Дисертації з теми "Galaxy feedback"

Feedback from the most massive components of a young stellar cluster deeply affects the surrounding ISM driving an expanding over-pressured hot gas cavity in it. In spiral galaxies these structures may have sufficient energy to break the disk and eject large amount of material into the halo. The cycling of this gas, which eventually will fall back onto the disk, is known as galactic fountains. We aim at better understanding the dynamics of such fountain flow in a Galactic context, frame the problem in a more dynamic environment possibly learning about its connection and regulation to the local driving mechanism and understand its role as a metal diffusion channel. The interaction of the fountain with a hot corona is hereby analyzed, trying to understand the properties and evolution of the extraplanar material. We perform high resolution hydrodynamical simulations with the moving-mesh code AREPO to model the multi-phase ISM of a Milky Way type galaxy. A non-equilibrium chemical network is included to self consistently follow the evolution of the main coolants of the ISM. Spiral arm perturbations in the potential are considered so that large molecular gas structures are able to dynamically form here, self shielded from the interstellar radiation field. We model the effect of SN feedback from a new-born stellar cluster inside such a giant molecular cloud, as the driving force of the fountain. Passive Lagrangian tracer particles are used in conjunction to the SN energy deposition to model and study diffusion of freshly synthesized metals. We find that both interactions with hot coronal gas and local ISM properties and motions are equally important in shaping the fountain. We notice a bimodal morphology where most of the ejected gas is in a cold $10^4$ K clumpy state while the majority of the affected volume is occupied by a hot diffuse medium. While only about 20\% of the produced metals stay local, most of them quickly diffuse through this hot regime to great scales.

Several hundred million years after the Big Bang, the Epoch of Reionisation(EoR) started as the photons from the first objects ionised neutral baryons in the Universe. The observations such as the Gunn-Peterson troughs in quasar absorption spectra and the linear polarisation of the cosmic microwave background (CMB) impose strong constraints on reionisation models of the EoR. Recent data provide the rest-frame ultraviolet luminosity of galaxies up to redshift 10. However, the observation of star formations in low mass galaxies is still not practicable. Their star formations are expected to be suppressed by the increase of ionised baryons and greatly affect the reionisation models. We develop a flexible pipeline which utilises the Munich Semi-Analytic Model of galaxy formation, L-Galaxies, and a semi-numerical modelling of cosmic reionisation. This combination allows us to create a self-consistent reionisation simulation in computational models of galaxy formation. We use this pipeline on a high resolution cosmological Nbody simulation to produce the redshift evolution of the star forming galaxies during the EoR. Comparisons of the properties of mock galaxies and the growth of ionised hydrogen bubbles suggest that the reionisation history heavily depends on the suppression models used in the modeling of dwarf galaxy formation. During this research, some numerical flaws of merger tree generation algorithms were identified. We investigated the origins of these problems and present suggestions for solving them.

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.

The evolution of galaxy cluster counts is a powerful probe of several fundamental cosmological parameters. A number of recent studies using this probe have claimed tension with the cosmology preferred by the analysis of the Planck primary CMB data, in the sense that there are fewer clusters observed than predicted based on the primary CMB cosmology. One possible resolution to this problem is systematic errors in the absolute halo mass calibration in cluster studies, which is required to convert the standard theoretical prediction (the halo mass function) into counts as a function of the observable (e.g., X-ray luminosity, Sunyaev-Zel'dovich flux, optical richness). Here I propose an alternative strategy, which is to directly compare predicted and observed cluster counts as a function of the one-dimensional velocity dispersion of the cluster galaxies. I show that the velocity dispersion of groups/clusters can be theoretically predicted as robustly as mass but, unlike mass, it can also be directly observed, thus circumventing the main systematic bias in traditional cluster counts studies. With the aid of the BAHAMAS suite of cosmological hydrodynamical simulations, I demonstrate the potential of the velocity dispersion counts for discriminating even similar ΛCDM models. Then, I compare the abundance of groups in the GAMA survey to the predictions from BAHAMAS to constrain the values of several cosmological parameters. Additionally, I investigate the role of active galactic nuclei (AGN) in galaxy evolution. The color bimodality of galaxy populations roughly divides galaxies into two groups: blue, star-forming galaxies, and red, quiescent galaxies. One theory that explains how high-mass, red, non-star-forming galaxies maintain this condition is the duty cycle hypothesis. This hypothesis invokes AGN feedback from low luminosity radio-loud AGN (LERGs) to deposit mechanical heating into the intergalactic medium, thus preventing star formation. I test this hypothesis by comparing the half-light radii of quiescent elliptical galaxies with LERG host galaxies using a large multi-wavelength sample from two surveys, UKIDSS/UDS, and ULTRAVISTA/COSMOS. The radius distribution of the two groups are similar, thus providing evidence for the duty cycle hypothesis. I also check the star formation activity of the LERGs. For the duty cycle to hold, LERGs should reside within non-star-forming galaxies. However, I find that a subset of LERGs appear to be dusty star forming galaxies.

Questo lavoro di tesi presenta uno studio sull'identificazione e l’analisi delle principali proprietà di strutture di gas sottodenso che potrebbero essere prodotte da esplosioni multiple di supernove, le cosiddette “super-bubbles”. Tali strutture sono generate dalla simulazione numerica di una galassia simile alla Via Lattea eseguita tramite il modello SMUGGLE che implementa i principali meccanismi fisici alla base dei processi di formazione galattica. Viene proposto un metodo di identificazione basato su considerazioni statistiche che permette di stimare le proprietà (come raggio, tasso di espansione, età, e massa spazzata) delle strutture candidate ad essere possibili super-bubbles e tale approccio viene applicato sia all'intero campione di strutture identificate sia ad un sotto-campione di cinque candidati. Inoltre il presente lavoro di tesi indaga la capacita del modello SMUGGLE di stabilire una relazione tra il numero di supernovae esplose all'interno di una super-bubble e le sue dimensioni, sia in maniera statistica sia considerando singolarmente alcuni particolari strutture. Infine i risultati ottenuti sono stati confrontati sia con i dati osservativi della Via Lattea ed altre galassie a spirale sia con la teoria sviluppata per l'evoluzione delle super-bubbles.

Active galaxies have been in the forefront of astronomic research since their first discovery, at least 50 years ago (e.g. Schmidt, 1963; Matthews & Sandage, 1963). The putative supermassive black hole (SMBH) at their center characterizes their properties and regulates the evolution of these objects. In this thesis, I study the 'demographics' and 'ecology' of active galactic nuclei (AGN) in the context of their evolution and the interaction with their environments (mainly their host galaxy). The number density of AGN has been found to peak at 1 < z < 3 (e.g. Ueda et al., 2003; Hasinger et al., 2005; Richards et al., 2005; Aird et al., 2010), similar to the star formation history (e.g. Silverman et al., 2008a; Aird et al., 2010). However, when taking into account obscuration, faint AGN are found to peak at lower redshift (z ≤ 2) than that of bright AGN (z ≈ 2 - 3; e.g. Hasinger et al., 2005; Hopkins et al., 2007; Xue et al., 2011). This qualitative behaviour is also broadly seen in star-forming galaxies (e.g. Cowie et al., 1996) and is often referred to as 'cosmic downsizing', although this term has developed a number of usages with respect to galaxies (e.g. Bundy et al., 2006; Cimatti et al., 2006; Faber et al., 2007; Fontanot et al., 2009). Though this behaviour is well established up to z ≈ 3, the nature of how and when the initial seed of these AGNs were formed remains an open question. For this study, I use Chandra surveys to study some of the most distant AGN in the Universe (z > 3). The combination of two different size and depth Chandra surveys (Chandra-COSMOS and ChaMP) provides me with the largest to-date z > 3 AGN sample, over a wide range of rest-frame 2-10 keV luminosities [log (Lₓ/erg s⁻¹) = 43.3-46.0] and obscuration (NH = 10²⁰ - 10²³ cm⁻²). I find strong evidence about a strong decline in number density of X-ray AGN above z ≈ 3, and also the association of this decline with a luminosity-dependent density evolution (LDDE; e.g. Gilli et al., 2007). Especially at high redshifts, the different evolution models predict quite different numbers of AGNs. The large size and the wide X-ray luminosity range of this sample reduces the uncertainties of previous studies at similar redshifts making it possible to distinguish between the different models and suggest that observations appear to favour the LDDE model. The observed AGN downsizing behaviour seen via the measured X-ray luminosity function (XLF) could arise due to changes in the mass of the typical active SMBH and/or changes in the typical accretion rate. But how does the growth of SMBHs over cosmic time influence its environment? A powerful way to address this question is to compare the host galaxy properties over a wide range of AGN and accretion rate types. Radio-jets are one of the most prominent constituents of AGN as they can interact directly with the host galaxy. Although AGN with radio jets are rare (they make up to 10 per cent of the total AGN population) radio galaxies make up over 30 per cent of the massive galaxy population and it is likely that all massive galaxies go through a radio-loud phase, as the activity is expected to be cyclical (e.g Best et al., 2005). It is therefore, important to investigate the impact of radio jets on the host galaxy and particularly the star formation. The method I follow focuses on the comparison of the host galaxy properties between optically selected quasar samples, with and without strong radio emission associated with powerful radio-jets, matched in AGN luminosity. Herschel far-infrared observations are used to trace the star formation in the host galaxy, providing minimal AGN contamination. In my first approach, I have constructed a sample of radio-loud and radio-quiet quasars from the Faint Images Radio Sky at Twenty-one centimetres (FIRST) and the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), over the H-ATLAS Phase 1 Area (9h, 12h and 14.5h). The main result of this work is that RLQs at lower AGN luminosities tend to have on average higher FIR and 250-μm luminosity with respect to RQQs matched in AGN luminosity and redshift. However, evolution effects could be strong as the quasars in this sample cover a wide range of redshifts (0.4 < z < 5). Therefore, I follow a second approach with the advantage of a QSO sample selection at a single redshift epoch, decomposing the evolution effects from the AGN/star-formation study. The results indicate that radio-jets in powerful QSOs can both suppress and enhance the star formation in their host galaxies. These fundings are consistent with a galaxy mass and jet-power dependence model. Then we expect more massive galaxies to have more star-formation for a given jet-power because their star-formation is more enhanced by the jet. Although radio-jets are the best candidates for a direct AGN impact to the host galaxy, many models refer to an AGN feedback associated with energetic AGN winds and outflows which are expected to suppress the star formation in powerful AGN when compared to the overall galaxy population. My results do not suggest star formation is suppressed in the hosts of optically selected QSOs at z ≈ 1, with more than 30 per cent of them being associated with strong star formation rates (SFR ≈ 350 M⊙ yr⁻¹). Although different interpretations are possible, this result can be explained through periods of enhanced AGN activity and star-forming bursts, possibly through major mergers. However, optical QSOs comprise only a small fraction of the total AGN population. Even if the 'unified model' predicts that the host galaxy properties should not be affected by the viewing angle (type-1 vs. type-2 AGN), several studies have shown results supporting a scenario departing from the basic model. Investigating star formation in the hosts of 24 μm selected type-1&2 AGN, I found that the type-2 AGNs display on average higher star-formation rate than type-1 AGNs. This result is in agreement with previous studies suggesting an undergoing transition between a hidden growth phase and an unobscured AGN phase.

Most of the ordinary matter of Galaxy Clusters is in the form of a rarefied and ionized gas, called the Intra Cluster Medium (ICM). The ICM is believed to be constantly in motion, mainly due to turbulence. This thesis analyzes an highly resolved cosmological simulation of the gas dynamics of a single galaxy cluster, which included several dynamical effects for the realistic evolution of the ICM: gravity, matter accretions, shock waves, turbulence, magnetic fields and a central Active Galactic Nucleus (AGN). The AGN has been resimulated five different times by modifying the power of its jets. This promotes the onset of a different long-term dynamics of the ICM. Once the jets are injected, they release relativistic electrons. It is possible to follow the trajectories of such particles through tracers, that are tracked from the beginning to the end of the simulation to study the dynamics they are subject to throughout the life of the cluster. The first purpose of this Thesis is to establish how much intracluster volume can be typically refilled by relativistic electrons expelled by jets, the electrons kinematics and the dependency between jet power and the kinematic of tracers. The second purpose of this thesis is to produce a systematic comparison between observable-like estimates of the age and energetics of X-ray cavities produced by AGN in clusters, and their true value in the simulation. To this end, I analysed X-ray maps in a way that is as similar as possible to the procedure adopted in the real observations. The statistical results of cosmological simulations are also compared with the results obtained by analyzing a second fully idealized simulation, to assess to which extend can my result be affected by specific choices in the assumed numerical and physical setup.

L’évolution des galaxies reste assez mal comprise. La densité de formation piquant entre 1
The evolution of galaxies is something that is still not well understood. The cosmic star formation rate density peaks between 1 < z < 3. It is therefore important to investigate the high-z Universe and the mechanisms which triggers or quenches star-formation in galaxies. In this thesis I investigate the effects of AGN feedback by studying high-z radio galaxies (HzRGs). For a sample of 25 HzRGs at 1 < z < 5.2, I add new ALMA data and determine the star formation rates (SFR) by multi-wavelength spectral energy distribution fitting. The ALMA data reveals that the mm continuum emission can be complicated, with contributions from several thermal dust emission components and/or synchrotron emission. The new estimated SFRs are 7 times lower than in previous studies. We might therefore be observing the effect of the AGN suppressing the growth of the host galaxy. For one source I explore the possibilities of constraining the gas Physics of the host galaxy and the halo gas by combining MUSE and ALMA data cubes. Quiescent ionized halo gas is detected, which coincide with a molecular gas reservoir detected with ALMA in [C I]. These observations probe the complex multi-phase halo gas and show the power of multi-wavelength observations

Feedback -- from stars and supermassive black holes -- is the bottleneck of our understanding on galaxy formation: it is likely to be critical, but neither the working mechanism nor the impact is clear. Supernovae (SNe), the dominant feedback force associated with stars, is the subject of this thesis. We use high-resolution, 3D hydrodynamic simulations to study: (i) how a SN blast wave imparts energy to a multiphase ISM; (ii) how multiple SNe regulate a multiphase ISM; (iii) how SNe drive galactic outflows. We focus on better understanding the physics, quantifying the impacts, and testing the simulations against observations.

Understanding how the baryonic physics affects the formation and evolution of galaxies is one of the most critical questions in modern astronomy. Significant progress in understanding stellar feedback and modeling them explicitly in simulations have made it possible to reproduce a wide range of observed galaxy properties. However, there are still various pieces of missing physics and uncertainties in galaxies of different mass range.

In this thesis, I will explore these missing pieces in baryonic physics on top of the Feedback in Realistic Environments (FIRE) stellar feedback in the cosmological hydrodynamic zoom-in simulations (FIRE-2 suite) and isolated galaxy simulations. These high-resolution simulations with FIRE physics capture multi-phase realistic interstellar medium (ISM) with gas cooling down to 10K, and star formations in dense clumps in giant molecular clouds. They are, therefore, an ideal tool for investigating the missing pieces in baryonic physics.

In the first part of the thesis, Chapter 2, I will focus on the discrete effects of stellar feedback like individual supernovae, hypernovae, and initial mass function (IMF) sampling in dwarfs (10⁹-10¹⁰ M_⊙). These discrete processes of stellar feedback can have maximum effects on the small galaxies without being averaged out. I will show that the discretization of supernovae (SNe) is absolutely necessary, while the effects from IMF sampling and hypernovae (HNe) is not apparent, due to the strong clustering nature of star formation.

In the second part of the thesis, Chapter 3-4, I will focus on fluid microphysics, exploring their effects on galaxy properties and their interplay with stellar feedback in sub-L* galaxies. I will demonstrate that, once the stellar feedback is explicitly implemented as FIRE stellar feedback model, fluid microphysics such as magnetic fields, conduction, and viscosity only have minor effects on the galaxy properties like star formation rate (SFR), phase structure, or outflows. Stellar feedback also strongly alters the amplifications and morphology of the magnetic fields, resulting in much more randomly-oriented field lines. However, despite the stellar feedback, the amplification of magnetic fields in ISM gas is primarily dominated by flux-freezing compression.

In the final part of my thesis, I focus on the massive cluster ellipticals of 10¹²-10¹⁴ M_⊙, where the physical mechanisms that regulate the observation-inferred cooling flows are highly uncertain -- the classic "cooling flow problem". I showed that solutions in the literature not associated with an active galactic nucleus (AGN), including stellar feedback, the cosmic ray from stellar feedback, magnetic fields, conduction, and morphological quenching, cannot possibly quench the galaxies, mostly because of the insufficient energy and the limited size of the affected region. After ruling out the non-AGN feedback solutions to the cooling flow problem, I will go into the most accessible, and perhaps promising solution: "AGN feedback", exploring the generic classes of AGN feedback models proposed in the literature. I am going to show that enhancing turbulence and injecting cosmic ray are probably the most important aspects of AGN feedback in galaxy quenching. Since they provide non-thermal pressure support that stably suppresses the core density, they can stably reduce the cooling flows without overheating the galactic cores.

Solving the cooling flow problem in cool-core galaxy clusters is critical to under- standing the largest structures in the universe. In addition, cool-core systems are the only places where we have observed direct evidence of AGN feedback, and thus provide the unique opportunity to test models of AGN feedback and various other physical processes. In this thesis we study the influence of momentum-driven AGN feedback on cool-core clusters using high-resolution adaptive mesh refinement (AMR) simulations. We find that run-away cooling first happens only in the central 50 pc region while no local instability develops outside the very center of the cluster. The gas is accreted onto the super-massive black hole (SMBH) which powers AGN jets at an increasing rate as the entropy continues to decrease in the core. The ICM first cools into clumps along the propagation direction of the AGN jets due to the non-linear perturbation. As the jet power increases, gas condensation occurs isotropically, forming spatially extended (up to a few tens kpc) structures that resemble the observed Hα filaments in Perseus and many other cool-core cluster. Jet heating elevates the gas entropy and cooling time, halting clump formation. The cold gas that is not accreted onto the SMBH settles into a rotating disk. In the last few Gyr, the ICM cools onto the disk directly while the innermost region of the disk continues to accrete onto the SMBH, powering the AGN jets to achieve a thermal balance. The mass cooling rate averaged over 7 Gyr is &sim 30 solarmass/yr, an order of magnitude lower than the classic cooling flow value (which we obtain in runs without the AGN). Owing to its self-regulating mechanism, AGN feedback can successfully balance cooling with a wide range of model parameters. Besides suppressing cooling, our model produces cold structures in early stages (up to &sim 2 Gyr) that are in good agreement with the observations. However, the long-lived massive cold disk is unrealistic, suggesting that additional physical processes are still needed. Our recent investigation shows that star formation may play an important role.

Aiming at understanding the role of stellar feedback in galaxy evolution, I present a study of the hot interstellar medium in several representative galaxies, based primarily on X-ray observations as well as theoretical modelling. I find that, in the massive disk galaxies NGC2613 and M104, the observed amount of hot gas is much less than that predicted by current galaxy formation models. Such a discrepancy suggests a lack of appropriate treatments of stellar/AGN feedback in these models. I also find that stellar feedback, primarily in the form of mass loss from evolved stars and energy released from supernovae, and presumably consumed by the hot gas, is largely absent from the inner regions of M104, a galaxy of a substantial content of evolved stars but little current star formation. A natural understanding of this phenomenon is that the hot gas is in the form of a galactic-scale outflow, by which the bulk of the stellar feedback is transported to the outer regions and perhaps into the intergalactic space. A comparison between the observed sub-galactic gas structures and model predictions indicate that this outflow is probably subsonic rather than being a classical supersonic galactic wind. Such outflows are likely prevalent in most early-type galaxies of intermediate masses in the present-day universe and thus play a crucial role in the evolution of such galaxies. For the first time I identify the presence of diffuse hot gas in and around the bulge of the Andromeda Galaxy (M31), our well-known neighbor. Both the morphology and energetics of the hot gas suggest that it is also in the form of a large-scale outflow. Assisted with multiwavelength observations toward the circumnuclear regions of M31, I further reveal the relation between the hot gas and other cooler phases of the interstellar medium. I suggest that thermal evaporation, mostly likely energized by Type Ia supernovae, acts to continuously turn cold gas into hot, a process that naturally leads to the inactivity of the central supermassive blackhole as well as the launch of the hot gas outflow. Such a mechanism plays an important role in regulating the multi-phase interstellar medium in the circumnuclear environment and transporting stellar feedback to the outer galactic regions.

Aiming at understanding the role of stellar feedback in galaxy evolution, I present a study of the hot interstellar medium in several representative galaxies, based primarily on X-ray observations as well as theoretical modelling. I find that, in the massive disk galaxies NGC2613 and M104, the observed amount of hot gas is much less than that predicted by current galaxy formation models. Such a discrepancy suggests a lack of appropriate treatments of stellar/AGN feedback in these models. I also find that stellar feedback, primarily in the form of mass loss from evolved stars and energy released from supernovae, and presumably consumed by the hot gas, is largely absent from the inner regions of M104, a galaxy of a substantial content of evolved stars but little current star formation. A natural understanding of this phenomenon is that the hot gas is in the form of a galactic-scale outflow, by which the bulk of the stellar feedback is transported to the outer regions and perhaps into the intergalactic space. A comparison between the observed sub-galactic gas structures and model predictions indicate that this outflow is probably subsonic rather than being a classical supersonic galactic wind. Such outflows are likely prevalent in most early-type galaxies of intermediate masses in the present-day universe and thus play a crucial role in the evolution of such galaxies. For the first time I identify the presence of diffuse hot gas in and around the bulge of the Andromeda Galaxy (M31), our well-known neighbor. Both the morphology and energetics of the hot gas suggest that it is also in the form of a large-scale outflow. Assisted with multiwavelength observations toward the circumnuclear regions of M31, I further reveal the relation between the hot gas and other cooler phases of the interstellar medium. I suggest that thermal evaporation, mostly likely energized by Type Ia supernovae, acts to continuously turn cold gas into hot, a process that naturally leads to the inactivity of the central supermassive blackhole as well as the launch of the hot gas outflow. Such a mechanism plays an important role in regulating the multi-phase interstellar medium in the circumnuclear environment and transporting stellar feedback to the outer galactic regions.

Galactic bulges of early-type spirals and elliptical galaxies comprise primarily old stars, which account for more than half of the total stellar mass in the local Universe. These stars collectively generate a long-lasting feedback via stellar mass loss and Type Ia supernovae. According to the empirical stellar mass loss and supernova rates, the stellar ejecta can be heated to more than 107 K, forming a very hot, diffuse, and iron-rich interstellar medium. Conventionally a strong galactic wind is expected, especially in low- and intermediate-mass early-type galaxies which have a relatively shallow potential well. X-ray observations, however, have revealed that both the temperature and iron abundance of the interstellar medium in such galaxies are unexpectedly low, leading to the so-called “missing feedback” and “missing metal” problems. As an effort to address the above outstanding issues, we have carried out a series of hydrodynamic simulations of galactic bulge feedback on various scales. On galactic halo scales, we demonstrate that the feedback from galactic bulges can play an essential role in the halo gas dynamics and the evolution of their host galaxies. We approximately divide the bulge stellar feedback into two phases: (1) a starbusrt-induced blastwave from the formation of the bulge built up through frequent major mergers at high redshifts; and (2) a gradual feedback from long-lived low mass stars. The combination of the two can heat the surrounding gas beyond the virial radius and stop further gas accretion, which naturally produces a baryon deficit around Milky Way-like galaxies and explains the lack of large-scale X-ray halos. On galactic bulge scales, we study the collective 3-dimensional effects of supernovae with their blastwaves resolved. We find that the sporadic explosions of supernovae can produce a wealth of substructures in the diffuse hot gas and significantly affect the spectroscopic properties of the X-ray-emitting gas. The differential emission measure in the temperature space has a broad lognormal-like distribution. Such distribution enhances the X-ray emission at both low and high energy bands. We further show that the SN Ia ejecta is not well-mixed with the ambient medium and the X-ray emission is primarily from the shocked stellar wind materials which in general have low metallicities. These 3-dimensional effects provide a promising explanation to the above “missing feedback” and “missing metal” problems. In addition, we demonstrate that the supernova iron ejecta forms a very hot bubbles, which have relatively larger radial velocities driven by buoyancy, resulting in a smaller iron mass fraction in the bulk outflow. These distinct properties give a natural explanation to the observed positive iron abundance gradient which has been a puzzle for decades.

abstract: Galaxy formation is a complex process with aspects that are still very uncertain or unknown. A mechanism that has been utilized in simulations to successfully resolve several of these outstanding issues is active galactic nucleus (AGN) feedback. Recent work has shown that a promising method for directly measuring this energy is by looking at small increases in the energy of cosmic microwave background (CMB) photons as they pass through ionized gas, known as the thermal Sunyaev-Zel’dovich (tSZ) effect. In this work, I present stacked CMB measurements of a large number of elliptical galaxies never before measured using this method. I split the galaxies into two redshift groups, "low-z" for z=0.5-1.0 and “high-z” for z=1.0-1.5. I make two independent sets of CMB measurements using data from the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT), respectively, and I use data from the Planck telescope to account for contamination from dust emission. With SPT I find average thermal energies of 7.6(+3.0/−2.3) × 10^60 erg for 937 low-z galaxies, and 6.0(+7.7/−6.3) × 10^60 erg for 240 high-z galaxies. With ACT I find average thermal energies of 5.6(+5.9/−5.6) × 10^60 erg for 227 low-z galaxies, and 7.0(+4.7/−4.4) × 10^60 erg for 529 high-z galaxies. I then attempt to further interpret the physical meaning of my observational results by incorporating two large-scale cosmological hydrodynamical simulations, one with (Horizon-AGN) and one without (Horizon-NoAGN) AGN feedback. I extract simulated tSZ measurements around a population of galaxies equivalent to those used in my observational work, with matching mass distributions, and compare the results. I find that the SPT measurements are consistent with Horizon-AGN, falling within 0.4σ at low-z and 0.5σ at high-z, while the ACT measurements are very different from Horizon-AGN, off by 6.9σ at low-z and 14.6σ at high-z. Additionally, the SPT measurements are loosely inconsistent with Horizon-NoAGN, off by 1.8σ at low-z but within 0.6σ at high-z, while the ACT measurements are loosely consistent with Horizon-NoAGN (at least much more so than with Horizon-AGN), falling within 0.8σ at low-z but off by 1.9σ at high-z.
Dissertation/Thesis
Doctoral Dissertation Astrophysics 2017

Galaxy clusters, the largest gravitationally-bound structures in the universe, are composed of 50-1000s of galaxies, hot X-ray emitting gas, and dark matter. They grow in size over time through cluster and group mergers. The merger history of a cluster can be imprinted on the hot gas, known as the intracluster medium (ICM). Merger signatures include shocks, cold fronts, and sloshing of the ICM, which can form spiral structures. Some clusters host double-lobed radio sources driven by active galactic nuclei (AGN). First, I will present a study of the galaxy cluster Abell 2029, which is very relaxed on large scales and has one of the largest continuous sloshing spirals yet observed in the X-ray, extending outward approximately 400 kpc. The sloshing gas interacts with the southern lobe of the radio galaxy, causing it to bend. Energy injection from the AGN is insufficient to offset cooling. The sloshing spiral may be an important additional mechanism in preventing large amounts of gas from cooling to very low temperatures. Next, I will present a study of Abell 98, a triple system currently undergoing a merger. I will discuss the merger history, and show that it is causing a shock. The central subcluster hosts a double-lobed AGN, which is evacuating a cavity in the ICM. Understanding the physical processes that affect the ICM is important for determining the mass of clusters, which in turn affects our calculations of cosmological parameters. To further constrain these parameters, as well as models of galaxy evolution, it is important to use a large sample of galaxy clusters over a range of masses and redshifts. Bent, double-lobed radio sources can potentially act as tracers of galaxy clusters over wide ranges of these parameters. I examine how efficient bent radio sources are at tracing high-redshift (z>0.7) clusters. Out of 646 sources in our high-redshift Clusters Occupied by Bent Radio AGN (COBRA) sample, 282 are candidate new, distant clusters of galaxies based on measurements of excess galaxy counts surrounding the radio sources in Spitzer infrared images.

Computational hydrodynamical simulations are a very useful tool for understanding how galaxies form and evolve over cosmological timescales not easily revealed through observations. However, they are only useful if they reproduce the sorts of galaxies that we see in the real universe. One of the ways in which simulations of this sort tend to fail is in the prescription of stellar feedback, the process by which nascent stars return material and energy to their immediate environments. Careful treatment of this interaction in subgrid models, so-called because they operate on scales below the resolution of the simulation, is crucial for the development of realistic galaxy models. Equally important is developing effective methods for comparing simulation data against observations to ensure galaxy models which mimic reality and inform us about natural phenomena. This thesis examines the formation and evolution of galaxies and the observable characteristics of the resulting systems. We employ extensive use of cosmological hydrodynamical simulations in order to simulate and interpret the evolution of massive spiral galaxies like our own Milky Way. First, we create a method for producing synthetic photometric images of grid-based hydrodynamical models for use in a direct comparison against observations in a variety of filter bands. We apply this method to a simulation of a cluster of galaxies to investigate the nature of the red-sequence/blue-cloud dichotomy in the galaxy color-magnitude diagram. Second, we implement several subgrid models governing the complex behavior of gas and stars on small scales in our galaxy models. Several numerical simulations are conducted with similar initial conditions, where we systematically vary the subgrid models, afterward assessing their efficacy through comparisons of their internal kinematics with observed systems. Third, we generate an additional method to compare observations with simulations, focusing on the tenuous circumgalactic medium. Informed by our previous studies, we investigate the sensitivity of this new mode of comparison to hydrodynamical subgrid prescription. Finally, we synthesize the results of these studies and identify future avenues of research.

This thesis presents a study of the optical emission-line regions of low-redshift brightest cluster galaxies (BCGs) and how these are connected to energy feedback in cluster cores. The cores of massive galaxy clusters appear to undergo cycles of self-regulating energy feedback, in which the output of nuclear activity in BCGs offsets much of the hot cluster gas cooling. Many of the BCGs in these `cool-core' clusters are surrounded by extended filamentary nebulae that produce characteristic low-ionisation emission spectra. The origin and sources of excitation of these structures are not well understood. A sample of BCGs was observed with a new wide-field, optical integral-field spectrograph. These galaxies produce strong line emission with a range of integrated luminosities. Maps of the morphology and velocities of the line-emitting gas are constructed from these data. A deep observation of the nearby galaxy NGC4696, provides a detailed view of the complex emission nebula. In the inner nebula, a bright spiral filament coincides closely with a prominent dust lane. The data reveal velocities in this structure consistent with infalling, orbit-like trajectories encircling the nucleus. A model is presented in which some of the gas in the filaments originated in the accretion of a gas-rich dwarf galaxy. It is suggested that the filaments are excited by ~200km/s ram-pressure-driven shocks as the gas clouds fall through the halo. The shock models described reproduce the optical spectrum of the nebula. Three BCGs with intermediate line luminosities are studied to investigate how the filament structures are related to the processes of feedback in the cluster cores. Two systems, Abell 3581 and Sersic 159-03, appear to be experiencing phases of feedback dominated by AGN heating. The structure and kinematics of the emitting gas in Abell 3581 show evidence for significant interaction between the radio outflows and the optical nebula, and possible accretion flows into the nucleus. X-ray and radio data indicate that Sersic 159-03 is dominated by AGN feedback, but the kinematics of the optical nebula are consistent with infall or outflow along the filaments. The third system, 2A0335+096, is dominated by a cooling and accumulation phase of feedback. The outer nebula forms a disk-like structure, ~14kpc in radius, with a velocity field of rotation about the BCG. Overall, our data are consistent with ongoing AGN-driven feedback in these systems. A comparative study of the line-luminous galaxies in the sample is also presented. The data for several systems show evidence for a correlation between the velocity dispersions and line flux ratios in the nebulae, suggesting a strong contribution from shocks to the excitation. The large-scale excitation properties of the nebulae are assessed using optical diagnostic diagrams and discussed in the context of mixing between shock and stellar photoionisation excitation models. It is suggested that both shock and photoionisation excitation contribute significantly to the properties of the emission in these BCGs, to a degree that varies among the galaxies.

We investigate three separate topics associated with the formation and evolution of the stellar mass component in galaxy clusters. The work presented herein is based primarily on optical imaging and spectra taken with, respectively, the Canada-France-Hawaii Telescope and Gemini North/South. We confront the result from the optical data analysis with the results from the analysis of high-resolution X-ray data taken with the Chandra and XMM-Newton space observatories. Confirming earlier results, we find that 22% of brightest cluster galaxies (BCGs) show central inversions in their optical color profiles (blue-cores), indicative of recent star formation or AGN activity. Based on the extended sizes of the blue-core regions we favour recent star formation. Comparison with the host cluster central entropies (and other X-ray properties) demonstrates that the source of cold gas required to fuel the recent activity in BCG cores is direct condensation from the rapidly cooling intra-cluster medium. We measure the giant-to-dwarf ratio (GDR) of red sequence galaxies in a sample of 97 clusters to constrain its evolution over the redshift range 0.05 < z < 0.55. We find that the GDR is evolving and can be parameterized by GDR=(0.88 +/- 0.15)z+(0.44 +/- 0.03). We find that the intrinsic scatter in this relation is consistent with zero, after accounting for measurement error, Poisson noise and contributions from large-scale structure. After correcting for cluster mass effects we investigate the evolution of the individual dwarf and giant populations in order to probe the source of the observed GDR evolution. Beyond z=0.25 the GDR evolution is driven by an increase in the number of dwarfs (consistent with interpretations from the literature), however, below z=0.2 the GDR evolution is caused by a significant reduction in the number of giants. We interpret this a evidence for a significant number of major mergers in the giant population at late times. This is supported by the relatively short dynamical friction timescales for these galaxies. We use velocity-broadened stellar template models to fit the optical spectra of 19 BCGs in order to measure their the line-of-sight component of their central velocity dispersions (sigma). The sigma values are combined with previous measurements of effective radii re and effective surface brightness e to investigate the properties of the BCG fundamental plane. We measure a BCG fundamental plane parameterized by log( re )= alpha log( sigma ) + beta log( e ) + gamma, with best fit parameters alpha = 1.24 +/- 0.08, beta = -0.80 +/- 0.1 and gamma = (0.3 +/- 2.0)x10-4. We constrain the intrinsic scatter in this relation to be deltaint = 0.066 +/- 0.010 in re, consistent with previous measures of the scatter in the fundamental plane for regular cluster ellipticals. Comparing the slope parameters (alpha, beta) of the BCG FP to those from previous studies of the FP for regular cluster ellipticals, we find that there is no conclusive evidence for curvature in the unified FP. We use the sigma measurements to estimate the BCG dynamical masses Mdyn. Comparing these estimates with mass proxies for the clusters (Tx, ng) we find that BCG mass is independent of cluster mass with Mdyn = (2.9 +/- 1.8)x1012 solar masses.
Graduate
0606
0605
bildfell@uvic.ca