques numériques utilisées : le code de simulations astrophysiques RAMSES et le code de transfert radiatif Cloudy, que j’ai utilisé pour développer une méthode de calcul de l’état d’ionisation d’une galaxie, détaillée au Chapitre 3. Le Chapitre 4 étudie le couplage entre les trous noirs actifs et les étoiles, avec le projet POGO, Origines Physiques des Vents Galactiques. Durant cette thèse, j’ai montré que les trous noirs actifs n’étaient pas en mesure de tuer subitement leur hôte, même en prenant en compte la rétroaction des étoiles, et que leur couplage peut réduire ou renforcer les vents dans les galaxies en fonction de leur masse. Le Chapitre 5 fait un état de l’art du domaine avant et pendant mon doctorat, reprend les conclusions de cette thèse et donne quelques perspectives, notamment en ce qui concerne le rôle additionnel des rayons cosmiques dans la mort des galaxies
In the Universe, we observe galaxies forming no, or almost no, stars anymore, but astrophysicists do not know yet what physical mechanisms cause their “death”. To give clues to solve the problem, I studied feedback processes from stars and active supermassive black holes, star formation and galactic outflows. Chapter 1 presents all the notions to understand the problem: the characteristics of typical galaxies in the local and distant Universe, galactic outflows, galaxy death, active supermassive black holes, stars, and their feedback processes. In Chapter 2, I describe the numerical techniques I used: the simulation code RAMSES, and the radiative transfer code Cloudy, which I used to develop a computation method to get the ionization state of an entire galaxy. This method is presented in Chapter 3. Chapter 4 studies the coupling between the feedback processes of active supermassive black holes and stars, with the POGO project, Physical Origins of Galactic Outflows. During this thesis, I showed that typical active supermassive black hole cannot suddenly kill their host, even when stellar feedback processes are accounted for, and that their coupling either reduces or enhances the mass outflow rate depending on the mass of the host. In Chapter 5, I give a state-of-the-art about active supermassive black holes before and during my thesis, sum up the conclusions of the work, and give perspectives to enlarge the scope of the study, especially regarding the additional role of cosmic rays in the death of galaxies

Neste trabalho estudamos a dinâmica de discos torcidos finos e espessos para compreender melhor a propagação da deformação nestes discos. No caso dos discos finos, estudamos a física do efeito Bardeen-Petterson e aplicamos este modelo para explicar o jato em escalas de parsec e kilo-parsec da galáxia NGC 1275. Encotramos que o efeito Bardeen-Petterson reproduziu muito bem a forma do jato e com isto derivamos os parâmetros do disco como raio, valores das viscosidades azimutal e vertical, lei de potência da densidade superficial e spin do buraco negro. Para uma melhor compreensão da física destes discos, realizamos simulações GRMHD de discos moderadamente finos tanto planos como inclinados para estudar a evolução do ângulo de inclinação entre os momentos angular do buraco negro e do disco de acresção assim como o ângulo de torção que está associado com a precessão do disco. Encontramos que quando o disco de acresção e o buraco negro rotacionam no mesmo sentido, o ângulo de inclinação entre os momentos angular apresentou um comportamento oscilatório na parte interna do disco e permaneceu constante na parte externa em acordo com as previsões teóricas. Já quando o buraco negro rotacina no sentido oposto ao disco de acresção, encontramos pela primeira vez numa simulação GRMHD evidências de alinhamento, ocorrendo um alinhamento de 10\\% do angulo entre os momentos angulares do disco e buraco negro. Além disso, comprovamos pela primeira vez numa simulação GRMHD a não isotropia do stress. Utilizando um modelo semi-analítico, comparamos os resultados de nossas simulações com este modelo, utilizando os dados da simulações de disco plano como entrada e obitivemos os mesmos comportamentos das simulações tanto no caso prógrado quanto no caso retrógrado mostrando que o alinhamento é devido ao regime onda.
In this work we studied the dynamics of twisted thin and thick disks to better understand how the warp propagates in these discs. In the case of thin discs, we studied the physics of the Bardeen-Petterson effect and we applied this model to explain the shape of the jet in both parsec and kilo-parsec scales of the galaxy NGC 1275. We found that the Bardeen-Petterson effect could explain very well the shape of the jet and with that we derived the disc parameters such as its radius, the values of the kinematic azimutal and vertical viscosities, the power-law of the surface density and the spin of the black hole. To better understand the physics of such discs, we have performed GRMHD simulations of moderatelly thin tilted disks to study the evolution of the tilt angle between the angular momentum of the accretion disk and black hole and also the twist angle which is associated with the precession of the disc. We found that when the accretion disc and the black hole are rotating in the same direction, the tilt angle showed an oscillatory behavior in the inner parts of the disk while in the outer parts it remained constant in agreement with the theorical modelos. However, when both rotate in the opposite direction, we found for the very first time in a GRMHD simulation, evidences of alignment of 10\\% of the tilt angle. Besides that, we prove for the first time in a GRMHD simulation that the stress is far from being isotropic. Using a semi-analitic model, we compared the results of our simulations with this model, using the datas of the untilted simulations as inputs and we found the same behaviors found in the simulations even in prograde case as in the retrograde case showing that the alignment is due to bending waves.

In this thesis we present numerical and analytical models of supermassive black hole (SMBH) feeding, via deposition of gas, in galactic nuclei. Through simulations, we consider the environment of galactic centres, starting at sub-parsec scales within our own Milky Way, and moving upwards in scale and outwards in generality to scales of hundreds of parsecs in typical galaxies and finally to dark matter halos within which galaxies reside. We find that the stellar features observed in our own Galactic centre are likely explained by a collision between two molecular clouds at a distance of a few parsecs from the central black hole, Sgr A*. The amount of gas transported to small radii is large, occurring on a timescale close to dynamical. The disordered nature of the flow leads to the formation of a gaseous disc around Sgr A* that in some cases remains small-scale, undergoing complex, time-varying evolution in its orientation. Such a disc would efficiently feed the SMBH, if replenished from larger scales. We develop a model for ballistic accretion onto an SMBH at the centre of a typical galaxy, from scales of ~ hundreds of parsecs. We invoke turbulence in the gas, assumed to be driven by feedback from supernovae, as the means to create such a flow. The accretion mode is again dominated, soon after the initial turbulent kick, by the dynamical timescale for the gas in the angular momentum loss-cone, resulting in an accretion rate at or near Eddington, >~ 1Mסּ yr−1. At the largest scale, we critically evaluate the current state-of-the-art prescription for SMBH growth in cosmological simulations, finding that in general it lacks a physically consistent basis. We propose an alternative, motivated by our analytical estimates and numerical simulations, that is based on the free-fall time.

Ventos (em inglês outflows) de ampla abertura e larga escala sâo uma característica comum em galáxias ativas, como as galáxias Seyfert. Em sistemas como este, onde buracos negros supermassivos (em inglês super massive black holes, SMBHs) de núcleos galácticos ativos de galáxias (em inglês active galactic nuclei, AGN) coexistem com regiões de formação estelar (em inglês star forming, SF), nâo está claro das observações se o AGN SMBH ou o SF (ou ambos) são responsaveis pela indução desses ventos. Neste trabalho, estudamos como ambos podem influenciar a evolução da galáxia hospedeira e seus outflows, considerando galáxias tipo Seyfert nas escalas de kilo-parsec (kpc). Para este objetivo, estendemos o trabalho anterior desenvolvido por Melioli & de Gouveia Dal Pino (2015), que considerou ventos puramente hidrodinâmicos impulsionados tanto pela SF quanto pelo AGN, mas levando em conta para este último apenas ventos bem estreitos (colimados). A fim de obter uma melhor compreensão da influencia (feedback) desses mecanismos sobre a evolução da galáxia e seus outflows, incluímos também os efeitos de ventos de AGN com maior ângulo de abertura, já que ventos em forma de cone podem melhorar a interação com o meio interestelar da galáxia e assim, empurrar mais gás nos outflows. Além disso, incluímos também os efeitos dos campos magnéticos no vento, já que estes podem, potencialmente, ajudar a preservar as estruturas e acelerar os outflows. Realizamos simulações tridimensionais magneto-hidrodinâmicas (MHD) considerando o resfriamento radiativo em equilíbrio de ionização e os efeitos dos ventos do AGN com dois diferentes ângulos de abertura (0º e 10º) e razões entre a pressão térmica e a pressão magnética beta=infinito, = 300 e 30, correspondentes a campos magnéticos 0, 0,76 micro-Gauss e 2,4 micro-Gauss respectivamente. Os resultados de nossas simulações mostram que os ventos impulsionados pelos produtos de SF (isto é, pelas explosões de supernovas, SNe) podem direcionar ventos com velocidades 100-1000 km s¹, taxas de perda de massa da ordem de 50 Massas solares/ano, densidades de ~1-10 cm-3 e temperaturas entre 10 e 10 K, que se assemelham às propriedades dos denominados absorvedores de calor (em inglês warm absorbers, WAs) e também são compatíveis com as velocidades dos outflows moleculares observadas. No entanto, as densidades obtidas nas simulações são muito pequenas e as temperaturas são muito grandes para explicar os valores observados nos outflows moleculares (que têm n ~150-300 cm³ e T<1000 K). Ventos colimados de AGN (sem a presença de ventos SF) também são incapazes de conduzir outflows, mas podem acelerar estruturas a velocidades muito altas, da ordem de ~10.000 km s¹ e temperaturas T> 10 K, tal como observado em ventos ultra rapidos (em inglês, ultra-fast outflows, UFOs). A introdução do vento de AGN, particularmente com um grande ângulo de abertura, causa a formação de estruturas semelhantes a fontes galácticas. Isso faz com que parte do gás em expansão (que está sendo empurrado pelo vento de SF) retorne para a galáxia, produzindo um feedback \'positivo\' na evolução da galáxia hospedeira. Descobrimos que esses efeitos são mais pronunciados na presença de campos magnéticos, devido à ação de forças magnéticas extras pelo vento AGN, o qual intensifica o efeito de retorno do gás (fallback), e ao mesmo tempo reduz a taxa de perda de massa nos outflows por fatores de até 10. Além disso, a presença de um vento de AGN colimado (0º) causa uma remoção significativa da massa do núcleo da galáxia em poucos 100.000 anos, mas este é logo reabastecido pelo de gás acretante proveniente do meio interestelar (ISM) à medida que as explosões de SNe se sucedem. Por outro lado, um vento de AGN com um grande ângulo de abertura, em presença de campos magnéticos, remove o gás nuclear inteiramente em alguns 100.000 anos e não permite o reabastecimento posterior pelo ISM. Portanto, extingue a acreção de combustível e de massa no SMBH. Isso indica que o ciclo de trabalho desses outflows é de cerca de alguns 100.000 anos, compatível com as escalas de tempo inferidas para os UFOs e outflows moleculares observados. Em resumo, os modelos que incluem ventos de AGN com um ângulo de abertura maior e campos magnéticos, levam a velocidades médias muito maiores que os modelos sem vento de AGN, e também permitem que mais gás seja acelerado para velocidades máximas em torno de ~10 km s¹, com densidades e temperaturas compatíveis com aquelas observadas em UFOs. No entanto, as estruturas com velocidades intermediárias de vários ~100 km s¹ e densidades até uns poucos 100 cm³, que de fato poderiam reproduzir os outflows moleculares observados, têm temperaturas que são muito grandes para explicar as características observadas nos outflows moleculares, que tem temperaturas T< 1000 K. Além disso, estes ventos de AGN não colimados em presença de campos magnéticos entre T< 1000 K. Alem disso, estes grandes ventos AGN de angulo de abertura em fluxos magnetizados reduzem as taxas de perda de massa dos outflows para valores menores que aqueles observados tanto em outflows moleculares quanto em UFOs. Em trabalhos futuros, pretendemos estender o espaço paramétrico aqui investigado e também incluir novos ingredientes em nossos modelos, como o resfriamento radioativo fora do equilíbrio, a fim de tentar reproduzir as características acima que não foram explicadas pelo modelo atual.
Large-scale broad outflows are a common feature in active galaxies, like Seyfert galaxies. In systems like this, where supermassive black hole (SMBH) active galactic nuclei (AGN) coexist with star-forming (SF) regions it is unclear from the observations if the SMBH AGN or the SF (or both) are driving these outflows. In this work, we have studied how both may influence the evolution of the host galaxy and its outflows, considering Seyfert-like galaxies at kilo-parsec (kpc) scales. For this aim, we have extended previous work developed by Melioli & de Gouveia Dal Pino (2015), who considered purely hydrodynamical outflows driven by both SF and AGN, but considering for the latter only very narrow (collimated) winds. In order to achieve a better understanding of the feedback of these mechanisms on the galaxy evolution and its outflows, here we have included the effects of AGN winds with a larger opening angle too, since conic-shaped winds can improve the interaction with the interstellar medium of the galaxy and thus push more gas into the outflows. Besides, we have also included the effects of magnetic fields in the flow, since these can potentially help to preserve the structures and speed up the outflows. We have performed three-dimensional magneto-hydrodynamical (MHD) simulations considering equilibrium radiative cooling and the effects of AGN-winds with two different opening angles (0º and 10º), and thermal pressure to magnetic pressure ratios of beta=infinite, 300 and 30 corresponding to magnetic fields 0, 0.76 micro-Gauss and 2.4 micro-Gauss, respectively. The results of our simulations show that the winds driven by the products of SF (i.e., by explosions of supernovae, SNe) alone can drive outflows with velocities ~100-1000 km s¹, mass outflow rates of the order of 50 Solar Masses yr¹, densities of ~1-10 cm³, and temperatures between 10 and 10 K, which resemble the properties of warm absorbers (WAs) and are also compatible with the velocities of the observed molecular outflows. However, the obtained densities from the simulations are too small and the temperatures too large to explain the observed values in molecular outflows (which have n ~ 150-300 cm³ and T<1000 K). Collimated AGN winds alone (without the presence of SF-winds) are also unable to drive hese outflows, but they can accelerate structures to very high speeds, of the order of ~ 10.000 km s¹, and temperatures T> 10 K as observed in ultra-fast outflows (UFOs). The introduction of an AGN wind, particularly with a large opening angle, causes the formation of fountain-like structures. This makes part of the expanding gas (pushed by the SF-wind) to fallback into the galaxy producing a \'positive\' feedback on the host galaxy evolution. We have found that these effects are more pronounced in presence of magnetic fields, due to the action of extra magnetic forces by the AGN wind producing enhanced fallback that reduces the mass loss rate in the outflows by factors up to 10. Furthermore, the presence of a collimated AGN wind (0º) causes a significant removal of mass from the core region in a few 100.000 yr, but this is soon replenished by gas inflow from the interstellar medium (ISM) when the SNe explosions fully develop. On the other hand, an AGN wind with a large opening angle in presence of magnetic fields is able to remove the nuclear gas entirely within a few 100.000 yr and does not allow for later replenishment. Therefore, it quenches the fueling and mass accretion onto the SMBH. This indicates that the duty cycle of these outflows is around a few 100.000 yr, compatible with the time-scales inferred for the observed UFOs and molecular outflows. In summary, models that include AGN winds with a larger opening angle and magnetic fields, lead to to be accelerated to maximum velocities around 10 km s¹ (than models with collimated AGN winds), with densities and temperatures which are compatible with those observed in UFOs. However, the structures with intermediate velocities of several ~100 km s¹ and densities up to a few 100 cm3, that in fact could reproduce the observed molecular outflows, have temperatures which are too large to explain the observed molecular features, which have temperatures T<1000 K. Besides, these large opening angle AGN winds in magnetized flows reduce the mass loss rates of the outflows to values smaller than those observed both in molecular outflows and UFOs. In future work, we intend to extend the parametric space here investigated and also include new ingredients in our models, such as non-equilibrium radiative cooling, in order to try to reproduce the features above that were not explained by the current model.

The hard X-ray spectrum from black hole candidates, such as 1E1740.7-2942 and Cygnus X-1, has been attributed to an inner hot (Tₑ ≈ 10⁹ K) two-temperature disk which Comptonizes externally produced soft photons from the outer disk. We developed a natural extension of this model, wherein the innermost region of the two-temperature disk, is much hotter (Tₑ ≈ 5 x 10⁹ K) since it is shielded from the external photons and is forced to cool via bremsstrahlung self-comptonization. The emission from this region can account for the long term γ-ray variability in Cygnus X-1. The e⁺e⁻ pairs produced above the hot plasma give rise to the annihilation line observed in 1E1740.7-2942 and the residual pairs form the extended radio jets observed in this source. These early successes called for more detailed modeling of the hot disk. The effects of e⁺e⁻ pairs produced inside the disk were investigated using a better technique for the Comptonization process than what had been reported previously in the literature. This has important quantitative (but no qualitative implications) on the model. Another crucial effect is that of the proton thermal energy being advected to smaller radii. This makes the disk sensitive to the outer boundary conditions (i.e the structure of the transition zone between the outer cold disk and the inner hot region). To determine the physics of the transition region we have developed a scheme for the radiative cooling which is valid at all optical depths. The application of this scheme revealed that the transition region is an extended one. The disk has a hot two-temperature configuration even without the assumption that instabilities in the disk drive the cold disk to this stage (which has been doubted in the past). Moreover, the spectrum from the transition zone matches well with the observed X-ray spectrum of Cygnus X-1 and variations in the magnetic field can account for the two X-ray states. We conclude from this new self-consistent model that the soft X-ray photons observed arise from the cold disk, the X-ray continuum is produced in the transition zone, while the γ-rays and associated phenomena (like the e⁺e⁻ line and the radio jets) are due to the inner hot disk.

Optical observations of a sample of narrow-line and classical broad-line Seyfert 1 galaxies are presented. Off-nuclear spectra of the bulge were used to obtain the line-of-sight stellar velocity dispersion σ_*. Nuclear spectra were used to remove residual nuclear emission from the bulge spectra and to calculate single-epoch virial black hole mass estimates. The two samples were augmented using objects taken from the literature. The optical flux variability of the least-luminous known Seyfert 1 galaxy, NGC 4395, has been a matter of controversy, with a number of apparently contradictory reports. An observed short time-scale continuum variability is consistent with NGC 4395 having an intermediate-mass (~ 10⁵M_⊙) central supermassive black hole, rather than a very low accretion rate. Comparison with the Seyfert 1 galaxy NGC 5548 shows that the observed variability seems to scale with black hole mass in roughly the manner expected in accretion models. However the absolute timescale of variability differs by several orders of magnitude from that expected in simple accretion disc models in both cases.

Galactic black hole X-ray transients (BHXRTs) represent the ideal opportunity to study accretion physics in extreme environments. Questions relating to accretion geometry, the mass of the compact object, how transient outbursts start and how jets form and evolve can only be answered by observing BHXRTs across multiple wavelengths. In this thesis I take a multi-wavelength approach to studying BHXRTs, and attempt to reconcile many aspects of accretion physicsusing a number of techniques. I have used timing analysis techniques to discover a super-orbital periodicity of 420 days in the X-ray and optical light curves of the BHXRT Swift J1753.5-0127, whilst also uncovering the likely orbital period of the candidate BHXRT MAXI J1305-704. X-ray spectral observations of Swift J1753.5-0127 revealed its first ever transition to a soft accretion state. This was found to be one of the lowest luminosity soft states ever recorded in such a system, at < 1% the Eddingtonluminosity, which proved crucial in the subsequent radio observations. Collaborators and I found that the compact jet had been quenched by a factor > 25, indicating that jet quenching was not dependent on accretion rate. In the optical regime, spectroscopy can reveal details about the mass of the compact object, which is important for population studies of black holes (BHs). I place a lower limit on the mass of Swift J1753.5-0127at M1 > 7:41:2M, conrming it as a BH and removing it from the so-called 'mass gap'. However, optical observations can also reveal details about the accretion geometryof a source, as in the case of the BHXRT V404 Cyg, in which collaborators and I combined optical spectroscopy and photometry and found evidence for the 2015 outburst initiating 1 week before the X-ray outburst was detected. Our results were found to be consistent with the current disc instability model for transient outbursts. I summarise these ndings and suggest avenues for future work in the closing pages of this thesis. Overall, this work shows that we can only truly understand BHXRTs once we have studied them across multiple wavelengths, and only in doing so can we discuss the underlying physics behind some of the most extreme regions of the Universe.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.
Includes bibliographical references (p. 183).
In rare cases, optical observations of Galactic binary star systems which are bright in the X-ray portion of the electromagnetic spectrum dynamically constrain the mass of one component to be well above theoretical limits for a neutron star. These systems - and systems with similar X-ray properties - are classified as black hole binaries. In this thesis, I report on observations of black hole binaries made with satellite observatories in the X-ray band. The region closest to the black hole is revealed in X-rays due to the viscous heating of matter that is accreted from the companion star. X-ray observations of these systems may therefore reveal General Relativistic effects. A fundamental and testable prediction of General Relativity is that matter may orbit more closely around black holes with significant angular momentum. I have investigated the possibility of black hole "spin" and the geometry of accretion flows in these systems using X-ray continuum spectroscopy, fast variability studies, and the shape of iron fluorescent emission lines in this band. I present evidence for black hole spin in XTE J1550-564, XTE J1650-500, and XTE J1748-248. Spin is not required by high-resolution spectral analysis of the archetypical Galactic black hole - Cygnus X-1 but a thermal accretion disk plus hot corona geometry is confirmed. Studies of XTE J1118+480 and GRS 1758-258 at low X-ray luminosity reveal that models for radiatively-inefficient accretion do not satisfactorily describe the geometry in these systems.
y Jon Matthew Miller.
Ph.D.

Thesis (Ph. D.) -- University of Maryland, College Park, 2007.
Thesis research directed by: Dept. of Astronomy. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.