Growth Of Massive Black Holes Research Articles

Connecting low-redshift LISA massive black hole mergers to the nHz stochastic gravitational wave background

Pulsar Timing Array (PTA) experiments worldwide recently reported evidence of a nHz stochastic gravitational wave background (sGWB) compatible with the existence of slowly inspiralling massive black hole (MBH) binaries (MBHBs). The shape of the signal contains valuable information about the evolution of $z<1$ MBHs above $ odot $, suggesting a faster dynamical evolution of MBHBs towards the gravitational-wave-driven inspiral or a larger MBH growth than usually assumed. In this work, we investigate if the nHz sGWB could also provide constraints on the population of merging lower-mass MBHBs ($ odot $) detectable by LISA. To this end, we use the L-Galaxies semi-analytical model applied to the Millennium suite of simulations. We generate a population of MBHs compatible simultaneously with current electromagnetic and nHz sGWB constraints by including the possibility that, in favourable environments, MBHs can accrete gas beyond the Eddington limit. The predictions of this new model for the sGWB show that the global (integrated up to high-$z$) LISA detection rate is not significantly affected when compared to a fiducial model whose nHz sGWB signal is $ sim \,2$ times smaller. In both cases, the global rate yields sim $ and is dominated by systems of $ odot $. The main differences are limited to low-$z$ ($z<3$), high-mass ($ odot $) LISA MBHBs. The model compatible with the latest PTA results predicts up to $ sim \,1.6$ times more detections, with a rate of sim erg/s$. Hence, in case PTA results are confirmed and given the current MBH modelling, our findings suggest there will be higher chances to perform multimessenger studies with LISA MBHB than previously expected.

The Redshift Evolution of the M •–M ⋆ Relation for JWST’s Supermassive Black Holes at z > 4

JWST has detected many overmassive galactic systems at z > 4, where the mass of the black hole, M •, is 10–100 times larger than expected from local relations, given the host’s stellar mass, M ⋆. This paper presents a model to describe these overmassive systems in the high-z Universe. We suggest that the black hole mass is the main driver of high-z star formation quenching. Supermassive black holes globally impact their high-z galaxies because their hosts are physically small, and the black holes have duty cycles close to unity at z > 4. In this regime, we assume that black hole mass growth is regulated by the quasar’s output, while stellar mass growth is quenched by it and uncorrelated to the global properties of the host halo. We find that the ratio M •/M ⋆ controls the average star formation efficiency: if M •/M ⋆ > 8 × 1018(nΛ/ f Edd)[(Ω b M h )/(Ω m M ⋆) − 1], then the galaxy is unable to form stars efficiently. Once this ratio exceeds the threshold, a runaway process brings the originally overmassive system toward the local M •–M ⋆ relation. Furthermore, the M •–M ⋆ relation evolves with redshift as ∝(1 + z)5/2. At z ∼ 5, we find an overmassive factor of ∼55, in excellent agreement with current JWST data and the high-z relation inferred from those. Extending the black hole horizon farther in redshift and lower in mass will test this model and improve our understanding of the early coevolution of black holes and galaxies.

Physical Properties of Hyperluminous, Dust-obscured Quasars at z ∼ 3: Multiwavelength Spectral Energy Distribution Analysis and Cold Gas Content Revealed by ALMA

We present a UV to millimeter spectral energy distribution (SED) analysis of 16 hyperluminous, dust-obscured quasars at z ∼ 3, selected by the Wide-field Infrared Survey Explorer. We aim to investigate the physical properties of these quasars, with a focus on their molecular gas content. We decompose the SEDs into three components: stellar, cold dust, and active galactic nucleus (AGN). By doing so, we are able to derive and analyze the relevant properties of each component. We determine the molecular gas mass from CO line emission based on Atacama Large Millimeter/submillimeter Array (ALMA) observations. By including ALMA observations in the multiwavelength SED analysis, we derive the molecular gas fractions, gas depletion timescales, and star formation efficiencies (SFEs). Their sample median and 16th–84th quartile ranges are fgas∼0.33−0.17+0.33 , t depl ∼ 39−28+85 Myr, and SFE ∼ 297−195+659 K km s−1 pc−2, respectively. Compared to main-sequence galaxies, they have lower molecular gas content and higher SFEs, similar to quasars in the literature. This suggests that the gas in these quasars is rapidly depleted, likely as the result of intense starburst activity and AGN feedback. The observed correlations between these properties and the AGN luminosities further support this scenario. Additionally, we infer the black hole to stellar mass ratio and black hole mass growth rate, which indicate significant central black hole mass assembly over short timescales. Our results are consistent with the scenario that our sample represents a short transition phase toward unobscured quasars.

How does the presence of bar affects the fueling of supermassive black holes? An IllustrisTNG100 perspective

ABSTRACT We conducted a statistical study of black hole masses of barred and unbarred galaxies in the IllustrisTNG100 cosmological magnetohydrodynamical simulations. This work aims to understand the role of the bars in the growth of central supermassive black hole mass and its implications on AGN fueling. Our sample consists of 1191 barred galaxies and 2738 unbarred galaxies in the IllustrisTNG100 simulations. To have an unbiased study, we perform our analysis with an equal number of barred and unbarred galaxies by using various controlled parameters like total galaxy mass, stellar mass, gas mass, dark matter halo mass, etc. Except for the stellar mass controlling, we find that the median of the black hole mass distribution for barred galaxies is higher than that of the unbarred ones, indicating that stellar mass is a key parameter influencing the black hole growth. The higher mean accretion rate of the black holes in barred galaxies, averaged since the bar forming epoch (z ∼ 2), explains the higher mean black hole masses in barred galaxies. Further, we also test that these results are unaffected by other environmental processes like minor/major merger histories and neighbouring gas density of black hole. Although the relationship between stellar mass, bar formation, and black hole growth is complex, with various mechanisms involved, our analysis suggests that bars can play a crucial role in feeding black holes, particularly in galaxies with massive stellar discs.

Can Cosmologically Coupled Mass Growth of Black Holes Solve the Mass Gap Problem?

Observations of elliptical galaxies suggest that black holes (BHs) might serve as dark energy candidates, coupled to the expansion of the Universe. According to this hypothesis, the mass of a BH could increase as the Universe expands. BH low-mass X-ray binaries (LMXBs) in the Galactic disk were born several gigayears ago, making the coupling effect potentially significant. In this work, we calculate the evolution of BH binaries with a binary population synthesis method to examine the possible influence of cosmologically coupled growth of BHs, if it really exists. The measured masses of the compact objects in LMXBs show a gap around ∼2.5–5 M ⊙, separating the most-massive neutron stars from the least-massive BHs. Our calculated results indicate that considering the mass growth seems to (partially) account for the mass gap and the formation of compact BH LMXBs, alleviating the challenges in modeling the formation and evolution of BH LMXBs with traditional theory. However, critical observational evidence like the detection of intermediate-mass BH binaries is required to test this hypothesis.

The AGN Feedback in Compact Galaxies: On the Impact of a More Massive Central Black Hole

We conduct high-resolution hydrodynamical simulations using the MACER framework to investigate the interplay between the interstellar medium, active galactic nuclei (AGN) feedback and black hole (BH) feeding in a massive compact galaxy, with an emphasis on the impact of different central BH masses. We find that with a more massive central BH, high-speed outflows are more prominent, and the gas fraction in the compact galaxy is reduced. Due to the lower gas density and higher gas temperature, the compact galaxy with a more massive BH (MAS galaxy) remains predominantly single-phase with the cooling time t cool ≳ 100t ff. In contrast, the compact galaxy with the reference BH mass (REF galaxy) maintains a higher gas fraction with a shorter cooling time, slightly more multiphase gas and less prominent outflows. We further demonstrate that the difference in gas thermal states and kinematics is caused by the stronger AGN feedback in the compact galaxy with a more massive BH, where the AGN wind power is twice as much as that with the reference BH. Since the AGN feedback efficiently suppresses the inflow rate and the BH feeding rate, the BH mass growth is significant in neither the compact galaxy with the reference BH nor that with the more massive BH, only by 24% and 11% of the initial BH mass, respectively, over the entire evolution time of 10 Gyr. We thus posit that without ex situ mass supply from mergers, the massive BHs in compact galaxies cannot grow significantly via gas accretion during the late phase, but might have already formed by the end of the rapid early phase of galaxy formation.

Massive black hole growth using the star gulping mechanism

"The evolution of merging black holes (BHs) and their associated processes will be the center-piece study for the upcoming gravitational waves (GWs) observatories, including the LISA Space Mission from ESA. We ran simulations specifically for studying the BH growth, in order to provide estimates regarding the detection rates for the future GW laboratories. Here, we describe the results of our investigations for a new implementation regarding the BH growth, star gulping (SG), in which stars are absorbed by BHs. The current scientific literature emphasizes two main mechanisms for the growth of BHs: accretion of surrounding material and merging with another BH. By taking into account both methods, one can begin to model the growth processes of the most massive BHs (MBHs) from the moment of their origin to current observations. Our previous work showed that even if you consider these processes at their maximum, it is very difficult to grow BHs to the masses and concentrations that we measure today. Therefore, we performed detailed simulations in order to study another growth mechanism, star gulping (SG), that has the potential to add more mass to BHs."

Critical accretion rates for rapidly growing massive Population III stars

Efforts to understand the origin and growth of massive black holes observed in the early Universe have spurred significant interest in the evolution and fate of rapidly accreting primordial (metal-free) stars. Here, we investigate the evolution of such Population III (PopIII) stars under variable accretion rates, focusing on the thermal response and stellar structure, the impact of the luminosity wave encountered early in the pre-main sequence (pre-MS) phase, and the influence of accretion on their subsequent evolution. We employ the Geneva stellar evolution code and simulate ten models with varying accretion histories, covering a final mass range from 491 M⊙ to 6127 M⊙. Our findings indicate that the critical accretion rate delineating the red and blue supergiant regimes during the pre-MS evolution is approximately 2.5 × 10−2 M⊙ yr−1. Once core hydrogen burning commences, the value of this critical accretion rate drops to 7.0 × 10−3 M⊙ yr−1. Moreover, we also confirm that the Kelvin–Helmholtz timescale in the outer surface layers is the more relevant timescale for determining the transition between red and blue phases. Regarding the luminosity wave, we find that it affects only the early pre-MS phase of evolution and does not directly influence the transition between red and blue phases, which primarily depends on the accretion rate. Finally, we demonstrate that variable accretion rates significantly impact the lifetimes, surface enrichment, and final mass of the PopIII stars, as well as the time they spend in the red phase. Our study provides a comprehensive understanding of the intricate evolutionary patterns of PopIII stars subjected to variable accretion rates.

A Candidate Dual QSO at Cosmic Noon

We report the discovery of a candidate dual QSO at z = 1.889, a redshift that is in the era known as “cosmic noon” where most of the universe’s black hole and stellar mass growth occurred. The source was identified in Hubble Space Telescope WFC3/IR images of a dust-reddened QSO that showed two closely separated point sources at a projected distance of 0.″26, or 2.2 kpc. This red QSO was targeted for imaging to explore whether red QSOs are hosted by merging galaxies. We subsequently obtained a spatially resolved Space Telescope Imaging Spectrograph spectrum of the system, covering the visible spectral range, and verifying the presence of two distinct QSO components. We also obtained high-resolution radio continuum observations with the Very Long Baseline Array at 1.4 GHz (21 cm L band) and found two sources coincident with the optical positions. The sources have similar black hole masses, bolometric luminosities, and radio-loudness parameters. However, their colors and reddenings differ significantly. The redder QSO has a higher Eddington ratio, consistent with previous findings. We consider the possibility of gravitational lensing and find that it would require extreme and unlikely conditions. If confirmed as a bona fide dual QSO, this system would link dust reddening to galaxy and supermassive black hole mergers, opening up a new population in which to search for samples of dual active galactic nuclei.

Constraints on the cosmological coupling of black holes from Gaia

Recent work has suggested that black holes (BHs) could be cosmologically coupled to the accelerated expansion of the Universe, potentially becoming a candidate for origin of dark energy. This would imply that BH mass growth follows cosmological expansion, with the masses of individual BHs growing as MBH ∝ (1 + z)3. In this Letter, we discuss the binary systems Gaia BH1 and Gaia BH2, which contain ∼9 M⊙ BHs orbited by ∼1 M⊙ stars in widely separated orbits. The ages of both systems can be constrained by the properties of the luminous stars. If BH masses are indeed growing as (1 + z)3, the masses of both BHs at formation would have been significantly smaller than today. We find a 77% probability that the mass of the BH in Gaia BH2 would have been below 2.2 M⊙ at formation. This is below the classical Tolman–Oppenheimer–Volkov limit, though it is not yet clear whether or not BHs subject to cosmological coupling should obey this limit. For Gaia BH1, the same probability is 70%. This analysis is consistent with results from two BHs in the globular cluster NGC 3201, but unlike the NGC 3201 BHs, the Gaia BHs have well-constrained inclinations and therefore firm upper mass limits. The discovery of more BHs in binary systems with Gaia astrometry in the coming years will allow us to test the cosmological coupling hypothesis decisively.

AGN Sub-populations Important for Black Hole Mass Growth: A Rule of Thumb

For building the super-massive black hole population within a Hubble time, only sub-populations with more than objects on the sky are relevant, where is the sample-averaged bolometric luminosity.

The co-evolution of supermassive black holes and galaxies in luminous AGN over a wide range of redshift

It is well known that supermassive black holes (SMBHs) and their host galaxies co-evolve. A manifestation of this co-evolution is the correlation that has been found between the SMBH mass, MBH, and the galaxy bulge or stellar mass, M*. The cosmic evolution of this relation, though, is still a matter of debate. In this work, we examine the MBH − M* relation, using 687 X-ray luminous (median log [LX,2−10 keV(erg s−1)] = 44.3), broad-line active galactic nuclei (AGN), at 0.2 < z < 4.0 (median z ≈ 1.4) that lie in the XMM-XXL field. Their MBH and M* range from 7.5 < log [MBH (M⊙)] < 9.5 and 10 < log [M*(M⊙)] < 12, respectively. Most of the AGN live in star-forming galaxies and their Eddington ratios range from 0.01 to 1, with a median value of 0.06. Our results show that MBH and M* are correlated (r = 0.47 ± 0.21, averaged over different redshift intervals). Our analysis also shows that the mean ratio of the MBH and M* does not evolve with redshift, at least up to z = 2 and has a value of log(MBH/M*)= − 2.44. The majority of the AGN (75%) are in a SMBH mass growth-dominant phase. In these systems, the MBH − M* correlation is weaker and their M* tends to be lower (for the same MBH) compared to systems that are in a galaxy mass growth phase. Our findings suggest that the growth of black hole mass occurs first, while the early stellar mass assembly may not be so efficient.

The growth of intermediate mass black holes through tidal captures and tidal disruption events

ABSTRACTWe present N-body simulations, including post-Newtonian dynamics, of dense clusters of low-mass stars harbouring central black holes (BHs) with initial masses of 50, 300, and 2000 M⊙. The models are evolved with the N-body code bifrost to investigate the possible formation and growth of massive BHs by the tidal capture of stars and tidal disruption events (TDEs). We model star–BH tidal interactions using a velocity-dependent drag force, which causes orbital energy and angular momentum loss near the BH. About ∼20–30 per cent of the stars within the spheres of influence of the black holes form Bahcall–Wolf cusps and prevent the systems from core collapse. Within the first 40 Myr of evolution, the systems experience 500–1300 TDEs, depending on the initial cluster structure. Most (>95 per cent) of the TDEs originate from stars in the Bahcall–Wolf cusp. We derive an analytical formula for the TDE rate as a function of the central BH mass, density, and velocity dispersion of the clusters ($\dot{N}_{\mathrm{TDE}} \propto M\mathrm{_{BH}}\rho \sigma ^{-3}$). We find that TDEs can lead a 300 M⊙ BH to reach $\sim 7000 \, \mathrm{{M}_{\odot }}$ within a Gyr. This indicates that TDEs can drive the formation and growth of massive BHs in sufficiently dense environments, which might be present in the central regions of nuclear star clusters.

Approximately stealth black hole in higher-order scalar-tensor theories

We investigate a generic quadratic higher-order scalar-tensor theory with a scordatura term, which is expected to provide a consistent perturbative description of stealth solutions with a timelike scalar field profile. In the DHOST subclass, exactly stealth solutions are known to yield perturbations infinitely strongly coupled and thus cannot be trusted. Beyond DHOST theories with the scordatura term, such as in ghost condensation and U-DHOST, we show that stealth configurations cannot be realized as exact solutions but those theories instead admit approximately stealth solutions where the deviation from the exactly stealth configuration is controlled by the mass scale M of derivative expansion. The approximately stealth solution is time-dependent, which can be interpreted as the black hole mass growth due to the accretion of the scalar field. From observed astrophysical black holes, we put an upper bound on M as ĉ D1 1/2 M ≲ 2 × 1011 GeV, where ĉ D1 is a dimensionless parameter of order unity that characterizes the scordatura term. As far as M is sufficiently below the upper bound, the accretion is slow and the approximately stealth solutions can be considered as stealth at astrophysical scales for all practical purposes while perturbations are weakly coupled all the way up to the cutoff M and the apparent ghost is as heavy as or heavier than M.

Supermassive black hole mass growth in infrared-luminous gas-rich galaxy mergers and potential power of (sub)millimeter H2O megamaser observations

AbstractWe present our systematic infrared and (sub)millimeter spectroscopic observations of gas/dust-rich merging ultraluminous infrared galaxies (ULIRGs) to scrutinize deeply buried AGNs (mass-accreting supermassive black holes [SMBHs]). We have found signatures of optically elusive, but intrinsically luminous buried AGNs in a large fraction of nearby (z < 0.3) ULIRGs, suggesting that SMBH mass growth is ongoing in the ULIRG population. Using ALMA, we have detected compact (<100 pc), very luminous (>104Lʘ), AGN-origin, 183 GHz (1.6 mm) H2O megamaser emission in one merging ULIRG, demonstrating that the megamaser emission can be a very powerful tool to dynamically estimate SMBH masses, with the smallest modeling uncertainty of kpc-wide stellar and gas mass distribution, at dusty ULIRGs’ nuclei, because of minimum extinction effects at millimeter. We present our current results and future prospect for the study of the SMBH mass growth in gas/dust-rich galaxy mergers, using (sub)millimeter AGN-origin H2O megamaser emission lines.

Black hole and host galaxy growth in an isolated z ∼ 6 QSO observed with ALMA

The outstanding mass growth of supermassive black holes (SMBHs) at the epoch of reionisation and its relation to the concurrent growth of their host galaxies poses challenges to theoretical models aimed at explaining how these systems formed on short timescales (< 1 Gyr). To trace the average evolutionary paths of quasi-stellar objects (QSOs) and their host galaxies in the plane of BH mass to host mass (Mdyn), we compare the star formation rate (SFR), derived from the accurate estimate of the dust temperature and the dust mass (Tdust, Mdust) based on infrared and sub-millimeter (sub-mm) spectral energy distribution (SED), with the BH accretion rate, derived from Lbol based on X-ray and optical and ultraviolet SED. To this aim, we analysed a deep ALMA observation of the sub-mm continuum, [CII], and H2O of the z ∼ 6 QSO J2310+1855 with a resolution of 900 pc, which enabled a detailed study of dust properties and cold gas kinematics. We performed an accurate SED analysis obtaining a dust temperature of Tdust = 71 ± 4 K, dust mass Mdust = (4.4 ± 0.7)×108 M⊙, and total far-infrared luminosity of LTIR = 2.5−0.5+0.6 × 1013 L⊙. The implied active galactic nuclei (AGN) – corrected SFR = 1240−260+310 M⊙ yr−1 is a factor of 2 lower than previously reported for this QSO. We measured a gas-to-dust ratio of GDR = 101 ± 20. The dust continuum and [CII] surface brightness profiles are spatially extended out to r ∼ 6.7 kpc and r ∼ 5 kpc, respectively, with half-light radii of 0.9 and 1.1 kpc for the dust and gas, respectively. The derived gas surface density, Σgas, and star formation rate density, ΣSFR, place the J2310+1855 host galaxy above the Kennicutt-Schmidt relation. We derived a best estimate of the dynamical mass Mdyn = 5.2 × 1010 M⊙ within r = 1.7 kpc based on a dynamical model of the system with a rotating disk inclined at i = 25 deg. The Toomre parameter profile across the disk is Qgas ∼ 3 and implies that the disk is unstable. We found that SFR/Mdyn > ṀBH/MBH, suggesting that AGN feedback might be efficiently acting to slow down the SMBH accretion, while stellar mass assembly is still vigorously taking place in the host galaxy. In addition, we were also able to detect high-velocity emission on the red and blue sides of the [CII] emission line that is not consistent with disk rotation and traces a gaseous outflow. We derived an outflow mass Mout = 3.5 × 108 M⊙, and a mass outflow rate in the range Ṁout = 1800 − 4500 M⊙ yr−1. The implied Ėout ∼ 0.0005 − 0.001 Lbol is in agreement with the values observed for ionised winds. For the first time, we mapped a spatially resolved water vapour disk through the H2O v = 0 3(2, 2) − 3(1, 3) emission line detected at νobs = 274.074 GHz, whose kinematic properties and size are broadly consistent with those of the [CII] disk. The luminosity ratio LH2O/LTIR = 1.4 × 10−5 is consistent with line excitation by dust-reprocessed star formation in the interstellar medium of the host galaxy.

AGN accretion and black hole growth across compact and extended galaxy evolution phases

ABSTRACT The extent of black hole growth during different galaxy evolution phases and the connection between galaxy compactness and active galactic nucleus (AGN) activity remain poorly understood. We use Hubble Space Telescope imaging of the CANDELS fields to identify star-forming and quiescent galaxies at z = 0.5–3 in both compact and extended phases and use Chandra X-ray imaging to measure the distribution of AGN accretion rates and track black hole growth within these galaxies. We show that accounting for the impact of AGN light changes ∼20 per cent of the X-ray sources from compact to extended galaxy classifications. We find that ∼10–25 per cent of compact star-forming galaxies host an AGN, a mild enhancement (by a factor ∼2) compared to extended star-forming galaxies or compact quiescent galaxies of equivalent stellar mass and redshift. However, AGNs are not ubiquitous in compact star-forming galaxies and this is not the evolutionary phase, given its relatively short time-scale, where the bulk of black hole mass growth takes place. Conversely, we measure the highest AGN fractions (∼10–30 per cent) within the relatively rare population of extended quiescent galaxies. For massive galaxies that quench at early cosmic epochs, substantial black hole growth in this extended phase is crucial to produce the elevated black hole mass-to-galaxy stellar mass scaling relation observed for quiescent galaxies at z ∼ 0. We also show that AGN fraction increases with compactness in star-forming galaxies and decreases in quiescent galaxies within both the compact and extended subpopulations, demonstrating that AGN activity depends closely on the structural properties of galaxies.

Landscape of massive black-hole spectroscopy with LISA and the Einstein Telescope

Measuring the quasi-normal mode~(QNM) spectrum emitted by a perturbed black-hole~(BH) --~also known as BH spectroscopy~-- provides an excellent opportunity to test the predictions of general relativity in the strong-gravity regime. We investigate the prospects and precision of BH spectroscopy in massive binary black hole ringdowns, one of the primary science objectives of the future Laser Interferometric Space Antenna~(LISA) mission. We simulate various massive binary BH population models, featuring competing prescriptions for the Delays between galaxy and BH mergers, for the impact of supernova feedback on massive BH growth, and for the initial population of high redshift BH seeds (light versus heavy seeds). For each of these scenarios, we compute the average number of expected events for precision BH spectroscopy using a Fisher-matrix analysis. We find that, for any heavy seed scenario, LISA will measure the dominant mode frequency within ${\cal O}(0.1) \%$ relative uncertainty and will estimate at least 3 independent QNM parameters within $1 \%$ error. The most optimistic heavy seed scenarios produce $\mathcal{O}(100)$ events with $1 \%$ measurability for 3 or more QNM quantities during LISA's operational time. On the other hand, light seed scenarios produce lighter merger remnants, which ring at frequencies higher than LISA's sensitivity. Interestingly, the light seed models give rise to a fraction of mergers in the band of Einstein Telescope, allowing for the measurement of 3 QNM parameters with $\sim 10 \%$ relative errors in approximately a few to ten events/yr. More precise BH spectroscopy in the light seed scenarios would require instruments operating in the deciHertz band.

Stellar and black hole assembly inz< 0.3 infrared-luminous mergers: intermittent starbursts versus super-Eddington accretion

ABSTRACTWe study stellar and black hole mass assembly in a sample of 42 infrared-luminous galaxy mergers at z < 0.3 by combining results from radiative transfer modelling with archival measures of molecular gas and black hole mass. The ratios of stellar mass, molecular gas mass, and black hole mass to each other are consistent with those of massive gas-rich galaxies at z < 0.3. The advanced mergers may show increased black hole mass to stellar mass ratios, consistent with the transition from active galactic nucleus (AGN) to ellipticals and implying substantial black hole mass growth over the course of the merger. Star formation rates are enhanced relative to the local main sequence, by factors of ∼100 in the starburst and ∼1.8 in the host, respectively. The starburst star formation rates appear distinct to star formation in the main sequence at all redshifts up to at least z ∼ 5. Starbursts may prefer late-stage mergers, but are observed at any merger stage. We do not find evidence that the starbursts in these low-redshift systems substantially increase the total stellar mass, with a soft upper limit on the stellar mass increase from starburst activity of about a factor of two. In contrast, 12 objects show evidence for super-Eddington accretion, associated with late-stage mergers, suggesting that many AGN in infrared-luminous mergers go through a super-Eddington phase. The super-Eddington phase may increase black hole mass by up to an order of magnitude at an accretion efficiency of $42\pm 33{{\ \rm per\ cent}}$ over a period of 44 ± 22 Myr. Our results imply that super-Eddington accretion is an important black hole growth channel in infrared-luminous galaxies at all redshifts.

Revisiting Black Hole Hyperaccretion in the Center of Gamma-Ray Bursts for the Lower Mass Gap

The ultrarelativistic jets triggered by neutrino annihilation processes or Blandford–Znajek (BZ) mechanisms in stellar-mass black hole (BH) hyperaccretion systems are generally considered to power gamma-ray bursts (GRBs). Due to the high accretion rate, the central BHs might grow rapidly on a short timescale, providing a new way to understand the lower mass gap problem. In this paper, we use the BH hyperaccretion model to investigate BH mass growth based on observational GRB data. The results show that (i) if the initial BH mass is set as 3 M ⊙, the neutrino annihilation processes are capable of fueling the BHs to escape the lower mass gap for more than half of long-duration GRBs (LGRBs), while the BZ mechanism is inefficient in triggering BH growth for LGRBs; (ii) the mean BH mass growth in the case of LGRBs without observable supernova (SN) association is much larger than that in the case of LGRBs associated with SNe for both mechanisms, which implies that more massive progenitors or lower SN explosion energies prevail throughout the former cases; (iii) for the short-duration GRBs, the mean BH mass growth is satisfied with the mass supply limitation in the scenario of compact object mergers, but the hyperaccretion processes are unable to rescue BHs from the gap in binary neutron star (NS) mergers or the initial BH mass being 3 M ⊙ after NS−BH mergers.