Population Of Holes Research Articles

Broad search for gravitational waves from subsolar-mass binaries through LIGO and Virgo’s third observing run

We present a search for gravitational waves from the coalescence of binaries which contain at least one subsolar-mass component using data from the LIGO and Virgo observatories through the completion of their third observing run. The observation of a merger with a component below $1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ would be a clear sign of either new physics or the existence of a primordial black hole population; these black holes could also contribute to the dark matter distribution. Our search targets binaries where the primary has mass ${M}_{1}$ between 0.1 and $100\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and the secondary has mass ${M}_{2}$ from 0.1 to $1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ for ${M}_{1}<20\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and 0.01 to $1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ for ${M}_{1}\ensuremath{\ge}20\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$. Sources with ${M}_{1}<7\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$, ${M}_{2}>0.5\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ are also allowed to have orbital eccentricity up to ${e}_{10}\ensuremath{\sim}0.3$. This search region covers from comparable to extreme mass ratio sources up to ${10}^{4}:1$. We find no statistically convincing candidates and so place new upper limits on the rate of mergers; our analysis sets the first limits for most subsolar sources with $7{M}_{\ensuremath{\bigodot}}<{M}_{1}<20\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and tightens limits by $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}(1.6\ifmmode\times\else\texttimes\fi{})$ where ${M}_{1}>20\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ (${M}_{1}<7\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$). Using these limits, we constrain the dark matter fraction to below $0.3(0.7)%$ for 1 $(0.5)\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ black holes assuming a monochromatic mass function. Due to the high merger rate of primordial black holes beyond the individual source horizon distance, we also use the lack of an observed stochastic background as a complementary probe to limit the dark matter fraction. We find that although the limits are, in general, weaker than those from the direct search, they become comparable at $0.1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$.

Signs of higher multipoles and orbital precession in GW151226

We present a reanalysis of GW151226, the second binary black hole merger discovered by the LIGO--Virgo Collaboration. Previous analysis showed that the best-fit waveform for this event corresponded to the merger of a $\sim 14 \, M_\odot$ black hole with a $\sim 7.5 \, M_\odot$ companion, and the posterior distribution in mass ratio ($q \leq 1$) is rather flat. In this work, we perform parameter estimation using a waveform model that includes the effects of orbital precession and higher-order radiative multipole modes, and we find that the source parameters of GW151226 shift towards the low $q$ and high effective spin ($\chi_{\rm eff}$) region and that $q$ is better measured. The new solution has a log likelihood roughly two points higher than when either higher multipoles or orbital precession is neglected and can alter the astrophysical interpretation of GW151226. Additionally, we find it useful to use a flat-in-$\chi{\rm eff}$ prior, which does not penalize the large $|\chi_{\rm eff}|$ region, in order to uncover the higher likelihood region for GW151226. Our solution has several interesting properties: (a) the secondary black hole mass is close to the upper limit of the hypothesized lower mass gap of astrophysical black hole population; and (b) orbital precession is driven by the primary black hole spin, which has a dimensionless magnitude as large as $\sim 0.85$ and is tilted away from the orbital angular momentum at an angle of $\sim 57^\circ$. Since GW151226 is a relatively weak signal, an unambiguous claim of the detection of these effects in the signal cannot be made.

Blazar nature of high-z radio-loud quasars

We report on the Swift/XRT observation and classification of eleven blazar candidates at z > 4. These sources were selected as part of a sample of extremely radio-loud quasars, with a focus on quasars with jets oriented roughly close to our line of sight. Deriving their viewing angles and their jets’ bulk Lorentz factors was crucial for a strict blazar classification, which was made possible only thanks to X-ray observations. Out of eleven sources, five show strong and hard X-ray fluxes that set the foundation for their blazar classification, while two are uncertain and three host relativistic jets that we observe just outside their beaming cone (i.e. are not strictly blazars), while one went undetected by Swift/XRT. Following this approach, we were able to trace the > ​109 M⊙ active super-massive black hole (SMBH) population hosted in jetted active galactic nuclei (AGN). At z ≥ 4, the massive jetted sources are likely predominant in the overall quasar population: this calls for a deep review of our understanding of the first SMBH formations and evolution. Jets are indeed key actors in fast accretion and must be searched for across the whole high-redshift quasar population. A note of caution must be added: radio-loudness and, in general, radio features at high redshifts seem do not seem to perfectly reflect high-energy properties. A strong effect attributed to the interaction with cosmic microwave background (CMB) radiation is surely at work, which quenches the radio emission with respect to the X-rays; however, in addition, more frequent occasions for the jet to be bent seem to play a relevant role in this regard. Thus, classifications and population studies must be carefully performed, so as to avoid interference resulting from these inconsistencies.

Suspicious Siblings: The Distribution of Mass and Spin across Component Black Holes in Isolated Binary Evolution

The LIGO and Virgo gravitational-wave detectors have uncovered binary black hole systems with definitively nonzero spins, as well as systems with significant spin residing in the more massive black hole of the pair. We investigate the ability of isolated binary evolution in forming such highly spinning, asymmetric-mass systems through both accretion onto the first-born black hole and tidal spin-up of the second-born black hole using a rapid population synthesis approach with detailed considerations of spin-up through tidal interactions. Even with the most optimistic assumptions regarding the efficiency at which an accreting star receives material from a donor, we find that it is difficult to form systems with significant mass asymmetry and moderate or high spins in the primary black hole component. Assuming efficient angular momentum transport within massive stars and Eddington-limited accretion onto black holes, we find that >1.5% of systems in the underlying binary black hole population have a primary black hole spin greater than 0.2 and a mass asymmetry of greater than 2:1 in our most optimistic models, with most models finding that this criteria is only met in ∼0.01% of systems. The production of systems with significant mass asymmetries and spin in the primary black hole component is thus an unlikely byproduct of isolated evolution unless highly super-Eddington accretion is invoked or angular momentum transport in massive stars is less efficient than typically assumed.

Are Binary Black Hole Mergers and Long Gamma-Ray Bursts Drawn from the Same Black Hole Population?

This paper compares the population of binary black hole (BBH) mergers detected by LIGO/Virgo with selected long gamma-ray burst (GRB) world models convolved with a delay function (LGRBs are used as a tracer of stellar-mass BH formation). The comparison involves the redshift distribution and the fraction of LGRBs required to produce the local rate of BBH mergers. We find that BBH mergers and LGRBs cannot have the same formation history, unless BBH mergers have a long coalescence time of several Gyr. This would imply that BHs born during the peak of long GRB formation at redshift z ≈ 2−3 merge within the horizon of current GW interferometers. We also show that LGRBs are more numerous than BBH mergers, meaning that most of them do not end their lives in BBH mergers. We interpret these results as an indication that BBH mergers and LGRBs constitute two distinct populations of stellar-mass BHs, with LGRBs being more frequent than BBH mergers. We speculate that the descendants of LGRBs may resemble galactic high-mass X-ray binaries more than BBH mergers. Finally, we discuss the possible existence of a subpopulation of fast-spinning LGRB descendants among BBH mergers, showing that this population, if it exists, is expected to become dominant beyond redshift z ≈ 1, leading to a change in the observed properties of BBH mergers.

The black hole population in low-mass galaxies in large-scale cosmological simulations

ABSTRACT 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.

Gravitational background from dynamical binaries and detectability with 2G detectors

We study the impact of young clusters on the gravitational wave background from compact binary coalescence. We simulate a catalog of sources from population I/II isolated binary stars and stars born in young clusters, corresponding to one year of observations with second-generation (2G) detectors. Taking into account uncertainties on the fraction of dynamical binaries and star formation parameters, we find that the background is dominated by the population of binary black holes, and we obtain a value of $\Omega_{gw}(25 \rm{Hz}) = 1.2^{+1.38}_{-0.65} \times 10^{-9}$ for the energy density, in agreement with the actual upper limits derived from the latest observation run of LIGO--Virgo. We demonstrate that a large number of sources in a specific corrected mass range yields to a bump in the background. This background could be detected with 8 years of coincident data by a network of 2G detectors.

Population inference of spin-induced quadrupole moments as a probe for nonblack hole compact binaries

Gravitational wave (GW) measurements of physical effects such as spin-induced quadrupole moments can distinguish binaries consisting of black holes from nonblack hole binaries. While these effects may be poorly constrained for single-event inferences with the second-generation detectors, combining information from multiple detections can help uncover features of nonblack hole binaries. The spin-induced quadrupole moment has specific predictions for different types of compact objects, and a generalized formalism must consider a population where different types of compact objects co-exist. In this study, we introduce a hierarchical mixture-likelihood formalism to estimate the fraction of nonbinary black holes in the population. We demonstrate the applicability of this method using simulated GW signals injected into Gaussian noise following the design sensitivities of the Advanced LIGO Advanced Virgo detectors. We compare the performance of this method with a traditionally-followed hierarchical inference approach. Both the methods are equally effective to hint at inhomogeneous populations, however, we find the mixture-likelihood approach to be more natural for mixture populations comprising compact objects of diverse classes. We also discuss the possible systematics in the mixture-likelihood approach, caused by several reasons, including the limited sensitivity of the second-generation detectors, specific features of the astrophysical population distributions, and the limitations posed by the waveform models employed. Finally, we apply this method to the LIGO-Virgo detections published in the second GW transient catalog (GWTC-2) and find them consistent with a binary black hole population with ${f}_{\mathrm{nbh}}$ estimated to be $\ensuremath{\le}0.29$ at 90% credibility.

The Imprint of Superradiance on Hierarchical Black Hole Mergers

Ultralight bosons are a proposed solution to outstanding problems in cosmology and particle physics: they provide a dark-matter candidate while potentially explaining the strong charge-parity problem. If they exist, ultralight bosons can interact with black holes through the superradiant instability. In this work we explore the consequences of this instability on the evolution of hierarchical black holes within dense stellar clusters. By reducing the spin of individual black holes, superradiance reduces the recoil velocity of merging binary black holes, which, in turn, increases the retention fraction of hierarchical merger remnants. We show that the existence of ultralight bosons with mass 2 × 10−14 ≲ μ/eV ≲ 2 × 10−13 would lead to an increased rate of hierarchical black hole mergers in nuclear star clusters. An ultralight boson in this energy range would result in up to ≈60% more present-day nuclear star clusters supporting hierarchical growth. The presence of an ultralight boson can also double the rate of intermediate-mass black hole mergers to ≈0.08 Gpc−3 yr−1 in the local universe. These results imply that a select range of ultralight boson masses can have far-reaching consequences for the population of black holes in dense stellar environments. Future studies into black hole cluster populations and the spin distribution of hierarchically formed black holes will test this scenario.

Detectability of a spatial correlation between stellar mass black hole mergers and active galactic nuclei in the local Universe

ABSTRACT The origin of the binary black hole (BBH) mergers detected through gravitational waves (GWs) by the LIGO-Virgo-KAGRA (LVK) collaboration remains debated. One fundamental reason is our ignorance of their host environment, as the typical size of an event’s localization volume can easily contain thousands of galaxies. A strategy around this is to exploit statistical approaches to assess the spatial correlation between these mergers and astrophysically motivated host galaxy types, such as active galactic nuclei (AGNs). We use a likelihood ratio method to infer the degree of GW–AGN connection out to z = 0.2. We simulate BBH mergers whose components’ masses are sampled from a realistic distribution of the underlying population of black holes (BHs). Localization volumes for these events are calculated assuming two different interferometric network configurations. These correspond to the configuration of the third (O3) and of the upcoming fourth (O4) LVK observing runs. We conclude that the 13 BBH mergers detected during the third observing run at z ≤ 0.2 are not enough to reject with a 3σ significance the hypothesis according to which there is no connection between GW and AGNs more luminous than $\approx 10^{44.3}\rm {erg}\ \rm {s}^{-1}$, that have number density higher than 10−4.75 Mpc−3. However, 13 detections are enough to reject this no-connection hypothesis when rarer categories of AGNs are considered, with bolometric luminosities greater than $\approx 10^{45.5}\rm {erg}\ \rm {s}^{-1}$. We estimate that O4 results will potentially allow us to test fractional contributions to the total BBH merger population from AGNs of any luminosity higher than $80{{\ \rm per\ cent}}$.

Modelling the formation of the first two neutron star–black hole mergers, GW200105 and GW200115: metallicity, chirp masses, and merger remnant spins

ABSTRACT The two neutron star–black hole mergers (GW200105 and GW200115) observed in gravitational waves by advanced LIGO and Virgo, mark the first ever discovery of such binaries in nature. We study these two neutron star–black hole systems through isolated binary evolution, using a grid of population synthesis models. Using both mass and spin observations (chirp mass, effective spin, and remnant spin) of the binaries, we probe their different possible formation channels in different metallicity environments. Our models only support LIGO data when assuming the black hole is non-spinning. Our results show a strong preference that GW200105 and GW200115 formed from stars with sub-solar metallicities Z ≲ 0.005. Only two metal-rich (Z = 0.02) models are in agreement with GW200115. We also find that chirp mass and remnant spins jointly aid in constraining the models, while the effective spin parameter does not add any further information. We also present the observable (i.e. post-selection effects) median values of spin and mass distribution from all our models, which may be used as a reference for future mergers. Further, we show that the remnant spin parameter distribution exhibits distinguishable features in different neutron star–black hole sub-populations. We find that non-spinning, first born black holes dominate significantly the merging neutron star–black hole population, ensuring electromagnetic counterparts to such mergers a rare affair.

Dynamical double black holes and their host cluster properties

ABSTRACT We investigate the relationship between the global properties of star clusters and their double black hole (DBH) populations. We use the code NBODY6 to evolve a suite of star cluster models with an initial mass of $\mathcal {O}(10^4)$M⊙ and varying initial parameters. We conclude that cluster metallicity plays the most significant role in determining the lifespan of a cluster, while the initial half-mass radius is dominant in setting the rate of BH exchange interactions in the central cluster regions. We find that the mass of interacting BHs, rather than how frequently their interactions with other BHs occur, is more crucial in the thermal expansion and eventual evaporation of the cluster. We formulate a novel approach to easily quantify the degree of BH-BH dynamical activity in each model. We report 12 in-cluster and three out-of-cluster (after ejection from the cluster) DBH mergers, of different types (inspiral, eccentric, and hierarchical) across the 10 N-body models presented. Our DBH merger efficiency is 3–4 × 10−5 mergers per M⊙. We note the cluster initial density plays the most crucial role in determining the number of DBH mergers, with the potential presence of a transitional density point (between 1.2 and 3.8 × 103 M⊙ pc−3) below which the number of in-cluster mergers increases with cluster density and above which the increased stellar density acts to prevent in-cluster BH mergers. The importance of the history of dynamical interactions within the cluster in setting up the pathways to ejected DBH mergers is also discussed.

Searching for a subpopulation of primordial black holes in LIGO-Virgo gravitational-wave data

With several dozen binary black hole events detected by LIGO/Virgo to date and many more expected in the next few years, gravitational-wave astronomy is shifting from individual-event analyses to population studies. Using the GWTC-2 catalog, we perform a hierarchical Bayesian analysis that for the first time combines several state-of-the-art astrophysical formation models with a population of primordial black holes (PBHs) and constrains the fraction of a putative subpopulation of PBHs in the data. We find that this fraction depends significantly on the set of assumed astrophysical models. While a primordial population is statistically favored against certain competitive astrophysical channels, such as globular clusters and nuclear stellar clusters, a dominant contribution from the stable-mass-transfer isolated formation channel drastically reduces the need for PBHs, except for explaining the rate of mass-gap events like GW190521. The tantalizing possibility that black holes formed after inflation are contributing to LIGO/Virgo observations could only be verified by further reducing uncertainties in astrophysical and primordial formation models, and it may ultimately be confirmed by third-generation interferometers.

A study of 1000 galaxies with unusually young and massive stars in the SDSS: a search for hidden black holes

ABSTRACT We select 1076 galaxies with extinction-corrected H α equivalent widths too large to be explained with a Kroupa initial mass function, and compare these with a control sample of galaxies that is matched in stellar mass, redshift, and 4000 Å break strength, but with normal H α equivalent widths. Our goal is to study how processes such as black hole growth and energetic feedback processes from massive stars differ between galaxies with extreme central H α emission and galaxies with normal young central stellar populations. The stellar mass distribution of H α excess galaxies is peaked at $3 \times 10^{10}\, {\rm M}_{\odot }$ and almost all fall well within the star-forming locus in the [O iii]/H β versus [N ii]/H α Baldwin, Philipps & Terlevich diagram. H α excess galaxies are twice as likely to exhibit H α line asymmetries and 1.55 times more likely to be detected at 1 GHz in the VLA FIRST survey compared to control sample galaxies. The radio luminosity per unit stellar mass decreases with the stellar age of the system. Using stacked spectra, we demonstrate that [Ne v] emission is not present in the very youngest of the radio-quiet H α excess galaxies with detectable Wolf–Rayet features, suggesting that black hole growth has not yet commenced in such systems. [Ne v] emission is detected in H α excess galaxies with radio detections and the strength of the line correlates with the radio luminosity. This is the clearest indication for a population of black holes that may be forming in a subset of the H α excess population.

Exploring Features in the Binary Black Hole Population

Abstract Vamana is a mixture model framework that infers the astrophysical distribution of chirp mass, mass ratio, and spin component aligned with the orbital angular momentum for the binary black holes (BBH) population. We extend the mixing components in this framework to also model the redshift evolution of merger rate and report all the major one- and two-dimensional features in the BBH population using the 69 gravitational-wave signals detected with a false alarm rate <1 yr−1 in the third Gravitational-Wave Transient Catalog (GWTC-3). Endorsing our previous report and a recent corroborating report from LIGO Scientific, Virgo, and KAGRA Collaborations, we observe the chirp mass distribution has multiple peaks and a lack of mergers with chirp masses 10–12 M ⊙. In addition, we observe that aligned spins show mass dependence with heavier binaries exhibiting larger spins, the mass ratio shows a dependence on the chirp mass but not on the aligned spin, and the redshift evolution of the merger rate for the peaks in the mass distribution is disparate. These features possibly reflect the astrophysics associated with the BBH formation channels. However, additional observations are needed to improve our limited confidence in them.

Abundance of LIGO/Virgo Black Holes from Microlensing Observations of Quasars with Reverberation Mapping Size Estimates

Assuming a population of black holes (BHs) with masses in the range inferred by LIGO/Virgo from BH mergers, we use quasar microlensing observations to estimate their abundances. We consider a mixed population of stars and BHs and the presence of a smooth dark matter component. We adopt reverberation mapping estimates of the quasar size. According to a Bayesian analysis of the measured microlensing magnifications, a population of BHs with masses ∼30M ⊙ constitutes less than 0.4% of the total matter at the 68% confidence level (less than 0.9% at the 90% confidence level). We have explored the whole mass range of LIGO/Virgo BHs, finding that this upper limit ranges from 0.5% to 0.4% at the 68% confidence level (from 1.1% to 0.9% at the 90% confidence level) when the BH masses change from 10 to 60M ⊙. We estimate a 16% contribution from the stars, in agreement with previous studies based on a single-mass population that do not explicitly consider the presence of BHs. These results are consistent with the estimates of BH abundances from the statistics of LIGO/Virgo mergers, and rule out primordial BHs (or any other types of compact object) in this mass range constituting a significant fraction of the dark matter.

Effects of an Immortal Stellar Population in AGN Disks

Stars are likely embedded in the gas disks of active galactic nuclei (AGN). Theoretical models predict that in the inner regions of the disk, these stars accrete rapidly, with fresh gas replenishing hydrogen in their cores faster than it is burned into helium, effectively stalling their evolution at hydrogen burning. We produce order-of-magnitude estimates of the number of such stars in a fiducial AGN disk. We find numbers of order 102–4, confined to the inner r cap ∼ 3000r s ∼ 0.03 pc. These stars can profoundly alter the chemistry of AGN disks, enriching them in helium and depleting them in hydrogen, both by order-unity amounts. We further consider mergers between these stars and other disk objects, suggesting that star–star mergers result in rapid mass loss from the remnant to restore an equilibrium mass, while star–compact object mergers may result in exotic outcomes and even host binary black hole mergers within themselves. Finally, we examine how these stars react as the disk dissipates toward the end of its life, and find that they may return mass to the disk fast enough to extend its lifetime by a factor of several and/or may drive powerful outflows from the disk. Post-AGN, these stars rapidly lose mass and form a population of stellar mass black holes around 10M ⊙. Due to the complex and uncertain interactions between embedded stars and the disk, their plausible ubiquity, and their order-unity impact on disk structure and evolution, they must be included in realistic disk models.

Signatures of the Many Supermassive Black Hole Mergers in a Cosmologically Forming Massive Early-type Galaxy

We model here the merger histories of the supermassive black hole (SMBH) population in the late stages of a cosmological simulation of a ∼ 2 × 1013 M ⊙ galaxy group. The gravitational dynamics around the several tens of SMBHs (M • > 7.5 × 107 M ⊙) hosted by the galaxies in the group is computed at high accuracy using regularized integration with the KETJU code. The 11 SMBHs that form binaries and a hierarchical triplet eventually merge after hardening through dynamical friction, stellar scattering, and gravitational wave (GW) emission. The binaries form at eccentricities of e ∼ 0.3–0.9, with one system evolving to a very high eccentricity of e = 0.998, and merge on timescales of a few tens to several hundred megayears. During the simulation, the merger-induced GW recoil kicks eject one SMBH remnant from the central host galaxy. This temporarily drives the galaxy off the M •–σ ⋆ relation; however, the galaxy returns to the relation due to subsequent galaxy mergers, which bring in new SMBHs. This showcases a possible mechanism contributing to the observed scatter of the M •–σ ⋆ relation. Finally, we show that pulsar timing arrays and LISA would be able to detect parts of the GW signals from the SMBH mergers that occur during the ∼4 Gyr time span simulated with KETJU.

3D hydrodynamical simulations of the impact of mechanical feedback on accretion in supersonic stellar-mass black holes

Context. Isolated stellar-mass black holes accrete gas from their surroundings, often at supersonic speeds, and can form outflows that may influence the accreted gas. The latter process, known as mechanical feedback, can significantly affect the accretion rate. Aims. We use hydrodynamical simulations to assess the impact of mechanical feedback on the accretion rate when the black hole moves supersonically through a uniform medium. Methods. We carried out three-dimensional (3D) hydrodynamical simulations of outflows fueled by accretion that interact with a uniform medium, probing scales equivalent to and larger than the accretor gravitational sphere of influence. In the simulations, the accretor is at rest and the medium moves at supersonic speeds. The outflow power is assumed to be proportional to the accretion rate. The simulations were run for different outflow-medium motion angles and velocity ratios. We also investigated the impact of different degrees of outflow collimation, accretor size, and resolution. Results. In general, the accretion rate is significantly affected by mechanical feedback. There is a minor reduction in accretion for outflows perpendicular to the medium motion, but the reduction quickly becomes more significant for smaller angles. Moreover, the decrease in accretion becomes greater for smaller medium-to-outflow velocity ratios. On the other hand, the impact of outflow collimation seems moderate. Mechanical feedback is enhanced when the accretor size is reduced. For a population of black holes with random outflow orientations, the average accretion rate drops by (low–high resolution) ∼0.2 − 0.4 and ∼0.1 − 0.2 for medium-to-outflow velocity ratios of 1/20 and 1/100, respectively, when compared to the corresponding cases without outflow. Conclusions. Our results strongly indicate that on the considered scales, mechanical feedback can easily reduce the energy available from supersonic accretion by at least a factor of a few. This aspect should be taken into account when studying the mechanical, thermal, and non-thermal output of isolated black holes.

Cosmology and modified gravitational wave propagation from binary black hole population models

A joint hierarchical Bayesian analysis of the binary black hole (BBH) mass function, merger rate evolution and cosmological parameters can be used to extract information on both the cosmological and population parameters. We extend this technique to include the effect of modified gravitational wave (GW) propagation. We discuss the constraints on the parameter $\Xi_0$ that describes this phenomenon (with $\Xi_0=1$ in General Relativity, GR) using the data from the GWTC-3 catalog. We find the constraints $\Xi_0 = 1.2^{+0.7}_{-0.7}$ with a flat prior on $\Xi_0$, and $\Xi_0 = 1.0^{+0.4}_{-0.8}$ with a prior uniform in $\log\Xi_0$ ($68\%$ C.L., maximum posterior and HDI), which only rely on the presence of a feature in the BBH mass distribution around $\sim 30-45 M_{\odot}$, and are robust to whether or not the event GW190521 is considered an outlier of the population. We then study in more detail the effects of modified GW propagation on population and cosmological analyses for LIGO/Virgo at design sensitivity. For a given data-taking period, the relative error $\Delta\Xi_0/\Xi_0$ has a significant dependence on the fiducial value of $\Xi_0$, since the latter has a strong influence on the detection rate. For five years of data, the accuracy ranges from $\sim 10\%$ on $\Xi_0$ when $\Xi_0=1$ to $\Delta\Xi_0/\Xi_0\sim 20\%$ for $\Xi_0=1.8$ - a large deviation from GR, still consistent with current limits and predicted by viable cosmological models. For the Hubble parameter, we forecast an accuracy of $\Delta H_0/H_0 \sim 20\%$, and an accuracy on $H(z)$ of $\sim7\%$ at a pivot redshift $z_*\sim 0.8$. We finally show that, if Nature is described by a modified gravity theory with a large deviation from the GR value $\Xi_0=1$, such as $\Xi_0=1.8$, analysing the data assuming GR produces a significant bias in the inferred values of the mass scales, Hubble constant, and BBH merger rate.