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JWST is discovering star-forming 'candidate' galaxies with photometric redshifts z > 9 and little attenuation. We model presumptive massive black holes (MBHs) in such galaxies and find that their unobscured emission is fainter than the galaxy starlight in JWST filters, and difficult to be detected via colour–colour selection, and X-ray and radio observations. Only MBHs overmassive relative to expected galaxy scaling relations, accreting at high Eddington rates, would be detectable. Their discovery would point to the presence of heavy MBH seeds, but care is needed to exclude the existence of lighter seeds as only overmassive MBHs are detectable in this type of galaxies. Conversely, if no overmassive MBHs are hosted in these galaxies, either there are no heavy seeds or they are rare. The most massive/highest redshift candidate galaxies can attain stellar masses in excess of |$5\times 10^{10}\, {\rm M_\odot }$| by z ∼ 6 if they grow along the star formation rate–mass sequence, and can nurse an MBH growing from |${\sim}10^5 \, {\rm M_\odot }$| up to |${\gt}3\times 10^7\, {\rm M_\odot }$| by z ∼ 6, to become hosts of some z > 6 quasars. Candidate galaxies of |$\log (M_{\rm gal}/\, {\rm M_\odot })\sim 8$| cannot grow their putative seeds fast, unless seeds are |${\gtrsim}10^6\, {\rm M_\odot }$|⁠. The number density of the JWST candidate galaxies far outnumbers that of the highest z quasar hosts and this allows for about only one bright z ∼ 6–7 quasar every 1000 of these galaxies. [ABSTRACT FROM AUTHOR]

The occurrence of dual active galactic nuclei (AGN) on scales of a few tens of kpc can be used to study merger-induced accretion on massive black holes (MBHs) and to derive clues on MBH mergers, using dual AGN as a parent population of precursors. We investigate the properties of dual AGN in the cosmological simulation Horizon-AGN. We create catalogs of dual AGN selected with distance and luminosity criteria, plus sub-catalogs where further mass cuts are applied. We divide the sample into dual AGN hosted in different galaxies, on the way to a merger, and into those hosted in one galaxy, after the galaxy merger has happened. We find that the relation between MBH and galaxy mass is similar to that of general AGN population and we compare the properties of dual AGN also with a control sample, discussing differences and similarities in masses and Eddington ratios. The typical mass ratio of galaxy mergers associated to dual AGN is 0.2, with mass loss in the smaller galaxy decreasing the mass ratio as the merger progresses. Between 30 and 80 per cent of dual AGN with separations between 4 and 30 kpc can be matched to an ensuing MBH merger. The dual AGN fraction increases with redshift and with separation threshold, although above 50 kpc the increase of multiple AGN limits that of duals. Multiple AGN are generally associated with massive haloes, and mass loss of satellites shapes the galaxy–halo relation. [ABSTRACT FROM AUTHOR]

Recent deep surveys have unravelled a population of faint active galactic nuclei (AGNs) in the high-redshift Universe, leading to various discussions on their nature and their role during the Epoch of Reionization. We use cosmological radiation hydrodynamics simulations of a bright galaxy at z ∼ 6 (⁠|${M_\star } \gtrsim 10^{10}\, {\rm M}_{\odot }$|⁠) hosting an actively growing supermassive black hole to study the properties of these objects. In particular, we study how the black hole and the galaxy coevolve and what is the relative contribution of the AGNs and of the stellar populations to the luminosity budget of the system. We find that the feedback from the AGN has no strong effect on the properties of the galaxy, and does not increase the total ionizing luminosity of the host. The average escape fraction of our galaxy is around |$f_{\rm esc} \sim 5{{\ \rm per\ cent}}$|⁠. While our galaxy would be selected as an AGN in deep X-ray surveys, most of the ultraviolet (UV) luminosity is originating from stellar populations. This confirms that there is a transition in the galaxy population from star-forming galaxies to quasar hosts, with bright Lyman-break galaxies with M UV around −22 falling in the overlap region. Our results also suggest that faint AGNs do not contribute significantly to reionizing the Universe. [ABSTRACT FROM AUTHOR]

High-redshift quasars are believed to reside in highly biased regions of the Universe, where black hole growth is sustained by an enhanced number of mergers and by being at the intersection of filaments bringing fresh gas. This assumption should be supported by an enhancement of the number counts of galaxies in the field of view of quasars. While the current observations of quasar environments do not lead to a consensus on a possible excess of galaxies, the future missions JWST, WFIRST , and Euclid will provide new insights on quasar environments, and will substantially increase the number of study-cases. We are in a crucial period, where we need to both understand the current observations and predict how upcoming missions will improve our understanding of BH environments. Using the large-scale simulation Horizon-AGN, we find that statistically the most massive BHs reside in environments with the largest galaxy number counts. However, we find a large variance in galaxy number counts, and some massive BHs do not show enhanced counts in their neighbourhood. Interestingly, some massive BHs have a very close galaxy companion but no further enhancement at larger scales, in agreement with recent observations. We find that AGN feedback in the surrounding galaxies is able to decrease their luminosity and stellar mass, and therefore to make them unobservable when using restrictive galaxy selection criteria. Radiation from the quasars can spread over large distances, which could affect the formation history of surrounding galaxies, but a careful analysis of these processes requires radiative transfer simulations. [ABSTRACT FROM AUTHOR]

Observations of high-redshift quasars provide information on the massive black holes (MBHs) powering them and the galaxies hosting them. Current observations of z ≳ 6 hosts, at sub-millimetre wavelengths, trace the properties of cold gas, and these are used to compare with the correlations between MBHs and galaxies characterizing the z  = 0 population. The relations at z  = 0, however, rely on stellar-based tracers of the galaxy properties. We perform a very high resolution cosmological zoom-in simulation of a z  = 7 quasar including state-of-the-art non-equilibrium chemistry, MBH formation, growth, and feedback, to assess the evolution of the galaxy host and the central MBH, and compare the results with recent ALMA observations of high-redshift quasars. We measure both the stellar-based quantities used to establish the z  = 0 correlations, as well as the gas-based quantities available in z ≳ 6 observations, adopting the same assumptions and techniques used in observational studies. The high-redshift studies argued that MBHs at high redshift deviate from the local MBH–galaxy correlations. In our analysis of the single galaxy we evolve, we find that the high-redshift population sits on the same correlations as the local one, when using the same tracers used at z  = 0. When using the gas-based tracers, however, MBHs appear to be overmassive. The discrepancy between local and high-redshift MBHs seems to be caused by the different tracers employed, and the necessary assumptions, and not by an intrinsic difference. Better calibration of the tracers, higher resolution data, and availability of facilities that can probe the stellar population will be crucial to assess precisely and accurately high-redshift quasar hosts. [ABSTRACT FROM AUTHOR]

The dynamics of black hole (BH) seeds in high-redshift galaxies is key to understand their ability to grow via accretion and to pair in close binaries during galactic mergers. To properly follow the dynamics of BHs we develop a physically motivated model to capture unresolved dynamical friction from stars, dark matter, and gas. We first validate the model and then we use it to investigate the dynamics of seed BHs born at z ∼ 9 in dwarf proto-galaxies. We perform a suite of zoom cosmological simulations with spatial resolution as high as 10 pc and with a stellar and dark matter mass resolution of |$2\times 10^3 \, \, $| and |$2\times 10^5 \, \, \mathrm{ M}_{\odot }$|⁠, respectively. We first explore the dynamics of a seed BH in the galaxy where it is born and show that it is highly erratic if the seed mass is less than |$10^5\, \, \mathrm{ M}_{\odot }$|⁠. The dynamics is dominated by the stellar component, whose distribution is irregular and patchy, thus inducing stochasticity in the orbits: the BH may be anywhere in the proto-galaxy. When this dwarf merges into a larger galaxy, it is paramount to simulate the process with very high spatial and mass resolution in order to correctly account for the stripping of the stellar envelope of the satellite BH. The outcome of the encounter could be either a tight binary or, at least temporary, a wandering BH, leading to multiple BHs in a galaxy, each inherited from a different merger. [ABSTRACT FROM AUTHOR]

The population of supermassive black holes (SMBHs) is split between those that are quiescent, such as those seen in local galaxies including the Milky Way, and those that are active, resulting in quasars and active galactic nuclei (AGN). Outside our neighborhood, all the information we have on SMBHs is derived from quasars and AGN, giving us a partial view. We study the evolution of the SMBH population, total and active, by the continuity equation, backwards in time from z = 0 to z = 4. Type-1 and type-2 AGN are differentiated in our model on the basis of their respective Eddington ratio distributions, chosen on the basis of observational estimates. The duty cycle is obtained by matching the luminosity function of quasars, and the average radiative efficiency is the only free parameter in the model. For higher radiative efficiencies (&0:07), a large fraction of the SMBH population, most of them quiescent, must already be in place by z = 4. For lower radiative efficiencies (~0:05), the duty cycle increases with the redshift and the SMBH population evolves dramatically from z = 4 onwards. The mass function of active SMBHs does not depend on the choice of the radiative efficiency or of the local SMBH mass function, but it is mainly determined by the quasar luminosity function once the Eddington ratio distribution is fixed. Only direct measurement of the total black-hole mass function at redshifts z ≳ 2 could break these degeneracies, offering important constraints on the average radiative efficiency. Focusing on type-1 AGN, for which observational estimates of the mass function and Eddington ratio distribution exist at various redshifts, models with lower radiative efficiencies better reproduce the high-mass end of the mass function at high z, but tend to over-predict it at low z, and vice-versa for models with higher radiative efficiencies. [ABSTRACT FROM AUTHOR]

We present a new suite of hydrodynamical simulations and use it to study, in detail, black hole and galaxy properties. The high time, spatial and mass resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable us to meaningfully compare star formation rate (SFR) and BH accretion rate (BHAR) time-scales, temporal behaviour, and relative magnitude. We find that (i) BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have different variability time-scales, except during the merger proper, lasting ~0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their variability are similar. Averaging over time, the merger phase leads typically to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and nuclear SFR are intrinsically proportional, but the correlation lessens if the long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones most likely to be selected as systems dominated by an active galactic nucleus, because of the long time spent in this phase. (iv) The time-scale over which a given diagnostic probes the SFR has a profound impact on the recovered correlations with BHAR, and on the interpretation of observational data. [ABSTRACT FROM AUTHOR]

The existence of massive black holes (MBHs) was postulated in the 1960s, when the first quasars were discovered. In the late 1990s their reality was proven beyond doubt in the Milky way and a handful nearby galaxies. Since then, enormous theoretical and observational efforts have been made to understand the astrophysics of MBHs. We have discovered that some of the most massive black holes known, weighing billions of solar masses, powered luminous quasars within the first billion years of the Universe. The first MBHs must therefore have formed around the time the first stars and galaxies formed. Dynamical evidence also indicates that black holes with masses of millions to billions of solar masses ordinarily dwell in the centers of today's galaxies. MBHs populate galaxy centers today, and shone as quasars in the past; the quiescent black holes that we detect now in nearby bulges are the dormant remnants of this fiery past. In this review we report on basic, but critical, questions regarding the cosmological significance of MBHs. What physical mechanisms led to the formation of the first MBHs? How massive were the initial MBH seeds? When and where did they form? How is the growth of black holes linked to that of their host galaxy? The answers to most of these questions are works in progress, in the spirit of these reports on progress in physics. [ABSTRACT FROM AUTHOR]

We assess the influence of massive black hole (MBH) ejections from galaxy centres due to gravitational radiation recoil, along the cosmic merger history of the MBH population. We discuss the ‘danger’ of recoil for MBHs as a function of different MBH spin-orbit configurations and of the host halo cosmic bias, and on how that reflects on the occupation fraction of MBHs. We assess ejection probabilities for mergers occurring in a gas-poor environment, in which the MBH binary coalescence is driven by stellar dynamical processes and the spin-orbit configuration is expected to be isotropically distributed. We contrast this case with the ‘aligned’ case. The latter is the more realistic situation for gas-rich, i.e. ‘wet’, mergers, which are expected for high-redshift galaxies. We find that if all haloes at host an MBH, the probability of the Milky Way (or similar size galaxy) to host an MBH today is less than 50 per cent, unless MBHs form continuously in galaxies. The occupation fraction of MBHs, intimately related to halo bias and MBH formation efficiency, plays a crucial role in increasing the retention fraction. Small haloes, with shallow potential wells and low escape velocities, have a high ejection probability, but the MBH merger rate is very low along their galaxy formation merger hierarchy: MBH formation processes are likely inefficient in such shallow potential wells. Recoils can decrease the overall frequency of MBHs in small galaxies to ∼60 per cent, while they have little effect on the frequency of MBHs in large galaxies (at most a 20 per cent effect). [ABSTRACT FROM AUTHOR]

We investigate the evolution of high-redshift seed black hole masses at late times and their observational signatures. The massive black hole seeds studied here form at extremely high redshifts from the direct collapse of pre-galactic gas discs. Populating dark matter haloes with seeds formed in this way, we follow the mass assembly of these black holes to the present time using a Monte Carlo merger tree. Using this machinery, we predict the black hole mass function at high redshifts and at the present time, the integrated mass density of black holes and the luminosity function of accreting black holes as a function of redshift. These predictions are made for a set of three seed models with varying black hole formation efficiency. Given the accuracy of present observational constraints, all three models can be adequately fitted. Discrimination between the models appears predominantly at the low-mass end of the present-day black hole mass function which is not observationally well constrained. However, all our models predict that low surface brightness, bulgeless galaxies with large discs are least likely to be sites for the formation of massive seed black holes at high redshifts. The efficiency of seed formation at high redshifts has a direct influence on the black hole occupation fraction in galaxies at . This effect is more pronounced for low-mass galaxies. This is the key discriminant between the models studied here and the Population III remnant seed model. We find that there exist a population of low-mass galaxies that do not host nuclear black holes. Our prediction of the shape of the MBH–σ relation at the low-mass end is in agreement with the recent observational determination from the census of low-mass galaxies in the Virgo cluster. [ABSTRACT FROM AUTHOR]

We investigate the consequences of a model of the assembly and growth of massive black holes (MBHs) from primordial seeds, remnants of the first generation of stars in a hierarchical structure formation scenario. Our model traces the build-up of MBHs from an early epoch, and follows the merger history of dark matter haloes and their associated holes via Monte Carlo realizations of the merger hierarchy, from early times to the present time, in aΛ cold dark matter cosmology. The sequence of minor and major mergers experienced by galactic haloes in their hierarchical growth affects the merger history of MBHs embedded in their nuclei. So, if the formation route for the assembly of supermassive black holes dates back to the early universe, a large number of black hole (BH) interactions is inevitable. The coalescence time-scales of binary black holes can be long enough for a third BH to fall in and interact with the central binary. These BH triple interactions lead typically to the final expulsion of one of the three bodies and to the recoil of the binary. Also, asymmetric emission of gravitational waves in the last stages of the BH merging can give a recoil velocity to the centre of mass of the coalescing binary. This scenario leads to the prediction of a population of intermediate-mass black holes (IMBHs) wandering in galaxy haloes at the present epoch. We compute the luminosity distribution produced by these IMBHs accreting from their circumstellar medium. We find that in a Milky Way-sized galaxy they are unable to account for sources with luminosities, unless they carry a baryonic remnant from which they are able to accrete for a long time. We also find that, for typical spiral galaxies, the bright end of the point-source distribution correlates with the mass of the galaxy and the most luminous sources are expected to be found in the disc. [ABSTRACT FROM AUTHOR]

Recent systematic searches for massive black holes (BHs) in local dwarf galaxies led to the discovery of a population of faint active galactic nuclei (AGNs). We investigate the agreement of the BH and AGN populations in the Illustris, TNG, Horizon-AGN, EAGLE, and SIMBA simulations with current observational constraints in low-mass galaxies. We find that some of these simulations produce BHs that are too massive, and that the BH occupation fraction (OF) at z  = 0 is not inherited from the simulation seeding modelling. The ability of BHs and their host galaxies to power an AGN depends on BH and galaxy subgrid modelling. The fraction of AGN in low-mass galaxies is not used to calibrate the simulations, and thus can be used to differentiate galaxy formation models. AGN fractions at z  = 0 span two orders of magnitude at fixed galaxy stellar mass in simulations, similarly to observational constraints, but uncertainties and degeneracies affect both observations and simulations. The agreement is difficult to interpret due to differences in the masses of simulated and observed BHs, BH OF affected by numerical choices, and an unknown fraction of obscured AGN. Our work advocates for more thorough comparisons with observations to improve the modelling of cosmological simulations, and our understanding of BH and galaxy physics in the low-mass regime. The mass of BHs, their ability to efficiently accrete gas, and the AGN fraction in low-mass galaxies have important implications for the build-up of the entire BH and galaxy populations with time. [ABSTRACT FROM AUTHOR]

The rate of tidal disruption events (TDEs) depends sensitively on the stellar properties of the central galactic regions. Simulations show that galaxy mergers cause gas inflows, triggering nuclear starbursts, increasing the central stellar density. Motivated by these numerical results, and by the observed overrepresentation of post-starburst galaxies among TDE hosts, we study the evolution of the TDE rate in high-resolution hydrodynamical simulations of a galaxy merger, in which we capture the evolution of the stellar density around the massive black holes (BHs). We apply analytical estimates of the loss-cone theory, using the stellar density profiles from simulations, to estimate the time evolution of the TDE rate. At the second pericentre, a nuclear starburst enhances the stellar density around the BH in the least massive galaxy, leading to an enhancement of the TDE rate around the secondary BH, although the magnitude and the duration of the increase depend on the stochasticity of star formation on very small scales. The central stellar density around the primary BH remains instead fairly constant, and so is its TDE rate. After the formation of the binary, the stellar density decreases, and so does the TDE rate. [ABSTRACT FROM AUTHOR]

Using a suite of three large cosmological hydrodynamical simulations, HORIZON-AGN, HORIZON-NOAGN (no AGN feedback) and HORIZON-DM (no baryons), we investigate how a typical sub-grid model for AGN feedback affects the evolution of the inner density profiles of massive dark matter haloes and galaxies. Based on direct object-to-object comparisons, we find that the integrated inner mass and density slope differences between objects formed in these three simulations (hereafter, HAGN, HnoAGN and HDM) significantly evolve with time. More specifically, at high redshift (z ∼ 5), the mean central density profiles of HAGN and HnoAGN dark matter haloes tend to be much steeper than their HDM counterparts owing to the rapidly growing baryonic component and ensuing adiabatic contraction. By z ∼ 1.5, these mean halo density profiles in HAGN have flattened, pummelled by powerful AGN activity ('quasar mode'): the integrated inner mass difference gaps with HnoAGN haloes have widened, and those with HDM haloes have narrowed. Fast forward 9.5 billion years, down to z = 0, and the trend reverses: HAGN halo mean density profiles drift back to a more cusped shape as AGN feedback efficiency dwindles ('radio mode'), and the gaps in integrated central mass difference with HnoAGN and HDM close and broaden, respectively. On the galaxy side, the story differs noticeably. Averaged stellar profile central densities and inner slopes are monotonically reduced by AGN activity as a function of cosmic time, resulting in better agreement with local observations. [ABSTRACT FROM AUTHOR]

The growth of the first supermassive black holes (SMBHs) at z > 6 is still a major challenge for theoretical models. If it starts from black hole (BH) remnants of Population III stars (light seeds with mass ∼100 M☉), it requires super-Eddington accretion. An alternative route is to start from heavy seeds formed by the direct collapse of gas on to an ∼105 M☉ BH. Here we investigate the relative role of light and heavy seeds as BH progenitors of the first SMBHs. We use the cosmological, data constrained semi-analytic model GAMETE/QSODUST to simulate several independent merger histories of z > 6 quasars. Using physically motivated prescriptions toform light and heavy seeds in the progenitor galaxies, we find that the formation of a few heavy seeds (between 3 and 30 in our reference model) enables the Eddington-limited growth of SMBHs at z > 6. This conclusion depends sensitively on the interplay between chemical, radiative and mechanical feedback effects, which easily erase the conditions that allow the suppression of gas cooling in the low-metallicity gas (Z < Zcr and JLW > Jcr). We find that heavy seeds cannot form if dust cooling triggers gas fragmentation above a critical dust-to-gas mass ratio (D ⩾ Dcr). In addition, the relative importance of light and heavy seeds depends on the adopted mass range for light seeds, as this dramatically affects the history of cold gas along the merger tree, by both SN- and AGN-driven winds. [ABSTRACT FROM AUTHOR]

The past decade has seen significant progress in understanding galaxy formation and evolution using large-scale cosmological simulations. While these simulations produce galaxies in overall good agreement with observations, they employ different sub-grid models for galaxies and supermassive black holes (BHs). We investigate the impact of the sub-grid models on the BH mass properties of the Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA simulations, focusing on the M BH − M ⋆ relation and the BH mass function. All simulations predict tight M BH − M ⋆ relations, and struggle to produce BHs of |$M_{\rm BH}\leqslant 10^{7.5}\, \rm M_{\odot }$| in galaxies of |$M_{\star }\sim 10^{10.5}\!-\!10^{11.5}\, \rm M_{\odot }$|⁠. While the time evolution of the mean M BH − M ⋆ relation is mild (⁠|$\rm \Delta M_{\rm BH}\leqslant 1\, dex$| for 0 |$\leqslant z \leqslant$| 5) for all the simulations, its linearity (shape) and normalization varies from simulation to simulation. The strength of SN feedback has a large impact on the linearity and time evolution for |$M_{\star }\leqslant 10^{10.5}\, \rm M_{\odot }$|⁠. We find that the low-mass end is a good discriminant of the simulation models, and highlights the need for new observational constraints. At the high-mass end, strong AGN feedback can suppress the time evolution of the relation normalization. Compared with observations of the local Universe, we find an excess of BHs with |$M_{\rm BH}\geqslant 10^{9}\, \rm M_{\odot }$| in most of the simulations. The BH mass function is dominated by efficiently accreting BHs (⁠|$\log _{10}\, f_{\rm Edd}\geqslant -2$|⁠) at high redshifts, and transitions progressively from the high-mass to the low-mass end to be governed by inactive BHs. The transition time and the contribution of active BHs are different among the simulations, and can be used to evaluate models against observations. [ABSTRACT FROM AUTHOR]

Mergers of galaxies are extremely violent events shaping their evolution. Such events are thought to trigger starbursts and, possibly, black hole accretion. Nonetheless, it is still not clear how to know the fate of a galaxy pair from the data available at a given time, limiting our ability to constrain the exact role of mergers. In this paper we use the light-cone of the horizon-agn simulation, for which we know the fate of each pair, to test three selection processes aiming at identifying true merging pairs. We find that the simplest one (selecting objects within two thresholds on projected distance d and redshift difference Δ z) gives similar results than the most complex one (based on a neural network analysing d , Δ z , redshift of the primary, masses/star formation rates/aspect ratio of both galaxies). Our best thresholds are |$d_\mathrm{th}\sim 100\, \mathrm{kpc}$| and Δ z th ∼ 10−3, in agreement with recent results. [ABSTRACT FROM AUTHOR]

In the local Universe, black holes of |$10^{5-6}\, {\rm M_\odot }$| are hosted in galaxies displaying a variety of stellar profiles and morphologies. These black holes are the anticipated targets of LISA, the Laser Interferometer Space Antenna that will detect the low-frequency gravitational-wave signal emitted by binary black holes in this mass interval. In this paper, we infer upper limits on the lifetime of binary black holes of |$10^{5-6}\, {\rm M_\odot }$| and up to |$10^8\, {\rm M_\odot }$|⁠ , forming in galaxy mergers, exploring two underlying stellar density profiles, by Dehnen and by Prugniel & Simien, and by exploiting local scaling relations between the mass of the black holes and several quantities of their hosts. We focus on the phase of the dynamical evolution when the binary is transitioning from the hardening phase ruled by the interaction with single stars to the phase driven by the emission of gravitational waves. We find that different stellar profiles predict very distinct trends with binary mass, with lifetimes ranging between fractions of a Gyr to more than 10 Gyr, and with a spread of about one order of magnitude, given by the uncertainties in the observed correlations, which are larger in the low-mass tail of the observed black hole population. [ABSTRACT FROM AUTHOR]

In this work, we include black hole (BH) seeding, growth, and feedback into our semi-analytic galaxy formation model, Delphi. Our model now fully tracks the accretion- and merger-driven hierarchical assembly of the dark matter halo, gas, stellar, and BH masses of high-redshift (z ≳ 5) galaxies. We explore a number of physical scenarios that include (i) two types of BH seeds (stellar and those from direct collapse BH); (ii) the impact of reionization; and (iii) the impact of instantaneous versus delayed galaxy mergers on the baryonic growth. Using a minimal set of mass- and z -independent free parameters associated with star formation and BH growth, and their associated feedback, and including the suppressed BH growth in lower mass galaxies, we show that our model successfully reproduces all available data sets for early galaxies and quasars. While both reionization and delayed galaxy mergers have no sensible impact on the evolving ultraviolet luminosity function, the impact of the former dominates in determining the stellar mass density for observed galaxies as well as the BH mass function. We then use this model to predict the LISA detectability of merger events at high- z. As expected, mergers of stellar BHs dominate the merger rates for all scenarios and our model predicts an expected upper limit of about 20 mergers using instantaneous merging and no reionization feedback over the 4-yr mission duration. Including the impact of delayed mergers and reionization feedback provides about 12 events over the same observational time-scale. [ABSTRACT FROM AUTHOR]