Search

This white paper explores the detectability of intermediate-mass black holes (IMBHs) wandering in the Milky Way (MW) and massive local galaxies, with a particular emphasis on the role of AXIS. IMBHs, ranging within 10 3 − 6 M ⊙ , are commonly found at the centers of dwarf galaxies and may exist, yet undiscovered, in the MW. By using model spectra for advection-dominated accretion flows (ADAFs), we calculated the expected fluxes emitted by a population of wandering IMBHs with masses of 10 5 M ⊙ in various MW environments and extrapolated our results to massive local galaxies. Around 40 % of the potential population of wandering IMBHs in the MW can be detected in an AXIS deep field. We proposed criteria to aid with selecting IMBH candidates using already available optical surveys. We also showed that IMBHs wandering in >200 galaxies within 10 Mpc can be easily detected with AXIS when passing within dense galactic environments (e.g., molecular clouds and cold neutral medium). In summary, we highlighted the potential X-ray detectability of wandering IMBHs in local galaxies and provided insights for guiding future surveys. Detecting wandering IMBHs is crucial for understanding their demographics and evolution and the merging history of galaxies. This white paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS white papers can be found at the AXIS website. [ABSTRACT FROM AUTHOR]

Recent dynamical measurements indicate the presence of a central supermassive black hole (SMBH) with mass |${\sim} 3\times 10^6\, {\rm M_\odot }$| in the dwarf galaxy Leo I, placing the system ∼50 times above the standard, local M BH– M ⋆ relation. While a few overmassive central SMBHs are reported in nearby isolated galaxies, this is the first one detected in a Milky Way satellite. We used the ASTRID and Illustris TNG50 lambda cold dark matter (LCDM) cosmological simulations to investigate the assembly history of galaxies hosting overmassive SMBHs. We estimate that, at the stellar mass of Leo I, |${\sim} 15~{{\ \rm per\ cent}}$| of galaxies above the M BH– M ⋆ relation lie >10 times above it. Leo I-like systems are rare but exist in LCDM simulations: they occur in |${\sim} 0.005~{{\ \rm per\ cent}}$| of all overmassive systems. Examining the properties of simulated galaxies harbouring overmassive central SMBHs, we find that: (i) stars assemble more slowly in galaxies above the M BH– M ⋆ relation; (ii) the gas fraction in these galaxies experiences a significantly steeper decline over time; and (iii) |$\gt 95~{{\ \rm per\ cent}}$| of satellite host galaxies in overdense regions are located above the M BH– M ⋆ relation. This suggests that massive satellite infall and consequent tidal stripping in a group/dense environment can drive systems away from the M BH– M ⋆ relation, causing them to become overmassive. As the merging histories of overmassive and undermassive systems do not differ, we conclude that additional environmental effects, such as being in overdense regions must play a crucial role. In the high- z Universe, central overmassive SMBHs are a signature of heavy BH seeds; we demonstrate, in contrast, that low- z overmassive systems result from complex environmental interactions. [ABSTRACT FROM AUTHOR]

Observational evidence is mounting regarding the population demographics of massive black holes (MBHs), from the most massive cluster galaxies down to the dwarf galaxy regime. However, the progenitor pathways from which these central MBHs formed remain unclear. Here, we report a potentially powerful observational signature of MBH formation in dwarf galaxies. We argue that a continuum in the mass spectrum of MBHs in (fossil) dwarf galaxies would be a unique signature of a heavy seed formation pathway. The continuum in this case would consist of the usual population of stellar mass black holes, formed through stellar evolution, plus a smaller population of heavy seed MBHs that have not yet sunk to the centre of the galaxy. Under the robust assumption of initial fragmentation of the parent gas cloud resulting in a burst of heavy seed production, a significant fraction of these seeds will survive to the present day as off-nuclear MBHs with masses less than that of the central object. Motivated by the recent discovery of an MBH in the relatively low central density Leo I galaxy, we show that such a continuum in MBH seed masses should persist from the lightest black hole masses up to the mass of the central MBH in contrast to the light seeding scenario where no such continuum should exist. The detection of off-centred MBHs and a central MBH would represent strong evidence of a heavy seeding pathway. [ABSTRACT FROM AUTHOR]

Intermediate-mass black holes (IMBHs, |$10^{3\!-\!6} \, {\rm M_\odot }$|⁠), are typically found at the centre of dwarf galaxies and might be wandering, thus far undetected, in the Milky Way (MW). We use model spectra for advection-dominated accretion flows to compute the typical fluxes, in a range of frequencies spanning from radio to X-rays, emitted by a putative population of |$10^5 \, {\rm M_\odot }$| IMBHs wandering in five realistic volume-weighted MW environments. We predict that |$\sim 27{{\ \rm per\ cent}}$| of the wandering IMBHs can be detected in the X-ray with Chandra , |$\sim 37{{\ \rm per\ cent}}$| in the near-infrared with the Roman Space Telescope , |$\sim 49{{\ \rm per\ cent}}$| in the sub-mm with CMB-S4, and |$\sim 57{{\ \rm per\ cent}}$| in the radio with ngVLA. We find that the brightest fluxes are emitted by IMBHs passing through molecular clouds or cold neutral medium, where they are always detectable. We propose criteria to facilitate the selection of candidates in multiwavelength surveys. Specifically, we compute the X-ray to optical ratio (αox) and the optical to sub-mm ratio, as a function of the accretion rate of the IMBH. We show that at low rates the sub-mm emission of IMBHs is significantly higher than the optical, UV, and X-ray emission. Finally, we place upper limits on the number N • of these objects in the MW: N • < 2000 and N • < 100, based on our detectability expectations and current lack of detections in molecular clouds and cold neutral medium, respectively. These predictions will guide future searches of IMBHs in the MW, which will be instrumental to understanding their demographics and evolution. [ABSTRACT FROM AUTHOR]

More than 200 quasars have been detected so far at z > 6, with only one showing clear signs of strong gravitational lensing. Some studies call for a missing population of lensed high- z quasars, but their existence is still in doubt. A large fraction of high- z quasars being lensed would have a significant effect on the shape of the intrinsic quasar luminosity function (QLF). Here, we perform the first systematic search for lensed X-ray-detected quasars at z ≳ 6 employing a Bayesian analysis, with the code baymax , to look for morphological evidence of multiple images that may escape a visual inspection. We analysed a sample of 22 quasars at z > 5.8 imaged by the Chandra X-ray observatory and found none with statistically significant multiple images. In the sub-sample of the eight sources with photon counts >20, we exclude multiple images with separations r > 1 arcsec and count ratios f > 0.4, or with separations as small as 0.7 arcsec and f > 0.7 at |$95{{\ \rm per\ cent}}$| confidence level. Comparing this non-detection with predictions from theoretical models suggesting a high- and a low-lensed fraction, we placed upper limits on the bright-end slope, β, of the QLF. Using only the sub-sample with eight sources, we obtain, in the high-lensing model, a limit β < 3.38. Assuming no multiple source is present in the full sample of 22 sources, we obtain β < 2.89 and β < 3.53 in the high- and low-lensing models, respectively. These constraints strongly disfavour steep QLF shapes previously proposed in the literature. [ABSTRACT FROM AUTHOR]

The detection of two z ∼ 13 galaxy candidates has opened a new window on galaxy formation at an era only 330 Myr after the big bang. Here, we investigate the physical nature of these sources: are we witnessing star forming galaxies or quasars at such early epochs? If powered by star formation, the observed ultraviolet (UV) luminosities and number densities can be jointly explained if: (i) these galaxies are extreme star-formers with star formation rates 5−24 × higher than those expected from extrapolations of average lower-redshift relations; (ii) the star formation efficiency increases with halo mass and is countered by increasing dust attenuation from z ∼ 10−5; (iii) they form stars with an extremely top-heavy initial mass function. The quasar hypothesis is also plausible, with the UV luminosity produced by black holes of |$\sim 10^8 \, \rm M_\odot$| accreting at or slightly above the Eddington rate (f Edd ∼ 1.0). This black hole mass at z ∼ 13 would require very challenging, but not implausible, growth parameters. If spectroscopically confirmed, these two sources will represent a remarkable laboratory to study the Universe at previously inaccessible redshifts. [ABSTRACT FROM AUTHOR]

The quest for high-redshift quasars has led to a series of record-breaking sources, with the current record holder at z  = 7.642. Here, we show how future detections of z > 8 quasars impact the constraints on the parameters for black hole growth and seed models. Using broad flat priors on the growth parameters (Eddington ratio |$\, {f_{\rm Edd}}$|⁠ , duty cycle |${\cal D}$|⁠ , seed mass M •,seed, and radiative efficiency ϵ), we show that the large uncertainties in their determination decrease by a factor ∼5 when a quasar's detection redshift goes from z  = 9 to z  = 12. In this high-redshift regime, ϵ tends to the lowest value allowed, and the distribution for M •,seed peaks well inside the heavy seed domain. Remarkably, two quasars detected at z > 7 with low accretion rates (J1243+0100 and J0313–1806) already tighten the available parameter space, requiring |$M_{\rm \bullet , seed} \gt 10^{3.5} \, {\rm M_\odot }$| and ϵ < 0.1. The radiative efficiency is a crucial unknown, with factor ∼2 changes able to modify the predicted mass by ∼3 orders of magnitude already at z ∼ 9. The competing roles of inefficient accretion (decreasing ϵ) and black hole spin-up (increasing ϵ) significantly impact growth models. Finally, we suggest that yields currently predicted by upcoming quasar surveys (e.g. Euclid) will be instrumental for determining the most-likely seed mass regime. For example, assuming thin-disc accretion, a detection of a quasar with |$M_\bullet \sim 10^{10} \, {\rm M_\odot }$| by z ∼ 9–10 would exclude the entire parameter space available for light seeds and dramatically reduce the one for heavy seeds. [ABSTRACT FROM AUTHOR]

Supermassive black holes (SMBHs) play a crucial role in the evolution of galaxies and are currently detected up to |$z$| ∼ 7.5. Theories describing black hole (BH) growth are challenged by how rapidly seeds with initial mass |$M_\bullet \lesssim 10^5 \, {\rm M_\odot }$|⁠ , formed at |$z$| ∼ 20–30, grew to |$M_\bullet \sim 10^9 \, {\rm M_\odot }$| by |$z$| ∼ 7. Here we study the effects of the value of the Hubble parameter, H 0, on models describing the early growth of BHs. First, we note that the predicted mass of a quasar at |$z$| = 6 changes by |$\gt 300{{\ \rm per\ cent}}$| if the underlying Hubble parameter used in the model varies from H 0 = 65 to H 0 = 74 km s−1Mpc−1, a range encompassing current estimates. Employing an MCMC approach based on priors from |$z$| ≳ 6.5 quasars and on H 0, we study the interconnection between H 0 and the parameters describing BH growth: seed mass M i and Eddington ratio f Edd. Assuming an Eddington ratio of f Edd = 0.7, in agreement with previous estimates, we find |$H_0 = 73.6^{+1.2}_{-3.3}$|  km s−1Mpc−1. In a second analysis, allowing all the parameters to vary freely, we find log (M i / M ⊙) > 4.5 (at 95 per cent CL), |$H_0 = 74^{+1.5}_{-1.4}$|  km s−1Mpc−1 and |$f_{\rm Edd}=0.77^{+0.035}_{-0.026}$| at 68 per cent CL. Our results on the typical Eddington ratio are in agreement with previous estimates. Current values of the Hubble parameter strongly favour heavy seed formation scenarios, with |$M_i \gtrsim 10^4 \, {\rm M_\odot }$|⁠. In our model, with the priors on BH masses of quasars used, light seed formation scenarios are rejected at ∼3σ. [ABSTRACT FROM AUTHOR]

Black holes across a broad range of masses play a key role in the evolution of galaxies. The initial seeds of black holes formed at z ∼ 30 and grew over cosmic time by gas accretion and mergers. Using observational data for quasars and theoretical models for the hierarchical assembly of dark matter halos, we study the relative importance of gas accretion and mergers for black hole growth, as a function of redshift (0 < z < 10) and black hole mass (). We find that (i) growth by accretion is dominant in a large fraction of the parameter space, especially at and z > 6; and (ii) growth by mergers is dominant at and z > 5.5, and at and z < 2. As the growth channel has direct implications for the black hole spin (with gas accretion leading to higher spin values), we test our model against ∼20 robust spin measurements available thus far. As expected, the spin tends to decline toward the merger-dominated regime, thereby supporting our model. The next generation of X-ray and gravitational-wave observatories (e.g., Lynx, AXIS, Athena, and LISA) will map out populations of black holes up to very high redshift (z ∼ 20), covering the parameter space investigated here in almost its entirety. Their data will be instrumental to providing a clear picture of how black holes grew across cosmic time. [ABSTRACT FROM AUTHOR]

There exist hitherto unexplained fluctuations in the cosmic infrared background on arcminute scales and larger. These have been shown to cross-correlate with the cosmic X-ray background, leading several authors to attribute the excess to a high-redshift growing black hole population. In order to investigate potential sources that could explain this excess, in this paper, we develop a new framework to compute the power spectrum of undetected sources that do not have constant flux as a function of halo mass. In this formulation, we combine a semi-analytic model for black hole growth and their simulated spectra from hydrodynamical simulations. Revisiting the possible contribution of a high-redshift black hole population, we find that too much black hole growth is required at early epochs for z > 6 accretion to explain these fluctuations. Examining a population of accreting black holes at more moderate redshifts, z ∼ 2–3, we find that such models produce a poor fit to the observed fluctuations while simultaneously overproducing the local black hole mass density. Additionally, we rule out the hypothesis of a missing Galactic foreground of warm dust that produces coherent fluctuations in the X-ray via reflection of Galactic X-ray binary emission. Although we firmly rule out accreting massive black holes as the source of these missing fluctuations, additional studies will be required to determine their origin. [ABSTRACT FROM AUTHOR]

The detection of gravitational waves (GWs) generated by merging black holes has recently opened up a new observational window into the Universe. The mass of the black holes in the first and third Laser Interferometer Gravitational Wave Observatory (LIGO) detections (36–29M⊚ and 32–19M⊚) suggests low-metallicity stars as their most likely progenitors. Based on high-resolution N-body simulations, coupled with state-of-the-art metal enrichment models, we find that the remnants of Pop III stars are preferentially located within the cores of galaxies. The probability of a GW signal to be generated by Pop III stars reaches ∼90 per cent at ∼0.5 kpc from the galaxy centre, compared to a benchmark value of ∼5 per cent outside the core. The predicted merger rates inside bulges is ∼60 × βIII Gpc-3 yr-1 (βIII is the Pop III binarity fraction). To match the 90 per cent credible range of LIGO merger rates, we obtain: 0.03 < βIII < 0.88. Future advances in GW observatories and the discovery of possible electromagnetic counterparts could allow the localization of such sources within their host galaxies. The preferential concentration of GW events within the bulge of galaxies would then provide an indirect proof for the existence of Pop III stars. [ABSTRACT FROM AUTHOR]

The peculiar emission properties of the z ~ 6.6 Lyα emitter COSMOS redshift 7 (CR7) have been initially interpreted with the presence of either a direct collapse black hole (DCBH) or a substantial mass of Pop III stars. Instead, updated photometric observations by Bowler et al. seem to suggest that CR7 is a more standard system. Here, we confirm that the original DCBH hypothesis is consistent also with the new data. Using radiation-hydrodynamic simulations, we reproduce the new infrared photometry with two models involving a Compton-thick DCBH of mass ≈7 × 106M☉ accreting (a) metal-free (Z = 0) gas with column density NH = 8 × 1025 cm-2 or (b) low-metallicity gas (Z = 5 × 10-3 Z☉) with NH = 3 × 1024 cm-2. The best-fitting model reproduces the photometric data to within 1σ. Such metals can be produced by weak star-forming activity occurring after the formation of the DCBH. The main contribution to the Spitzer/IRAC 3.6 μm photometric band in both models is due to He I/He II λλ4714, 4687 emission lines, while the contribution of [OIII] λλ4959, 5007 emission lines, if present, is sub-dominant. Spectroscopic observations with JWST will be required to ultimately clarify the nature of CR7. [ABSTRACT FROM AUTHOR]

The first black hole seeds, formed when the Universe was younger than ~500 Myr, are recognized to play an important role for the growth of early (z ~ 7) supermassive black holes. While progresses have been made in understanding their formation and growth, their observational signatures remain largely unexplored. As a result, no detection of such sources has been confirmed so far. Supported by numerical simulations, we present a novel photometric method to identify black hole seed candidates in deep multiwavelength surveys. We predict that these highly obscured sources are characterized by a steep spectrum in the infrared (1.6-4.5 µm), i.e. by very red colours. The method selects the only two objects with a robust X-ray detection found in the CANDELS/GOODS-S survey with a photometric redshift z ≲ 6. Fitting their infrared spectra only with a stellar component would require unrealistic star formation rates (≲2000 M⊙ yr-1). To date, the selected objects represent the most promising black hole seed candidates, possibly formed via the direct collapse black hole scenario, with predicted mass >105 M⊙. While this result is based on the best photometric observations of high-z sources available to date, additional progress is expected from spectroscopic and deeper X-ray data. Upcoming observatories, like the JWST, will greatly expand the scope of this work. [ABSTRACT FROM AUTHOR]

Massive black hole (MBH) seeds at redshift z ≳ 10 are now thought to be key ingredients to explain the presence of the supermassive (109-10M⊙) black holes in place <1Gyr after the big bang. Once formed, massive seeds grow and emit copious amounts of radiation by accreting the left-over halo gas; their spectrum can then provide crucial information on their evolution. By combining radiation-hydrodynamic and spectral synthesis codes, we simulate the time-evolving spectrum emerging from the host halo of a MBH seed with initial mass 105M⊙, assuming both standard Eddington-limited accretion, or slim accretion discs, appropriate for super-Eddington flows. The emission occurs predominantly in the observed infrared-submm (1-1000µm) and X-ray (0.1-100keV) bands. Such signal should be easily detectable by JWSTaround ~1µm up to z ~25, and by ATHENA (between 0.1 and 10keV, up to z ~ 15). Ultra-deep X-ray surveys like the Chandra Deep Field South could have already detected these systems up to z ~ 15. Based on this, we provide an upper limit for the z ≳ 6 MBH mass density of ρ• ≲ 2.5 × 10²M⊙ Mpc-3 assuming standard Eddington-limited accretion. If accretion occurs in the slim disc mode the limits are much weaker, ρ• ≳ 7.6 × 10³ M⊙Mpc-3 in the most constraining case. [ABSTRACT FROM AUTHOR]

The observational evidence that Super-Massive Black Holes (M. ~ 109-10 M⊙) are already in place less than 1 Gyr after the big bang poses stringent time constraints on the growth efficiency of their seeds. Among proposed possibilities, the formation of massive (~103-6 M⊙) seeds and/or the occurrence of super-Eddington (Ṁ>ṀEdd) accretion episodes may contribute to the solution of this problem. In this work, using a set of astrophysically motivated initial conditions, we analytically and numerically investigate the accretion flow on to high-redshift (z ~ 10) black holes to understand the physical requirements favouring rapid and efficient growth. Our model identifies a 'feeding-dominated' accretion regime and a 'feedback-limited' one, the latter being characterized by intermittent (duty cycles D≲0.5) and inefficient growth, with recurring outflow episodes. We find that low-mass seeds (≲103-4 M⊙) evolve in the feedback-limited regime, while more massive seeds (≳105-6 M⊙) grow very rapidly as they are found in the feeding-dominated regime. In addition to the standard accretion model with a fixed matter-energy conversion factor (ε = 0.1), we have also explored slim disc models, appropriate for super-Eddington accretion, where radiation is trapped in the disc and the radiative efficiency is reduced (ε ≲ 0.04), which may ensure a continuous growth with Ṁ≫ṀEdd (up to ~300ṀEdd in our simulations). Under these conditions, outflows play a negligible role and a black hole can accrete 80-100?per?cent of the gas mass of the host halo (~107 M⊙) in ~10 Myr, while in feedback-limited systems we predict that black holes can accrete only up to ~15 per cent of the available mass. [ABSTRACT FROM AUTHOR]

The possible formation of direct collapse black holes (DCBHs) in the first metal-free atomic cooling haloes at high redshifts (z ≥ 10) is nowadays object of intense study and several methods to prove their existence are currently under development. The abrupt collapse of a massive (~104-105M⊙) and rotating object is a powerful source of gravitational waves emission. In this work, we employ modern waveforms and the improved knowledge on the DCBHs formation rate to estimate the gravitational signal emitted by these sources at cosmological distances. Their formation rate is very high (~104 yr-1 up to z ~ 20), but due to a short duration of the collapse event (~2-30 s, depending on the DCBH mass), the integrated signal from these sources is characterized by a very low duty-cycle (D ~ 10-3), i.e. a shot-noise signal. Our results show that the estimated signal lies above the foreseen sensitivity of the Ultimate-Deci-hertz Interferometer Gravitational wave Observatory in the frequency range (0.8-300) mHz, with a peak amplitude Ωgw = 1.1 x 10-54 at vmax = 0.9 mHz and a peak signal-to-noise ratio SNR ~ 22 at v = 20 mHz. This amplitude is lower than the Galactic confusion noise, generated by binary systems of compact objects in the same frequency band. For this reason, advanced techniques will be required to separate this signal from background and foreground noise components. As a proof-of-concept, we conclude by proposing a simple method, based on the autocorrelation function, to recognize the presence of a D « 1 signal buried into the continuous noise. The aim of this work is to test the existence of a large population of high-z DCBHs, by observing the gravitational waves emitted during their infancy. [ABSTRACT FROM AUTHOR]

Theoretical models predict that a population of intermediate-mass black holes (IMBHs) of mass M• ≈ 104–5 M⊙ might form at high (z ≳ 10) redshift by different processes. Such objects would represent the seeds out of which z ≳ 6 supermassive black holes grow. We numerically investigate the radiation–hydrodynamic evolution governing the growth of such seeds via accretion of primordial gas within their parent dark matter halo of virial temperature Tvir ∼ 104 K. We find that the accretion on to a direct collapse black hole of initial mass M0 = 105 M⊙ occurs at an average rate $\skew3\dot{M}_{\bullet } \simeq 1.35 \, \skew3\dot{M}_{\rm Edd} \simeq 0.1 \, \mathrm{M_{{\odot }} \, yr^{-1}}$, is intermittent (duty cycle ≲ 50 per cent) and lasts ≈142 Myr; the system emits on average at super-Eddington luminosities, progressively becoming more luminous as the density of the inner mass shells, directly feeding the central object, increases. Finally, when ≈90 per cent of the gas mass has been accreted (in spite of an average super-Eddington emission) on to the black hole, whose final mass is ∼7 × 106 M⊙, the remaining gas is ejected from the halo due to a powerful radiation burst releasing a peak luminosity Lpeak ∼ 3 × 1045 erg s−1. The IMBH is Compton-thick during most of the evolution, reaching a column density NH ∼ 1025 cm−2 in the late stages of the simulation. We briefly discuss the observational implications of the model. [ABSTRACT FROM PUBLISHER]

The cosmological 21 cm signal is a physics-rich probe of the early Universe, encoding information about both the ionization and the thermal history of the intergalactic medium (IGM). The latter is likely governed by X-rays from star formation processes inside very high redshift (z ≳ 15) galaxies. Due to the strong dependence of the mean free path on the photon energy, the X-ray spectral energy distribution (SED) can have a significant impact on the interferometric signal from the cosmic dawn. Recent Chandra observations of nearby, star-forming galaxies show that their SEDs are more complicated than is usually assumed in 21 cm studies. In particular, these galaxies have ubiquitous, sub-keV thermal emission from the hot interstellar medium (ISM), which generally dominates the soft X-ray luminosity (with energies ≲1 keV, sufficiently low to significantly interact with the IGM). Using illustrative soft and hard SEDs, we show that the IGM temperature fluctuations in the early Universe would be substantially increased if the X-ray spectra of the first galaxies were dominated by the hot ISM, compared with X-ray binaries with harder spectra. The associated large-scale power of the 21 cm signal would be higher by a factor of ∼3. More generally, we show that the peak in the redshift evolution of the large-scale (k ∼ 0.2 Mpc−1) 21 cm power is a robust probe of the soft-band SED of the first galaxies, and importantly, is not degenerate with their bolometric luminosities. On the other hand, the redshift of the peak constrains the X-ray luminosity and halo masses which host the first galaxies. [ABSTRACT FROM PUBLISHER]

Massive black holes in the centres of galaxies today must have grown by several orders of magnitude from seed black holes formed at early times. Detecting a population of intermediate mass black holes (IMBHs) can provide constraints on these elusive BH seeds. Here, we use the large volume cosmological hydrodynamical simulation Astrid , which includes IMBH seeds and dynamical friction to investigate the population of IMBH seeds. Dynamical friction is largely inefficient at sinking and merging seed IMBHs at high-z. This leads to an extensive population (several hundred per galaxy) of wandering IMBHs in large haloes at |$z\sim 2$|⁠. A small fraction of these IMBHs are detectable as HLXs, Hyper Luminous X-ray sources. Importantly, at |$z\sim 2$|⁠ , IMBHs mergers produce the peak of GW events. We find close to a million GW events in Astrid  between |$z=\rm{2\!-\!3}$| involving seed IMBH mergers. These GW events (almost all detectable by LISA) at cosmic noon should provide strong constraints on IMBH seed models and their formation mechanisms. At the centre of massive galaxies, where the number of IMBHs can be as high as 10–100, SMBH-IMBH pairs can form. These Intermediate mass ratio inspirals (IMRIs) and extreme mass ratio inspirals (EMRIs), will require the next generation of milli- |$\mu$| Hz space-based GW interferometers to be detected. Large populations of IMBHs around massive black holes will probe their environments and MBH causal structure. [ABSTRACT FROM AUTHOR]

We use a semi-analytic model, Delphi, which jointly tracks the dark matter and baryonic assembly of high-redshift (z ≃4-20) galaxies to gain insight on the number density of dicollapse black hole (DCBH) hosts in three different cosmologies: the standard cold dark matter (CDM) model and two warm dark matter (WDM) models with particle masses of 3.5 and 1.5 keV. Using the Lyman-Werner (LW) luminosity of each galaxy from Delphi, we use a clustering bias analysis to identify all, pristine haloes with a virial temperature Tvir ≳104 K which are irradiated by an LW background above a critical value as DCBH hosts. In good agreement with previous studies, we find the DCBH number density rises from ~10-6.1 to ~10-3.5 cMpc-3 from z ≃17.5 to 8 in the CDM model using a critical LW background value of 30J21 (where J21 = 10-21 erg s-1 Hz-1 cm-2 sr-1). We find that a combination of delayed structure formation and an accelerated assembly of galaxies results in a later metal-enrichment and an accelerated build-up of the LW background in the 1.5 keV WDM model, resulting in DCBH hosts persisting down to much lower redshifts (z ≃5) as compared to CDM where DCBH hosts only exist down to z ≃8. We end by showing how the expected colours in three different bands of the Near Infrared Camera (NIRCam) onboard the forthcoming James Webb Space Telescope can be used to hunt for potential z ≃5-9 DCBHs, allowing hints on the WDM particle mass. [ABSTRACT FROM AUTHOR]

Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral at z > 7 and largely ionized by z ∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low ( x ¯ H I ≲ 10 − 3 ) or close to unity ( x ¯ H I ∼ 1). In particular, the neutral fraction evolution of the IGM at the critical redshift range of z = 6–7 is poorly constrained. We present new constraints on x ¯ H I at z ∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 < z < 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of "dark" pixels identified in the Ly α and Ly β forests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction at z ∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of x ¯ H I (z = 6.3) < 0.79 ± 0.04 (1 σ), x ¯ H I (z = 6.5) < 0.87 ± 0.03 (1 σ), and x ¯ H I (z = 6.7) < 0.94 − 0.09 + 0.06 (1 σ). The dark pixel fractions at z > 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018. [ABSTRACT FROM AUTHOR]

We present two bright galaxy candidates at z ∼ 12â€"13 identified in our H -dropout Lyman break selection with 2.3 deg2 near-infrared deep imaging data. These galaxy candidates, selected after careful screening of foreground interlopers, have spectral energy distributions showing a sharp discontinuity around 1.7 ÎĽ m, a flat continuum at 2â€"5 ÎĽ m, and nondetections at <1.2 ÎĽ m in the available photometric data sets, all of which are consistent with a z > 12 galaxy. An ALMA program targeting one of the candidates shows a tentative 4 σ [O iii ] 88 ÎĽ m line at z = 13.27, in agreement with its photometric redshift estimate. The number density of the z ∼ 12â€"13 candidates is comparable to that of bright z ∼ 10 galaxies and is consistent with a recently proposed double-power-law luminosity function rather than the Schechter function, indicating little evolution in the abundance of bright galaxies from z ∼ 4 to 13. Comparisons with theoretical models show that the models cannot reproduce the bright end of rest-frame ultraviolet luminosity functions at z ∼ 10â€"13. Combined with recent studies reporting similarly bright galaxies at z ∼ 9â€"11 and mature stellar populations at z ∼ 6â€"9, our results indicate the existence of a number of star-forming galaxies at z > 10, which will be detected with upcoming space missions such as the James Webb Space Telescope, Nancy Grace Roman Space Telescope, and GREX-PLUS. [ABSTRACT FROM AUTHOR]

We report new measurements of the intergalactic medium (IGM) Lyα and Lyβ effective optical depth at 5.3 < z < 6.5, using a new sample of quasar sight lines including 32 quasars at 6.308 ≤ z ≤ 7.00. These quasars provide a large statistical sample to measure the IGM evolution during the transition phase of the reionization epoch. We construct a data set of deep optical spectra of these quasars using VLT, Keck, Gemini, LBT, and MMT. We measure the Lyα effective optical depth at 5.36 < z < 6.57 using the Lyα forests of both individual spectra and the stacked spectrum. The large scatter of individual measurements is consistent with previous work, suggesting an inhomogeneous reionization process. Combining our new measurements and previous results, we obtain a best fit for the Lyα effective optical depth evolution at z > 5.3,. We then estimate the observed Lyβ effective optical depth using Lyβ forests and convert them to Lyα optical depth for comparison, which provides additional constraints on the evolution of the IGM optical depth. The Lyβ-based measurements are generally in agreement with the best-fit evolution obtained from Lyα forests. Using this new sample, we identify 389 Lyα and 50 Lyβ transmission spikes at 5.5 < z < 6.3. The upper limits of Lyα optical depth estimated using transmission spikes are well consistent with our best-fit evolution. The evolution in number density of these high-redshift transmission spikes suggests a rapid transition phase at the end of the reionization. Comparison of our optical depth measurements with hydrodynamical simulations indicates an IGM neutral hydrogen fraction at z = 6. [ABSTRACT FROM AUTHOR]

We present a multiwavelength analysis of 28 of the most luminous low-redshift narrow-line, ultra-hard X-ray-selected active galactic nuclei (AGN) drawn from the 70-month Swift /BAT all-sky survey, with bolometric luminosities of |$\log (L_{\rm bol} /{\rm erg\, s}^{-1}) \gtrsim 45.25$|⁠. The broad goal of our study is to determine whether these objects have any distinctive properties, potentially setting them aside from lower luminosity obscured AGN in the local Universe. Our analysis relies on the first data release of the BAT AGN Spectroscopic Survey (BASS/DR1) and on dedicated observations with the VLT, Palomar, and Keck observatories. We find that the vast majority of our sources agree with commonly used AGN selection criteria which are based on emission line ratios and on mid-infrared colours. Our AGN are pre-dominantly hosted in massive galaxies (9.8 ≲ log (M */M⊙) ≲ 11.7); based on visual inspection of archival optical images, they appear to be mostly ellipticals. Otherwise, they do not have distinctive properties. Their radio luminosities, determined from publicly available survey data, show a large spread of almost four orders of magnitude – much broader than what is found for lower X-ray luminosity obscured AGN in BASS. Moreover, our sample shows no preferred combination of black hole masses (M BH) and/or Eddington ratio (λEdd), covering 7.5 ≲ log (M BH/M⊙) ≲ 10.3 and 0.01 ≲ λEdd ≲ 1. Based on the distribution of our sources in the λEdd− N H plane, we conclude that our sample is consistent with a scenario where the amount of obscuring material along the line of sight is determined by radiation pressure exerted by the AGN on the dusty circumnuclear gas. [ABSTRACT FROM AUTHOR]

Local samples of quiescent galaxies with dynamically measured black hole masses (M bh) may suffer from an angular resolution-related selection effect, which could bias the observed scaling relations between M bh and host galaxy properties away from the intrinsic relations. In particular, previous work has shown that the observed M bh– M star relation is more strongly biased than the M bh–σ relation. Local samples of active galactic nuclei (AGN) do not suffer from this selection effect, as in these samples M bh is estimated from megamasers and/or reverberation mapping-based techniques. With the exception of megamasers, M bh estimates in these AGN samples are proportional to a virial coefficient f vir. Direct modelling of the broad-line region suggests that f vir ∼ 3.5. However, this results in an M bh– M star relation for AGN, which lies below and is steeper than the one observed for quiescent black hole samples. A similar though milder trend is seen for the M bh–σ relation. Matching the high-mass end of the M bh– M star and M bh–σ relations observed in quiescent samples requires f vir ≳ 15 and f vir ≳ 7, respectively. On the other hand, f vir ∼ 3.5 yields M bh–σ and M bh– M star relations for AGN, which are remarkably consistent with the expected 'intrinsic' correlations for quiescent samples (i.e. once account has been made of the angular resolution-related selection effect), providing additional evidence that the sample of local quiescent black holes is biased. We also show that, as is the case for quiescent black holes, the M bh– M star scaling relation of AGN is driven by σ, thus providing additional key constraints to black hole–galaxy co-evolution models. [ABSTRACT FROM AUTHOR]

We present a theoretical model embedding the essential physics of early galaxy formation (z ≃ 5–12) based on the single premise that any galaxy can form stars with a maximal limiting efficiency that provides enough energy to expel all the remaining gas, quenching further star formation. This simple idea is implemented into a merger-tree-based semi-analytical model that utilizes two mass and redshift-independent parameters to capture the key physics of supernova feedback in ejecting gas from low-mass haloes, and tracks the resulting impact on the subsequent growth of more massive systems via halo mergers and gas accretion. Our model shows that: (i) the smallest haloes (halo mass Mh ≤ 1010 M⊙) build up their gas mass by accretion from the intergalactic medium; (ii) the bulk of the gas powering star formation in larger haloes (Mh ≥ 1011.5 M⊙) is brought in by merging progenitors; (iii) the faint-end UV luminosity function slope evolves according to α = −1.75 log z − 0.52. In addition, (iv) the stellar mass-to-light ratio is well fitted by the functional form log M* = −0.38MUV − 0.13 z + 2.4, which we use to build the evolving stellar mass function to compare to observations. We end with a census of the cosmic stellar mass density (SMD) across galaxies with UV magnitudes over the range −23 ≤ MUV ≤ −11 spanning redshifts 5 < z < 12; (v) while currently detected LBGs contain ≈50 per cent (10 per cent) of the total SMD at z = 5 (8), the James Webb Space Telescope will detect up to 25 per cent of the SMD at z ≃ 9.5. [ABSTRACT FROM AUTHOR]