Academic literature on the topic 'Galactic Black Holes'

AbstractThe dynamical evidence for black holes (BHs) in galactic nuclei is reviewed, with emphasis on recent improvements in spatial resolution, methods for analyzing galaxy spectra and dynamical modeling. M31, M32 and M87 are discussed in some detail.

AbstractSupermassive black holes are common in centers of galaxies. Among the active galaxies, quasars are the most extreme, and their black hole masses range as high as to 6⋅1010M⊙. Binary black holes are of special interest but so far OJ287 is the only confirmed case with known orbital elements. In OJ287, the binary nature is confirmed by periodic radiation pulses. The period is twelve years with two pulses per period. The last four pulses have been correctly predicted with the accuracy of few weeks, the latest in 2007 with the accuracy of one day. This accuracy is high enough that one may test the higher order terms in the Post Newtonian approximation to General Relativity. The precession rate per period is 39°.1 ± 0°.1, by far the largest rate in any known binary, and the (1.83 ± 0.01)⋅1010M⊙primary is among the dozen biggest black holes known. We will discuss the various Post Newtonian terms and their effect on the orbit solution. The over 100 year data base of optical variations in OJ287 puts limits on these terms and thus tests the ability of Einstein's General Relativity to describe, for the first time, dynamic binary black hole spacetime in the strong field regime. The quadrupole-moment contributions to the equations of motion allows us to constrain the ‘no-hair’ parameter to be 1.0 ± 0.3 which supports the black hole no-hair theorem within the achievable precision.

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.