The Coevolution of Galaxies and Supermassive Black Holes: Insights from Surveys of the Contemporary Universe

Aims. Our goal is to carry out a comparative study of the properties of central galaxies hosting active galactic nuclei (AGN) in cosmic voids and their surrounding structures (i.e. filaments and walls) at z = 0, comparing them to non-AGN galaxies in similar environments. Methods. We used the central galaxies selected from the EAGLE project, combined with a void catalogue that identifies voids, filaments, and walls. We categorised our sample of central galaxies into four global environments based on their distance to the nearest void. We analysed several properties such as the star formation activity and black hole mass, as a function of stellar mass and environment for galaxies with and without AGN. Results. We found that the AGN fraction decreases as a function of void-centric distance, with void galaxies displaying the highest AGN fraction (12%), while galaxies in denser environments, display the lowest AGN fraction (6.7%), consistent with observations. The AGN fraction is particularly high in most massive void galaxies when controlling for stellar mass. When comparing AGN host galaxies to inactive ones, we find that AGN galaxies tend to have slightly more massive supermassive black holes (SMBHs), higher specific star formation rates (sSFRs), and a tendency to reside in higher mass haloes at a given stellar mass than non-AGN galaxies. At M ∗ > 10 10.2 M ⊙ , AGN hosts in voids tend to have slightly more massive SMBHs than those in denser environments. Otherwise, the AGN population does not show a clear trend in relation to the global environment. In contrast, non-AGN void galaxies host more massive SMBHs, slightly higher sSFRs, and are located in more massive haloes than those in denser environments. Analysing the recent merger histories of both AGN and non-AGN populations, we find that a larger fraction of massive AGN galaxies have undergone major mergers compared to non-AGN galaxies, regardless of environment. Notably, AGN galaxies in voids show a higher frequency of recent mergers (especially major mergers) than their counterparts in other environments, particularly at high stellar mass. Conclusions. Our results suggest that the evolution of SMBHs in voids is closely related to that of their host galaxies and their surrounding environment, while the most recent AGN activity is more strongly linked to recent interactions.

In this Thesis, we investigate the cosmological co-evolution of supermassive black holes (BHs), Active Galactic Nuclei (AGN) and their hosting dark matter (DM) halos and galaxies, within the standard CDM scenario. We analyze both analytic, semi-analytic and hybrid techniques and use the most recent observational data available to constrain the assumptions underlying our models. First, we focus on very simple analytic models where the assembly of BHs is directly related to the merger history of DM haloes. For this purpose, we implement the two original analytic models of Wyithe & Loeb 2002 and Wyithe & Loeb 2003, compare their predictions to the AGN luminosity function and clustering data, and discuss possible modifications to the models that improve the match to the observation. Then we study more sophisticated semi-analytic models in which however the baryonic physics is neglected as well. Finally we improve the hybrid simulation of De Lucia & Blaizot 2007, adding new semi-analytical prescriptions to describe the BH mass accretion rate during each merger event and its conversion into radiation, and compare the derived BH scaling relations, fundamental plane and mass function, and the AGN luminosity function with observations. All our results support the following scenario: • The cosmological co-evolution of BHs, AGN and galaxies can be well described within the CDM model. • At redshifts z & 1, the evolution history of DM halo fully determines the overall properties of the BH and AGN populations. The AGN emission is triggered mainly by DM halo major mergers and, on average, AGN shine at their Eddington luminosity. • At redshifts z . 1, BH growth decouples from halo growth. Galaxy major mergers cannot constitute the only trigger to accretion episodes in this phase. • When a static hot halo has formed around a galaxy, a fraction of the hot gas continuously accretes onto the central BH, causing a low-energy “radio” activity at the galactic centre, which prevents significant gas cooling and thus limiting the mass of the central galaxies and quenching the star formation at late time. • The cold gas fraction accreted by BHs at high redshifts seems to be larger than at low redshifts.

During galaxy mergers, gas and dust are driven toward the centers of merging galaxies, triggering enhanced star formation and supermassive black hole (SMBH) growth. Theory predicts that this heightened activity peaks at SMBH separations <20 kpc; if sufficient material accretes onto one or both of the SMBHs for them to become observable as active galactic nuclei (AGNs) during this phase, they are known as offset and dual AGNs, respectively. To better study these systems, we have built the ACS-AGN Merger Catalog, a large catalog (N = 204) of uniformly selected offset and dual AGN observed by the Hubble Space Telescope at 0.2 < z < 2.5 with separations <20 kpc. Using this catalog, we answer many questions regarding SMBH−galaxy coevolution during mergers. First, we confirm predictions that the AGN fraction peaks at SMBH pair separations <10 kpc; specifically, we find that the fraction increases significantly at pair separations of <4 kpc. Second, we find that AGNs in mergers are preferentially found in major mergers and that the fraction of AGNs found in mergers follows a logarithmic relation, decreasing as merger mass ratio increases. Third, we do not find that mergers (nor the major or minor merger subpopulations) trigger the most luminous AGNs. Finally, we find that nuclear column density, AGN luminosity, and host galaxy star formation rate have no dependence on SMBH pair separation or merger mass ratio in these systems, nor do the distributions of these values differ significantly from that of the overall AGN population.

Due to the co-evolution of supermassive black holes and their host galaxies, understanding the mechanisms that trigger active galactic nuclei (AGN) are imperative to understanding galaxy evolution and the formation of massive galaxies. It is observationally difficult to determine the trigger of a given AGN due to the difference between the AGN lifetime and triggering timescales. Here, we utilize AGN population synthesis modeling to determine the importance of different AGN triggering mechanisms. An AGN population model is computed by combining an observationally motivated AGN triggering rate and a theoretical AGN light curve. The free parameters of the AGN light curve are constrained by minimizing a \chi squared test with respect to the observed AGN hard X-ray luminosity function. The observed black hole space density, AGN number counts, and X-ray background spectrum are also considered as observational constraints. It is found that major mergers are not able to account for the entire AGN population. Therefore, non-merger processes, such as secular mechanisms, must also trigger AGN. Indeed, non-merger processes are the dominant AGN triggering mechanism at z \lesssim 1--1.5. Furthermore, the shape and evolution of the black hole mass function of AGN triggered by major mergers is intrinsically different from the shape and evolution of the black hole mass function of AGN triggered by secular processes.

The old, red stars that constitute the bulges of galaxies, and the massive black holes at their centres, are the relics of a period in cosmic history when galaxies formed stars at remarkable rates and active galactic nuclei (AGN) shone brightly as a result of accretion onto black holes. It is widely suspected, but unproved, that the tight correlation between the mass of the black hole and the mass of the stellar bulge results from the AGN quenching the surrounding star formation as it approaches its peak luminosity. X-rays trace emission from AGN unambiguously, whereas powerful star-forming galaxies are usually dust-obscured and are brightest at infrared and submillimetre wavelengths. Here we report submillimetre and X-ray observations that show that rapid star formation was common in the host galaxies of AGN when the Universe was 2-6 billion years old, but that the most vigorous star formation is not observed around black holes above an X-ray luminosity of 10(44) ergs per second. This suppression of star formation in the host galaxy of a powerful AGN is a key prediction of models in which the AGN drives an outflow, expelling the interstellar medium of its host and transforming the galaxy's properties in a brief period of cosmic time.

Context.The study of active galactic nuclei (AGNs) is fundamental to discern the formation and growth of supermassive black holes (SMBHs) and their connection with star formation and galaxy evolution. Due to the significant kinetic and radiative energy emitted by powerful AGNs, they are prime candidates to observe the interplay between SMBH and stellar growth in galaxies.Aims.We aim to develop a method to predict the AGN nature of a source, its radio detectability, and redshift purely based on photometry. The use of such a method will increase the number of radio AGNs, allowing us to improve our knowledge of accretion power into an SMBH, the origin and triggers of radio emission, and its impact on galaxy evolution.Methods.We developed and trained a pipeline of three machine learning (ML) models than can predict which sources are more likely to be an AGN and to be detected in specific radio surveys. Also, it can estimate redshift values for predicted radio-detectable AGNs. These models, which combine predictions from tree-based and gradient-boosting algorithms, have been trained with multi-wavelength data from near-infrared-selected sources in theHobby-EberlyTelescope Dark Energy Experiment (HETDEX) Spring field. Training, testing, calibration, and validation were carried out in the HETDEX field. Further validation was performed on near-infrared-selected sources in the Stripe 82 field.Results.In the HETDEX validation subset, our pipeline recovers 96% of the initially labelled AGNs and, from AGNs candidates, we recover 50% of previously detected radio sources. For Stripe 82, these numbers are 94% and 55%. Compared to random selection, these rates are two and four times better for HETDEX, and 1.2 and 12 times better for Stripe 82. The pipeline can also recover the redshift distribution of these sources withσNMAD = 0.07 for HETDEX (σNMAD = 0.09 for Stripe 82) and an outlier fraction of 19% (25% for Stripe 82), compatible with previous results based on broad-band photometry. Feature importance analysis stresses the relevance of near- and mid-infrared colours to select AGNs and identify their radio and redshift nature.Conclusions.Combining different algorithms in ML models shows an improvement in the prediction power of our pipeline over a random selection of sources. Tree-based ML models (in contrast to deep learning techniques) facilitate the analysis of the impact that features have on the predictions. This prediction can give insight into the potential physical interplay between the properties of radio AGNs (e.g. mass of black hole and accretion rate).

Over the past decade, research has revealed contradictory results on whether mergers of galaxies of similar mass, so-called major mergers, are the dominant or at least an essential mechanism for the formation of active galactic nuclei (AGN) and the growth of supermassive black holes (SMBH) in galaxies. In this work, it is investigated whether such a connection exists for the ‘most plausible’ objects – broad line AGN with the highest Eddington ratios. The first sample is based on HST/WFC3 observations and focuses on these objects that possess an Eddington Ratio >0.7 at z=2, the peak epoch of black hole activity. To validate the results and to identify possible discrepancies between observations and simulations, a comparison study is performed on similar objects modeled by the large-scale cosmological simulation ILLUSTRIS-TNG. An additional sample consists of local z=0.2 AGN with an Eddington ratio >0.3, observed with the VLT/FORS. All AGN samples are compared to inactive galaxies of similar stellar mass and redshift. For all three individual studies, the applied methodology is identical: sources are visually identified and classified into merging and unperturbed galaxies. An excess for the respective AGN host galaxies in the subsequently derived major merger fractions would then indicate that AGN are predominantly triggered by such galaxy mergers. The analysis of the observed galaxies at z=2 yields merger fractions of 0.24 ± 0.09 and 0.19 ± 0.04 for the AGN hosts and inactive galaxies, respectively. In addition to the visual assessment, the ILLUSTRIS-TNG data set is analyzed by evaluating the galaxies’ merger histories and accounting for typical observational uncertainties. The resulting best estimates give 0.27 ± 0.07 and 0.22 ± 0.01, indicating an excellent agreement between the two studies at this redshift. In contrast, with 0.41 ± 0.12 and 0.08 ± 0.06, the fractions at z=0.2 show a clear excess in the incidence of major mergers for the AGN host galaxies. The results are analyzed extensively, including an investigation of possible influences due to selection effects, a time lag between the visibility of galaxy mergers and the AGN, as well as the methodology used. While there is no significant evidence that such major mergers are the dominant process for the existence of the studied AGN at z=2, such gravitational encounters appear to be an important mechanism for comparable AGN at z=0.2. This may be due to the fact that the gas required for SMBH to achieve such high accretion rates is less abundant in galaxies at low redshifts, leading to major mergers playing a more dominant role in the local Universe. Nevertheless, regardless of redshift, for a minimum of 50% of this particular AGN population, the origin remains unclear.

The relation between the mass of the supermassive black hole (SMBH) in the center of galaxies and their bulge mass or central velocity dispersion is well known. This suggests a coevolution between the SMBHs and their galaxy hosts. Our aim is to study this relation, specifically, for radio loud galaxies, and as a function of redshiftz. We selected a sample of 42 radio galaxies and active galactic nuclei (AGN) with broad emission lines and spectroscopic redshifts betweenz = 0.3 − 4 by cross-matching the low radio frequency sources from Very Large Array (VLA) FIRST with spectroscopically confirmed galaxies from wide-field surveys, including Sloan Digital Sky Survey (SDSS) DR14 ugriz and Dark Energy Survey (DES) DR2 grzY in the optical, Wield Infrared Survey Explorer (WISE), and the Galaxy And Mass Assembly (GAMA) spectroscopic survey. We characterized the stellar mass (M⋆), star formation, and black hole properties (mass of the central SMBH, Eddington ratioη, and jet power,Qjet). The relation between SMBH mass,M⋆,η, andzis placed into context by comparing them with scaling relations (MBH–M⋆,MBH/M⋆–z,MBH–Qjet, andQjet–η) from the literature. On the basis of a multiwavelength spectral energy distribution modeling, our radio sources are broadly consistent with being on the star-forming main sequence. They have sub-Eddington accretion rates,η ≃ 1% on average, as typically found in type I AGN, while higher accretion rates favor more powerful jets to be launched by the central engine. We find overmassive SMBHs in (17 ± 5)% of our radio sources, similarly to previous studies on nearby early-type galaxies. Altogether, an evolutionary scenario in which radio-mode AGN feedback regulates the accretion onto the SMBHs and the stellar mass assembly of the radio sources is discussed, which may explain the observed phenomenology. This pilot study represents a benchmark for future studies using wide-field surveys such as those withEuclidand theVera RubinObservatory.

We model the cosmological co-evolution of galaxies and their central supermassive black holes (BHs) within a semi-analytical framework developed on the outputs of the Millennium Simulation. This model, described in detail by Croton et al. and De Lucia and Blaizot, introduces a ‘radio mode’ feedback from active galactic nuclei (AGN) at the centre of X-ray emitting atmospheres in galaxy groups and clusters. Thanks to this mechanism, the model can simultaneously explain: (i) the low observed mass dropout rate in cooling flows; (ii) the exponential cut-off in the bright end of the galaxy luminosity function and (iii) the bulge-dominated morphologies and old stellar ages of the most massive galaxies in clusters. This paper is the first of a series in which we investigate how well this model can also reproduce the physical properties of BHs and AGN. Here we analyse the scaling relations, the fundamental plane and the mass function of BHs, and compare them with the most recent observational data. Moreover, we extend the semi-analytic model to follow the evolution of the BH mass accretion and its conversion into radiation, and compare the derived AGN bolometric luminosity function with the observed one. While we find for the most part a very good agreement between predicted and observed BH properties, the semi-analytic model underestimates the number density of luminous AGN at high redshifts, independently of the adopted Eddington factor and accretion efficiency. However, an agreement with the observations is possible within the framework of our model, provided it is assumed that the cold gas fraction accreted by BHs at high redshifts is larger than at low redshifts.

Research over the past decade has shown diminishing empirical evidence for major galaxy mergers being a dominating or even important mechanism for the growth of supermassive black holes in galaxies and the triggering of optically or X-ray selected active galactic nuclei (AGN). We here for the first time test whether such a connection exists at least in the most plausible part of parameter space for this mechanism: the highest specific accretion rate broad-line AGNs at the peak epoch of black hole activity around z = 2. To that end we examine 21 galaxies hosting a high accreting black hole (L/Ledd > 0.7) observed with HST/WFC3 and 92 stellar mass- and redshift- matched inactive galaxies taken from the CANDELS survey. We removed the AGN point sources from their host galaxies and avoided bias in visual classification by adding and then subtracting mock point sources to and from the comparison galaxies, producing matched residual structures for both sets. The resulting samples were joined, randomized, and subsequently visually ranked with respect to perceived strength of structural distortions by 10 experts. The ensuing individual rankings were combined into a consensus sequence and from this we derived merger fractions for both samples. With the merger fractions f$_{m,agn}$ = 0.24 $\pm$ 0.09 for the AGN host galaxy sample and f$_{m,ina}$ = 0.19 $\pm$ 0.04 for the inactive galaxies, we find no significant difference between the AGN host galaxies and inactive galaxies. Also, both samples display comparable fractions of disk-dominated galaxies. These findings are consistent with previous studies for different AGN populations, and we conclude that even black hole growth at the highest specific accretion rates and at the peak of cosmic AGN activity is not predominantly caused by major mergers. (abriged)

The BAT AGN Spectroscopic Survey (BASS) is designed to provide a highly complete census of the key physical parameters of the supermassive black holes (SMBHs) that power local active galactic nuclei (AGNs) (z ≲ 0.3), including their bolometric luminosity (L bol), black hole (BH) mass (M BH), accretion rates (L bol/L Edd), line-of-sight gas obscuration (N H), and the distinctive properties of their host galaxies (e.g., star formation rates, masses, and gas fractions). We present an overview of the second data release of BASS (DR2), an unprecedented spectroscopic AGN survey in spectral range, resolution, and sensitivity, including 1449 optical (∼3200 Å–1 μm) and 233 near-IR (1–2.5 μm) spectra for the brightest 858 ultrahard X-ray (14–195 keV) selected AGNs across the entire sky and essentially all levels of obscuration. This release provides a highly complete set of key measurements (emission-line measurements and central velocity dispersions), with 99.9% measured redshifts and 98% BH masses estimated (for unbeamed AGNs outside the Galactic plane). The BASS DR2 AGN sample represents a unique census of nearby powerful AGNs, spanning over 5 orders of magnitude in AGN bolometric luminosity (L bol ∼ 1040–1047 erg s−1), BH mass (M BH ∼ 105–1010 M ⊙), Eddington ratio (L bol/L Edd ≳ 10−5), and obscuration (N H ∼ 1020–1025 cm−2). The public BASS DR2 sample and measurements can thus be used to answer fundamental questions about SMBH growth and its links to host galaxy evolution and feedback in the local universe, as well as open questions concerning SMBH physics. Here we provide a brief overview of the survey strategy, the key BASS DR2 measurements, data sets and catalogs, and scientific highlights from a series of DR2-based works pursued by the BASS team.

In a previous work, we have proposed a sub-grid model of active galactic nucleus (AGN) feedback by taking into account the state-of-the-art AGN physics, and used that model to study the effect of AGN feedback on the evolution of an isolated elliptical galaxy by performing 2D high-resolution (i.e. the Bondi radius is well resolved) simulations. In that work, typical values of model parameters were adopted. In this work, we extend that study by exploring the effects of uncertainties of parameter values. Such a study is also useful for us to understand the respective roles of various components of the model. These parameters include the mass flux and velocity of AGN wind and radiative efficiency in both the hot and cold feedback modes, and the initial black hole (BH) mass. We find that the velocity of AGN wind in the hot mode is the most important quantity to control the typical accretion rate and luminosity of AGN, and the mass growth of the BH. The effect of the wind on star formation is less sensitive. Within the limited parameter range explored in this work, a stronger AGN wind suppresses star formation within ∼100 pc but enhances star formation beyond this radius, while the star formation integrated over the evolution time and the whole galaxy roughly remain unchanged. AGN radiation suppresses the BH accretion in a mild way, but dust is not considered here. Finally, a smaller initial BH mass results in a more violent evolution of the BH accretion rate. The corresponding AGN spends more time in the high-luminosity state and the percentage of BH mass growth is higher. Our results indicate the robustness of AGN feedback in keeping the galaxy quenched.

Recent James Webb Space Telescope (JWST) observations have unveiled a numerous population of low-luminosity active galactic nuclei (AGNs) at $4\lesssim z\lesssim 10$, with space densities roughly an order of magnitude above pre-JWST estimates, and many of these AGNs have masses orders of magnitude above the local black hole mass–stellar mass ($M_{\rm BH}-M_{\star }$) scaling relations. We investigate the consistency of these observations within a data-driven framework that links the galaxy stellar mass function to the supermassive black hole (SMBH) mass function and AGN luminosity functions using different $M_{\rm BH}-M_{\star }$ relations and the observed Eddington-ratio distribution. By comparing our predictions against observed AGN luminosity functions at $z\sim 5.5$ we find that observations can be reproduced either by highly elevated $M_{\rm BH}-M_{\star }$ relations paired with low duty cycles ($f_{\rm AGN}\sim 0.08$), or moderate relations with higher duty cycles ($f_{\rm AGN}\sim 0.5$). Through the Sołtan argument, we find that $M_{\rm BH}-M_{\star }$ relations that are modestly above the local relation for AGNs produce consistency between multiple tracers of the SMBH demography at $z\sim 5.5$, while more extreme normalizations would require a weakly evolving luminosity function at $z\ge 5.5$. Continuity-equation modelling shows that initially high $M_{\rm BH}-M_{\star }$ relations predict a strong two-phase evolutionary scenario and very steep low-mass SMBH mass functions in tension with several current estimates, while more moderate relations generate local SMBH mass functions in better agreement with present determinations and near-constant scaling relations. Our results favour a scenario where SMBHs at $z \sim 5$ on average lie modestly above local AGN scaling relations, with elevated but physically plausible duty cycles. Future wide-field clustering and demographic studies will help break the remaining degeneracies between SMBH scaling relations and AGN duty cycles at early cosmic times.

We develop semi-empirical models of the supermassive black hole and active galactic nucleus (AGN) populations, which incorporate the black hole growth implied by the observed AGN luminosity function assuming a radiative efficiency ϵ and a distribution of Eddington ratios λ. By generalizing these continuity-equation models to allow a distribution P(λ | MBH, z), we are able to draw on constraints from observationally estimated λ distributions and active galaxy fractions while accounting for the luminosity thresholds of observational samples. We consider models with a Gaussian distribution of log λ and Gaussians augmented with a power-law tail to low λ. Within our framework, reproducing the high observed AGN fractions at low redshift requires a characteristic Eddington ratio λc that declines at late times, and matching observed Eddington ratio distributions requires a P(λ) that broadens at low redshift. To reproduce the observed increase of AGN fraction with black hole or galaxy mass, we also require a λc that decreases with increasing black hole mass, reducing the AGN luminosity associated with the most massive black holes. Finally, achieving a good match to the high-mass end of the local black hole mass function requires an increased radiative efficiency at high black hole mass. We discuss the potential impact of black hole mergers or a λ-dependent bolometric correction, and we compute evolutionary predictions for black hole and galaxy specific accretion rates. Despite the flexibility of our framework, no one model provides a good fit to all the data we consider; it is particularly difficult to reconcile the relatively narrow λ distributions and low duty cycles estimated for luminous broad-line AGN with the broader λ distributions and higher duty cycles found in more widely selected AGN samples, which typically have lower luminosity thresholds.

JWST has now revealed a population of broad-line active galactic nuclei (AGN) at z ≳ 4 characterized by a distinctive spectral energy distribution shape, with a very red rest-frame optical and very blue rest-frame UV continuum. While the optical continuum is thought to originate from the accretion disk, the origin of the UV continuum has been largely unclear. We report the detection of the strong rest-frame UV emission lines of C iii] λλ1907, 1909 and C iv λλ1549, 1551 in a “little red dot” AGN, COS-66964. Spectroscopically confirmed at z = 7.0371, COS-66964 exhibits broad Hα emission (FWHM ∼ 2000 km s−1) and weak broad Hβ, implying significant dust attenuation to the broad-line region ( A V = 3 . 9 − 0.9 + 1.7 ). The Hα line width implies a central supermassive black hole (SMBH) mass of M BH = 1 . 9 − 0.7 + 1.6 × 1 0 7 M ⊙ and an Eddington ratio λ ∼ 0.3–0.5. While marginal He ii λ4687 and [Fe x] λ6376 detections further indicate that the AGN dominates in the rest-frame optical, the nondetection of He ii λ1640 in the UV despite high EW C iii] and C iv (∼35 Å) is more consistent with photoionization by massive stars. The nondetection of Mg ii λλ2800 is similarly inconsistent with an AGN scattered light interpretation. Assuming the rest-frame UV is dominated by stellar light, we derive a stellar mass of log M ⋆ / M ⊙ ∼ 8.5 , implying an elevated M BH/M ⋆ ratio ∼2 orders of magnitude above the local relation, but consistent with other high-z AGN discovered by JWST. The source is unresolved in all bands, implying a very compact size ≲200 pc in the UV. This suggests that the simultaneous buildup of compact stellar populations (i.e., galaxy bulges) and the central SMBH is ongoing even at z ≳ 7.