arXiv subjects

Andrea Lapi

Publications and source records attributed to Andrea Lapi.

At least 91 records · Page 5Linked to original sources

arXiv2022

Searching for anisotropic stochastic gravitational-wave backgrounds with constellations of space-based interferometers

Many recent works have shown that the angular resolution of ground-based detectors is too poor to characterize the anisotropies of the stochastic gravitational-wave background (SGWB). For this reason, we asked ourselves if a constellation of space-based instruments could be more suitable. We consider the Laser Interferometer Space Antenna (LISA), a constellation of multiple LISA-like clusters, and the Deci-hertz Interferometer Gravitational-wave Observatory (DECIGO). Specifically, we test whether these detector constellations can probe the anisotropies of the SGWB. For this scope, we considered the SGWB produced by two astrophysical sources: merging compact binaries and a recently proposed scenario for massive black-hole seed formation through multiple mergers of stellar remnants. We find that measuring the angular power spectrum of the SGWB anisotropies is almost unattainable. However, it turns out that it could be possible to probe the SGWB anisotropies through cross-correlation with the CMB fluctuations. In particular, we find that a constellation of two LISA-like detectors and CMB-S4 can marginally constrain the cross-correlation between the CMB lensing convergence and the SGWB produced by the black hole seed formation process. Moreover, we find that DECIGO can probe the cross-correlation between the CMB lensing and the SGWB from merging compact binaries.

gr-qc↗

arXiv2022

The demographics of obscured AGN from X-ray spectroscopy guided by multiwavelength information

A complete census of Active Galactic Nuclei (AGN) is a prerequisite for understanding the growth of supermassive black holes across cosmic time. A significant challenge toward this goal is the whereabouts of heavily obscured AGN that remain uncertain. This paper sets new constraints on the demographics of this population by developing a methodology that combines X-ray spectral information with priors derived from multiwavelength observations. We select X-ray AGN in the Chandra COSMOS Legacy survey and fit their $2.2-500μm$ spectral energy distributions with galaxy and AGN templates to determine the mid-infrared ($6μm$) luminosity of the AGN component. Empirical correlations between X-ray and $6μm$ luminosities are then adopted to infer the intrinsic accretion luminosity at X-rays for individual AGN. This is used as prior information in our Bayesian X-ray spectral analysis to estimate physical properties, such as line-of-sight obscuration. Our approach breaks the degeneracies between accretion luminosity and obscuration that affect X-ray spectral analysis, particularly for the most heavily obscured (Compton-Thick) AGN with low photon counts X-ray spectra. The X-ray spectral results are then combined with the selection function of the Chandra COSMOS Legacy survey to derive the AGN space density and a Compton-Thick fraction of $21.0^{+16.1}_{-9.9}\%$ at redshifts $z<0.5$. At higher redshift, our analysis suggests upper limits to the Compton-Thick AGN fraction of $\le 40\%$. These estimates are at the low end of the range of values determined in the literature and underline the importance of multiwavelength approaches for tackling the challenge of heavily obscured AGN demographics.

astro-ph.GA↗

arXiv2022

Astroparticle Constraints from the Cosmic Star Formation Rate Density at High Redshift: Current Status and Forecasts for JWST

We exploit the recent determination of cosmic star formation rate (SFR) density at redshifts $z\gtrsim 4$ to derive astroparticle constraints on three common dark matter scenarios alternative to standard cold dark matter (CDM): warm dark matter (WDM), fuzzy dark matter ($ψ$DM) and self-interacting dark matter (SIDM). Our analysis relies on the UV luminosity functions measured by the Hubble Space Telescope out to $z\lesssim 10$ and down to UV magnitudes $M_{\rm UV}\lesssim -17$. We extrapolate these to fainter yet unexplored magnitude ranges, and perform abundance matching with the halo mass functions in a given DM scenario, so obtaining a relationship between the UV magnitude and the halo mass. We then compute the cosmic SFR density by integrating the extrapolated UV luminosity functions down to a faint magnitude limit $M_{\rm UV}^{\rm lim}$, which is determined via the above abundance matching relationship by two free parameters: the minimum threshold halo mass $M_{\rm H}^{\rm GF}$ for galaxy formation, and the astroparticle quantity $X$ characterizing each DM scenario (namely, particle mass for WDM and $ψ$DM, and kinetic temperature at decoupling $T_X$ for SIDM). We perform Bayesian inference on such parameters via a MCMC technique by comparing the cosmic SFR density from our approach to the current observational estimates at $z\gtrsim 4$, constraining the WDM particle mass to $m_X\approx 1.2^{+0.3\,(11.3)}_{-0.4\,(-0.5)}$ keV, the $ψ$DM particle mass to $m_X\approx 3.7^{+1.8\,(+12.9.3)}_{-0.4\,(-0.5)}\times 10^{-22}$ eV, and the SIDM temperature to $T_X\approx 0.21^{+0.04\,(+1.8)}_{-0.06\,(-0.07)}$ keV at $68\%$ ($95\%$) confidence level. We then forecast how such constraints will be strengthened by upcoming refined estimates of the cosmic SFR density, if the early data on the UV luminosity function at $z\gtrsim 10$ from JWST will be confirmed down to ultra-faint magnitudes.

astro-ph.CO↗

arXiv2022

Cosmic evolution of the incidence of Active Galactic Nuclei in massive clusters: Simulations versus observations

This paper explores the role of small-scale environment ($<1$ Mpc) in modulating accretion events onto supermassive black holes by studying the incidence of Active Galactic Nuclei (AGN) in massive clusters of galaxies. A flexible, data-driven semi-empirical model is developed based on a minimal set of parameters and under the zero order assumption that the incidence of AGN in galaxies is independent of environment. This is used to predict how the fraction of X-ray selected AGN among galaxies in massive dark matter halos ($\gtrsim 3\times 10^{14}\,M_{\odot}$) evolves with redshift and reveal tensions with observations. At high redshift, $z\sim1.2$, the model underpredicts AGN fractions, particularly at high X-ray luminosities, $L_X(\rm 2-10\,keV) \gtrsim 10^{44}\, erg \, s^{-1}$. At low redshift, $z\sim0.2$, the model estimates fractions of moderate luminosity AGN ($L_X(\rm 2-10\,keV) \gtrsim 10^{43}\, erg \, s^{-1}$) that are a factor of $2-3$ higher than the observations. These findings reject the zero order assumption on which the semi-empirical model hinges and point to a strong and redshift-dependent influence of the small-scale environment on the growth of black holes. Cluster of galaxies appear to promote AGN activity relative to the model expectation at $z\sim1.2$ and suppress it close to the present day. These trends could be explained by the increasing gas content of galaxies toward higher redshift combined with an efficient triggering of AGN at earlier times in galaxies that fall onto clusters.

astro-ph.GA↗

arXiv2022

A panchromatic view of infrared quasars: excess star formation and radio emission in the most heavily obscured systems

To understand the Active Galactic Nuclei (AGN) phenomenon and their impact on the evolution of galaxies, a complete AGN census is required; however, finding heavily obscured AGNs is observationally challenging. Here we use the deep and extensive multi-wavelength data in the COSMOS field to select a complete sample of 578 infrared (IR) quasars ($L_{\rm AGN,IR}>10^{45}\rm \: erg\: s^{-1}$) at $z<3$, with minimal obscuration bias, using detailed UV-to-far IR spectral energy distribution (SED) fitting. We complement our SED constraints with X-ray and radio observations to further investigate the properties of the sample. Overall, 322 of the IR quasars are detected by Chandra and have individual X-ray spectral constraints. From a combination of X-ray stacking and $L_{\rm 2-10\rm keV}$ - $L_{\rm 6\: μm}$ analyses, we show that the majority of the X-ray faint and undetected quasars are heavily obscured (many are likely Compton thick), highlighting the effectiveness of the mid-IR band to find obscured AGNs. We find that 355 ($\approx$61%) IR quasars are obscured ($N_{\rm H}>10^{22}\rm \: cm^{-2}$) and identify differences in the average properties between the obscured and unobscured quasars: (1) obscured quasars have star-formation rates $\approx 3$ times higher than unobscured systems for no significant difference in stellar mass and (2) obscured quasars have stronger radio emission than unobscured systems, with a radio-loudness parameter $\approx 0.2 \rm \: dex$ higher. These results are inconsistent with a simple orientation model but in general agreement with either extreme host-galaxy obscuration towards the obscured quasars or a scenario where obscured quasars are an early phase in the evolution of quasars.

astro-ph.GA↗

arXiv2022

Testing the key role of the stellar mass-halo mass relation in galaxy merger rates and morphologies via DECODE, a novel Discrete statistical sEmi-empiriCal mODEl

The relative roles of mergers and star formation in regulating galaxy growth are still a matter of intense debate. We here present our DECODE, a new Discrete statistical sEmi-empiriCal mODEl specifically designed to predict rapidly and efficiently, in a full cosmological context, galaxy assembly and merger histories for any given input stellar mass-halo mass (SMHM) relation. DECODE generates object-by-object dark matter merger trees (hence discrete) from accurate subhalo mass and infall redshift probability functions (hence statistical) for all subhaloes, including those residing within other subhaloes, with virtually no resolution limits on mass or volume. Merger trees are then converted into galaxy assembly histories via an input, redshift dependent SMHM relation, which is highly sensitive to the significant systematics in the galaxy stellar mass function and on its evolution with cosmic time. DECODE can accurately reproduce the predicted mean galaxy merger rates and assembly histories of hydrodynamic simulations and semi-analytic models, when adopting in input their SMHM relations. In the present work we use DECODE to prove that only SMHM relations implied by stellar mass functions characterized by large abundances of massive galaxies and significant redshift evolution, at least at $M_\star \gtrsim 10^{11} \, M_\odot$, can simultaneously reproduce the local abundances of satellite galaxies, the galaxy (major merger) pairs since $z \sim 3$, and the growth of Brightest Cluster Galaxies. The same models can also reproduce the local fraction of elliptical galaxies, on the assumption that these are strictly formed by major mergers, but not the full bulge-to-disc ratio distributions, which require additional processes.

astro-ph.CO↗

arXiv2022

Dust evolution with MUPPI in cosmological volumes

We study the evolution of dust in a cosmological volume using a hydrodynamical simulation in which the dust production is coupled with the MUPPI (MUlti Phase Particle Integrator) sub-resolution model of star formation and feedback. As for the latter, we keep as reference the model setup calibrated previously to match the general properties of Milky Way like galaxies in zoom-in simulations. However, we suggest that an increase of the star formation efficiency with the local dust to gas ratio would better reproduce the observed evolution of the cosmic star formation density. Moreover, the paucity of quenched galaxies at low redshift demands a stronger role of AGN feedback. We tune the parameters ruling direct dust production from evolved stars and accretion in the inter stellar medium to get scaling relations involving dust, stellar mass and metallicity in good agreement with observations. In low mass galaxies the accretion process is inefficient. As a consequence, they remain poorer in silicate and small grains than higher mass ones. We reproduce reasonably well the few available data on the radial distribution of dust outside the galactic region, supporting the assumption that the dust and gas dynamics are well coupled at galactic scales.

astro-ph.GA↗

arXiv2022

Empirical Evidence of Non-Minimally Coupled Dark Matter in the Dynamics of Local Spiral Galaxies?

We look for empirical evidence of a non-minimal coupling (NMC) between dark matter (DM) and gravity in the dynamics of local spiral galaxies. In particular, we consider a theoretically motivated NMC that may arise dynamically from the collective behavior of the coarse-grained DM field (e.g., via Bose-Einstein condensation) with averaging/coherence length $L$. In the Newtonian limit, this NMC amounts to modify the Poisson equation by a term $L^{2} \nabla^{2} ρ$ proportional to the Laplacian of the DM density itself. We show that such a term, when acting as a perturbation over the standard Navarro-Frenk-White (NFW) profile of cold DM particles, can substantially alter the dynamical properties of galaxies, in terms of their total radial acceleration within the disk and rotation velocity. Specifically, we find that this NMC model can properly fit the stacked rotation curves of local spiral galaxies with different velocities at the optical radius, including dwarfs and low-surface brightness systems, at a level of precision comparable to, and in some instances even better than, the phenomenological Burkert profile. Finally we show that, extrapolating down to smaller masses the scaling of $L$ vs. halo mass found from the above rotation curve analysis, the NMC model can adequately reproduce the radial acceleration relation (or RAR) in shape and normalization down to the dwarf spheroidal galaxy range, a task which constitutes a serious challenge for alternative DM models even inclusive of baryonic effects.

astro-ph.CO↗

arXiv2022

Detectability of the cross-correlation between CMB lensing and stochastic GW background from compact object mergers

The anisotropies of the Stochastic Gravitational-Wave Background (SGWB) produced by merging compact binaries constitute a possible new probe of the Large-Scale Structure (LSS). However, the significant shot noise contribution caused by the discreteness of the GW sources and the poor angular resolution of the instruments hamper the detection of the intrinsic anisotropies induced by the LSS. In this work, we investigate the potential of cross-correlating forthcoming high precision measurements of the SGWB energy density and the Cosmic Microwave Background (CMB) lensing convergence to mitigate the effect of shot noise. Combining a detailed model of stellar and galactic astrophysics with a novel framework to distribute the GW emitters in the sky, we compute the auto- and cross-correlation power spectra for the two cosmic fields, evaluate the shot noise contribution and predict the signal-to-noise ratio. The results of our analysis show that the SGWB energy density correlates significantly with the CMB lensing convergence and that the cross-correlation between these two cosmic fields reduces the impact of instrumental and shot noise. Unfortunately, the S/N is not high enough to detect the intrinsic SGWB anisotropies. Nevertheless, a network composed of both present and future generation GW interferometers, operating for at least 10 yrs, should be able to measure the shot noise contribution.

astro-ph.CO↗

arXiv2022

An Eddington ratio-driven origin for the ${\rm L}_{\rm X}-{\rm M}_{*}$ relation in quiescent and star forming active galaxies

A mild correlation exists in active galaxies between the mean black hole accretion, as traced by the mean X-ray luminosity $\left<{\rm L}_{\rm X}\right>$, and the host galaxy stellar mass M$_*$, characterised by a normalisation steadily decreasing with cosmic time and lower in more quiescent galaxies. We create comprehensive semi-empirical mock catalogues of active black holes to pin down which parameters control the shape and evolution of the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation of X-ray detected active galaxies. We find that the normalisation of the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation is largely independent of the fraction of active galaxies (the duty cycle), but strongly dependent on the mean Eddington ratio, when adopting a constant underlying M$_{\rm BH}-{\rm M}_*$ relation as suggested by observational studies. The data point to a decreasing mean Eddington ratio with cosmic time and with galaxy stellar mass at fixed redshift. Our data can be reproduced by black holes and galaxies evolving on similar M$_{\rm BH}-{\rm M}_*$ relations but progressively decreasing their average Eddington ratios, mean X-ray luminosities, and specific star formation rates, when moving from the starburst to the quiescent phase. Models consistent with the observed $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation and independent measurements of the mean Eddington ratios, are characterised by M$_{\rm BH}-{\rm M}_*$ relations lower than those derived from dynamically measured local black holes. Our results point to the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation as a powerful diagnostic to: 1) probe black hole-galaxy scaling relations and the level of accretion onto black holes; 2) efficiently break the degeneracies between duty cycles and accretion rates in cosmological models of black holes.

astro-ph.GA↗

arXiv2022

Gravitational waves $\times$ HI intensity mapping: cosmological and astrophysical applications

Two of the most rapidly growing observables in cosmology and astrophysics are gravitational waves (GW) and the neutral hydrogen (HI) distribution. In this work, we investigate the cross-correlation between resolved gravitational wave detections and HI signal from intensity mapping (IM) experiments. By using a tomographic approach with angular power spectra, including all projection effects, we explore possible applications of the combination of the Einstein Telescope and the SKAO intensity mapping surveys. We focus on three main topics: \textit{(i)} statistical inference of the observed redshift distribution of GWs; \textit{(ii)} constraints on dynamical dark energy models as an example of cosmological studies; \textit{(iii)} determination of the nature of the progenitors of merging binary black holes, distinguishing between primordial and astrophysical origin. Our results show that: \textit{(i)} the GW redshift distribution can be calibrated with good accuracy at low redshifts, without any assumptions on cosmology or astrophysics, potentially providing a way to probe astrophysical and cosmological models; \textit{(ii)} the constrains on the dynamical dark energy parameters are competitive with IM-only experiments, in a complementary way and potentially with less systematics; \textit{(iii)} it will be possible to detect a relatively small abundance of primordial black holes within the gravitational waves from resolved mergers. Our results extend towards $\mathrm{GW \times IM}$ the promising field of multi-tracing cosmology and astrophysics, which has the major advantage of allowing scientific investigations in ways that would not be possible by looking at single observables separately.

astro-ph.CO↗

arXiv2021

The weak dependence of velocity dispersion on disk fractions, mass-to-light ratio and redshift: Implications for galaxy and black hole evolution

Velocity dispersion ($σ$) is a key driver for galaxy structure and evolution. We here present a comprehensive semi-empirical approach to compute $σ$ via detailed Jeans modelling assuming both a constant and scale-dependent mass-to-light ratio $M^*/L$. We compare with a large sample of local galaxies from MaNGA and find that both models can reproduce the Faber-Jackson (FJ) relation and the weak dependence of $σ$ on bulge-to-total ratio $B/T$ (for $B/T\gtrsim 0.25$). The dynamical-to-stellar mass ratio within $R\lesssim R_e$ can be fully accounted for by a gradient in $M^*/L$. We then build velocity dispersion evolutionary tracks $σ_{ap}[M^*,z]$ (within an aperture) along the main progenitor dark matter haloes assigning stellar masses, effective radii and Sersic indices via a variety of abundance matching and empirically motivated relations. We find: 1) clear evidence for downsizing in $σ_{ap}[M^*,z]$ along the progenitor tracks; 2) at fixed stellar mass $σ\propto(1+z)^{0.2-0.3}$ depending on the presence or not of a gradient in $M^*/L$. We extract $σ_{ap}[M^*,z]$ from the TNG50 hydrodynamic simulation and find very similar results to our models with constant $M^*/L$. The increasing dark matter fraction within $R_e$ tends to flatten the $σ_{ap}[M^*,z]$ along the progenitors at $z \gtrsim 1$ in constant $M^*/L$ models, while $σ_{ap}[M^*,z]$ have a steeper evolution in the presence of a stellar gradient. We then show that a combination of mergers and gas accretion are likely responsible for the constant or increasing $σ_{ap}[M^*,z]$ with time. Finally, our $σ_{ap}[M^*,z]$ are consistent with a nearly constant and steep $M_{bh}-σ$ relation at $z\lesssim 2$, with black hole masses derived from the $L_X-M^*$ relation.

astro-ph.GA↗

arXiv2021

Modelling high-resolution ALMA observations of strongly lensed dustystar forming galaxies detected by Herschel

We present modelling of ~0.1arcsec resolution Atacama Large Millimetre/sub-millimeter Array imaging of seven strong gravitationally lensed galaxies detected by the Herschel Space Observatory. Four of these systems are galaxy-galaxy scale strong lenses, with the remaining three being group-scale lenses. Through careful modelling of visibilities, we infer the mass profiles of the lensing galaxies and by determining the magnification factors, we investigate the intrinsic properties and morphologies of the lensed sub-millimetre sources. We find that these sub-millimetre sources all have ratios of star formation rate to dust mass that is consistent with or in excess of the mean ratio for high-redshift sub-millimetre galaxies and low redshift ultra-luminous infrared galaxies. The contribution to the infrared luminosity from possible AGN is not quantified and so could be biasing our star formation rates to higher values. The majority of our lens models have mass density slopes close to isothermal, but some systems show significant differences.

astro-ph.GA↗

arXiv2021

The Black Hole Mass Function Across Cosmic Times I. Stellar Black Holes and Light Seed Distribution

This is the first paper in a series aimed at modeling the black hole (BH) mass function, from the stellar to the intermediate to the (super)massive regime. In the present work we focus on stellar BHs and provide an ab-initio computation of their mass function across cosmic times. Specifically, we exploit the state-of-the-art stellar and binary evolutionary code \texttt{SEVN}, and couple its outputs with redshift-dependent galaxy statistics and empirical scaling relations involving galaxy metallicity, star-formation rate and stellar mass. The resulting relic mass function ${\rm d}N/{\rm d}V{\rm d}\log m_\bullet$ as a function of the BH mass $m_\bullet$ features a rather flat shape up to $m_\bullet\approx 50\, M_\odot$ and then a log-normal decline for larger masses, while its overall normalization at a given mass increases with decreasing redshift. We highlight the contribution to the local mass function from isolated stars evolving into BHs and from binary stellar systems ending up in single or binary BHs. We also include the distortion on the mass function induced by binary BH mergers, finding that it has a minor effect at the high-mass end. We estimate a local stellar BH relic mass density of $ρ_\bullet\approx 5\times 10^7\, M_\odot$ Mpc$^{-3}$, which exceeds by more than two orders of magnitude that in supermassive BHs; this translates into an energy density parameter $Ω_\bullet\approx 4\times 10^{-4}$, implying that the total mass in stellar BHs amounts to $\lesssim 1\%$ of the local baryonic matter. We show how our mass function for merging BH binaries compares with the recent estimates from gravitational wave observations by LIGO/Virgo, and discuss the possible implications for dynamical formation of BH binaries in dense environments like star clusters. [abridged]

astro-ph.GA↗

arXiv2021

Probing modified gravity with magnetically levitated resonators

We present an experimental procedure, based on Meissner effect levitation of neodymium ferromagnets, as a method of measuring the gravitational interactions between mg masses. The scheme consists of two superconducting lead traps, with a magnet levitating in each trap. The levitating magnets behave as harmonic oscillators, and by carefully driving the motion of one magnet on resonance with the other, we find that it should be easily possible to measure the gravitational field produced by a 4~mg sphere, with the gravitational attraction from masses as small as 30~$μ$g predicted to be measurable within realistic a realistic measurement time frame. We apply this acceleration sensitivity to one concrete example and show the ability of testing models of modified Newtonian dynamics.

gr-qc↗

arXiv2021

Intensity and anisotropies of the stochastic Gravitational Wave background from merging compact binaries in galaxies

We investigate the isotropic and anisotropic components of the Stochastic Gravitational Wave Background (SGWB) originated from unresolved merging compact binaries in galaxies. We base our analysis on an empirical approach to galactic astrophysics that allows to follow the evolution of individual systems. We then characterize the energy density of the SGWB as a tracer of the total matter density, in order to compute the angular power spectrum of anisotropies with the Cosmic Linear Anisotropy Solving System (CLASS) public code in full generality. We obtain predictions for the isotropic energy density and for the angular power spectrum of the SGWB anisotropies, and study the prospect for their observations with advanced Laser Interferometer Gravitational-Wave and Virgo Observatories and with the Einstein Telescope. We identify the contributions coming from different type of sources (binary black holes, binary neutron stars and black hole-neutron star) and from different redshifts. We examine in detail the spectral shape of the energy density for all types of sources, comparing the results for the two detectors. We find that the power spectrum of the SGWB anisotropies behaves like a power law on large angular scales and drops at small scales: we explain this behaviour in terms of the redshift distribution of sources that contribute most to the signal, and of the sensitivities of the two detectors. Finally, we simulate a high resolution full sky map of the SGWB starting from the power spectra obtained with CLASS and including Poisson statistics and clustering properties.

gr-qc↗

arXiv2021

Growth of Massive Black Hole Seeds by Migration of Stellar and Primordial Black Holes: Gravitational Waves and Stochastic Background

We investigate the formation and growth of massive black hole (BH) seeds in dusty star-forming galaxies, relying and extending the framework proposed by Boco et al. 2020. Specifically, the latter envisages the migration of stellar compact remnants (neutron stars and stellar-mass black holes) via gaseous dynamical friction towards the galaxy nuclear region, and their subsequent merging to grow a massive central BH seed. In this paper we add two relevant ingredients: (i) we include primordial BHs, that could constitute a fraction $f_{\rm pBH}$ of the dark matter, as an additional component participating in the seed growth; (ii) we predict the stochastic gravitational wave background originated during the seed growth, both from stellar compact remnant and from primordial BH mergers. We find that the latter events contribute most to the initial growth of the central seed during a timescale of $10^6-10^7\,\rm yr$, before stellar compact remnant mergers and gas accretion take over. In addition, if the fraction of primordial BHs $f_{\rm pBH}$ is large enough, gravitational waves emitted by their mergers in the nuclear galactic regions could be detected by future interferometers like Einsten Telescope, DECIGO and LISA. As for the associated stochastic gravitational wave background, we predict that it extends over the wide frequency band $10^{-6}\lesssim f [{\rm Hz}]\lesssim 10$, which is very different from the typical range originated by mergers of isolated binary compact objects. On the one hand, the detection of such a background could be a smoking gun to test the proposed seed growth mechanism; on the other hand, it constitutes a relevant contaminant from astrophysical sources to be characterized and subtracted, in the challenging search for a primordial background of cosmological origin.

astro-ph.CO↗

arXiv2021

The impact of the FMR and starburst galaxies on the (low-metallicity) cosmic star formation history

The question how much star formation is occurring at low metallicity throughout the cosmic history appears crucial for the discussion of the origin of various energetic transients, and possibly - double black hole mergers. We revisit the observation-based distribution of birth metallicities of stars (f$_{\rm SFR}$(Z,z)), focusing on several factors that strongly affect its low metallicity part: (i) the method used to describe the metallicity distribution of galaxies (redshift-dependent mass metallicity relation - MZR, or redshift-invariant fundamental metallicity relation - FMR), (ii) the contribution of starburst galaxies and (iii) the slope of the MZR. We empirically construct the FMR based on the low-redshift scaling relations, which allows us to capture the systematic differences in the relation caused by the choice of metallicity and star formation rate (SFR) determination techniques and discuss the related f$_{\rm SFR}$(Z,z) uncertainty. We indicate factors that dominate the f$_{\rm SFR}$(Z,z) uncertainty in different metallicity and redshift regimes. The low metallicity part of the distribution is poorly constrained even at low redshifts (even a factor of $\sim$200 difference between the model variations) The non-evolving FMR implies a much shallower metallicity evolution than the extrapolated MZR, however, its effect on the low metallicity part of the f$_{\rm SFR}$(Z,z) is counterbalanced by the contribution of starbursts (assuming that they follow the FMR). A non-negligible fraction of starbursts in our model may be necessary to satisfy the recent high-redshift SFR density constraints.

astro-ph.GA↗