arXiv subjects

Andrea Lapi

Publications and source records attributed to Andrea Lapi.

At least 19 recordsLinked to original sources

arXiv2026

The ViSta method for optimized stacking of broadband interferometric data in the Fourier domain

We present the optimized version of ViSta, a visibility-domain stacking method that combines interferometric observations in the Fourier domain from radio to sub-millimeter wavelengths. By stacking visibilities directly and transforming them into the rest frame, ViSta enhances the signal, suppresses noise, and improves image reconstruction through extended uv-coverage. ViSta outperforms image stacking when individual sources are too faint to detect, achieving higher SNR in the low-signal and extended regime. This new optimized version features a C++/OpenMP kernel which replaces CASA demanding functions, enabling GPU acceleration while minimising memory usage and intermediate data products. The method is highly flexible, allowing stacking regardless of array configuration, spectral setup, or telescope. In the SKA era, where visibilities will not be routinely preserved, ViSta helps in assessing the information lost in the image-plane transition and in exploiting the vast amount of data still stored in interferometric archives.

astro-ph.IM↗

arXiv2026

Cosmography with DESI-DR1 Cosmic Chronometers: Direct H(z) measurements from Luminous Red Galaxy ages

Providing robust redshift estimates for almost 3 million luminous red galaxies (LRGs), the Dark Energy Spectroscopic Instrument (DESI) offers a unique opportunity to test the expansion rate of the Universe with independent approaches. We apply the cosmic chronometer method to derive new, independent constraints on the Hubble parameter at 0.3<z<1.2 from the differential age evolution of DESI LRGs. We select spectra applying spectroscopic cuts to ensure sample purity and remove contamination by star-forming objects, then build a robust sample of cosmic chronometers (CCs) by stacking to obtain stable, high signal-to-noise (S/N) spectra, which also serves as a democratic binning choice for the $t-z$ plane. Ages are estimated by measuring Lick indices on the stacked spectra and fitting them with a theoretical stellar population model. We obtain $t-z$ relations from which we derive $H(z)$ constraints via two independent approaches: a fit with a pivotal-redshift cosmography, and a direct estimate from the original CC method. The cosmographic fit yields posteriors for the kinematic parameters $\{H_{z_0}, q_{z_0}, j_{z_0}\}$ compatible with currently considered cosmologies, giving a precision-level estimate of $H(z)$. We provide the maximum-a-posteriori (MAP) $H(z)$ estimate, an array of the median confidence region in the $H-z$ plane, and its covariance matrix. We also leverage the redshift distributions of the $t-z$ relation for different velocity dispersion groups to obtain two independent local measurements using the discrete approximation $H(z) \approx -\Delta z/[\Delta t (1+z)]$; the one from the reddest envelope of CCs gives $H(z \approx 0.61) = 88.5^{+6.7}_{-12.6}$ (stat.) $\pm 8.1$ (syst.) km s$^{-1}$ Mpc$^{-1}$. Systematic uncertainties for both the cosmographic and discrete $H(z)$ measurements come from a comprehensive analysis of all methodological choices in the data treatment.

astro-ph.CO↗

arXiv2026

Cosmology with a Non-minimally Coupled Dark Matter Fluid II. Cosmological Perturbations

We extend our study of a cosmological scenario in which dark matter is non-minimally coupled to gravity at the fluid level. In previous work, we showed that this interaction can drive an early phase of accelerated expansion, addressing the horizon and flatness problems, and can also lead to a cosmological bounce in the presence of spatial curvature. Here we analyse the evolution of linear perturbations in this framework. We derive the equations governing scalar, vector and tensor perturbations, and obtain analytic solutions in the relevant cosmological regimes. We find that perturbations generated during the accelerated expansion phase produce a strongly blue scalar power spectrum and are therefore incompatible with observations. By contrast, in bouncing solutions primordial fluctuations can originate during the contracting phase before the bounce. In this case, the model yields an approximately scale-invariant scalar power spectrum while keeping the tensor-to-scalar ratio compatible with current bounds, without introducing additional scalar fields. Although our treatment relies on simplifying approximations that should be refined in future work, these results indicate that non-minimally coupled dark matter may provide a viable alternative mechanism for the generation of primordial cosmological perturbations.

gr-qc↗

arXiv2026

The impact of stellar binaries and star cluster dynamics on pair-instability supernovae

Pair-instability supernovae (PISNe) are among the most luminous transients in the Universe. However, they have never been confidently observed. Solving this puzzle would have key implications for several astrophysical topics, including galaxy chemical enrichment, the interpretation of gravitational waves from binary black hole mergers, and the nature of red dropout sources seen by JWST. With this aim, we present the first in-depth study of PISN occurrence in binary stars, both in isolation and in dense star clusters. We employ the SEVN code, with PARSEC stellar tracks, to evolve a suite of 35 synthetic binary populations, including variations on formation channels, cluster properties, and upper limit of the stellar initial mass function. We find that binary interactions can boost the PISN rate by up to threefold, relative to single stars, whereas binary hardening can either enhance or suppress PISN production, depending on whether the progenitors are primordial or dynamically formed. Moreover, we showcase how our comprehensive framework for the cosmic PISN rate can be used to constrain uncertain aspects of stellar and galaxy evolution models, via comparison with observations, including the recipes for stellar-wind mass loss in very-massive stars, and the galaxy metallicity distribution throughout the Universe.

astro-ph.HE↗

arXiv2026

How galaxies acquire their stellar mass at high redshift: High star formation efficiencies and the relative roles of dust and initial mass function

JWST has measured an unprecedented abundance of galaxies above $z\gtrsim 4-5$, whose formation and evolution are still difficult to reconcile within traditional galaxy evolution models in a $\Lambda$CDM framework. Here, we present a study on the star formation histories of these high-redshift galaxies between $z\simeq5-12$ via a data-driven semi-empirical model that uses the observed UV LFs as input to retrieve SFRs, naturally bypassing any uncertain modelling of cooling, feedback and/or stochastic processes. Galaxy stellar masses are progressively built in time by integrating their SFRs assigned along their progenitor haloes via the SFR-halo accretion rate relation, derived from abundance matching between the input observed UV LFs with the dark matter halo accretion rate distributions at each redshift. This makes the SFEs a full prediction of the model rather than a tuned input, serving as a natural baseline to test burstiness, dust attenuation, or IMF variations. Our approach naturally reproduces the total stellar mass function, the large-scale clustering, and the star-forming main sequence. We find that massive galaxies grew their stellar mass with a bursty star formation at $z\sim9-10$, broadly in agreement with the star formation histories inferred from spectral energy distribution fitting, with the SFE reaching high peaks of $0.8-0.9$ at $z>9$ and lowering to standard values of $0.2-0.3$ below $z\lesssim9$. We find that the presence of dust could enhance the predicted SFRs at $z\lesssim8$, better reproducing the observed SFRs of massive dusty galaxies, and increase the SFEs to values close to or even above unity at $z \gtrsim 8$. Finally, switching to top-heavy IMFs reduces the SFEs by a factor of $2-3$, highlighting the need for a variable IMF as an inevitable ingredient in the evolution of galaxies at high redshifts to avoid unphysical SFEs, especially in the presence of dust.

astro-ph.GA↗

arXiv2026

The impact of envelope binding energies on the merger rate density of binary compact objects

The common envelope (CE) phase plays a key role in the formation of binary compact object systems. Its final outcome strongly depends on the envelope binding energy, but this quantity is often estimated using fitting formulas that are not fully consistent with the underlying stellar evolution models adopted in population-synthesis codes. Here, we investigate envelope binding energies across the most extensive stellar grid considered to date. Our stellar tracks, evolved with PARSEC v2.0, include hydrogen (H) -rich stars with metallicities ranging from $Z = 10^{-11}$ (Population III stars) to $Z = 0.03$, and initial masses between 2 and 2000 M$_\odot$, as well as pure-helium stars with masses from 0.36 to 350 M$_\odot$. We examine the sensitivity of the envelope binding energies to the selected core-envelope boundary definition and to different internal energy source contributions. For H-rich stars, we find that internal energy sources can alter the envelope binding energy by more than an order of magnitude, whereas the core boundary criteria play a secondary role. In contrast, for pure helium stars, the core-boundary criterion becomes the dominant factor. The envelope binding energies derived from different stellar tracks can show deviations of several orders of magnitude, with larger differences for more massive stars and higher metallicities.Finally, by implementing our new envelope binding energy prescriptions into the binary population synthesis code SEVN, we show that the predicted merger rate densities of compact binaries can differ by more than an order of magnitude compared to previous models. Our results highlight the importance of using envelope binding energies that are consistent with the underlying stellar evolution models and caution against extrapolating empirical fits beyond the considered parameter space.

astro-ph.SR↗

arXiv2026

Painting a full radio sky -- Empirical mock catalogues with multiple source populations for future radio surveys

Upcoming radio surveys will probe the sky with unprecedented depth and sky coverage, enabling a broad range of cosmological and astrophysical applications, as well as powerful synergies with experiments at other wavelengths. The preparation and scientific exploitation of these surveys require realistic mock catalogues that capture the complexity of the radio sky and the interplay of its emitting components. We present a modular and extensible algorithm for generating empirical simulations over the full radio sky, i.e. a solid angle of $4\pi$ steradians ($f_{\rm sky}=1$), down to redshift $z=5$, comprising both radio continuum and line emission. The framework combines a simulated dark-matter light-cone with empirically sampled galaxy populations and a probabilistic galaxy-halo assignment scheme, producing self-consistent mock catalogues including multiple radio populations on the same light-cone. We release two public catalogues: a shallow catalogue, fully constrained by existing observational data and limited to flux thresholds of $S_\text{1.4 GHz}^\text{lim} \sim 8\times10^{-5}\ \text{Jy}$ at $1.4\ \text{GHz}$ and $S_\text{21}^\text{lim} \sim 2\ \text{Jy}\cdot\text{Hz}$ for the HI 21 cm line; and a deep catalogue extending the calibrated empirical model to better sensitivities, broadly matching future SKAO surveys, with flux limits of $S_\text{1.4 GHz}^\text{lim} \sim 4\times10^{-5}\ \text{Jy}$ and $S_\text{21}^\text{lim} \sim 0.3\ \text{Jy}\cdot\text{Hz}$. The catalogues include radio continuum active galactic nuclei and star-forming galaxies, together with HI-emitting galaxies, for a total of more than 260 million sources in the shallow catalogue and more than 1 billion in the deep catalogue. We validate the simulations by analysing their statistical properties: the mocks reproduce the targeted clustering and population statistics while retaining minimal physical assumptions.

astro-ph.GA↗

arXiv2026

Too many or too massive? Investigating the high-$z$ demography of active SMBHs from JWST

Recent JWST observations have unveiled a numerous population of low-luminosity active galactic nuclei (AGN) at $4< z<10$, with space densities roughly an order of magnitude above pre-JWST estimates, and many of these AGN have masses orders of magnitude above the local black hole mass-stellar mass ($M_{\rm BH}-M_{*}$) scaling relations. We investigate the consistency of these observations within a data-driven framework that links the galaxy stellar mass function to the supermassive black hole (SMBH) mass function and AGN luminosity functions using different $M_{\rm BH}-M_{*}$ relations and the observed Eddington-ratio distribution. By comparing our predictions against observed AGN luminosity functions at $z\sim 5.5$ we find that observations can be reproduced either by highly-elevated $M_{\rm BH}-M_{*}$ relations paired with low duty cycles, or moderate relations with higher duty cycles. Through the Soltan argument, we find that $M_{\rm BH}-M_{*}$ relations that are modestly above the local relation for AGN produce consistency between multiple tracers of the SMBH demography at $z\sim 5.5$, while more extreme normalisations would require a weakly-evolving luminosity function at $z> 5.5$. Continuity-equation modelling shows that initially high $M_{\rm BH}-M_{*}$ relations predict a strong two-phase evolutionary scenario and very steep low-mass SMBH mass functions in tension with several current estimates, while more moderate relations generate local SMBH mass functions in better agreement with present determinations and near-constant scaling relations. Our results favour a scenario where SMBHs at $z \sim 5$ on average lie modestly above local AGN scaling relations, with elevated but physically plausible duty cycles. Future wide-field clustering and demographic studies will help break the remaining degeneracies between SMBH scaling relations and AGN duty cycles at early cosmic times.

astro-ph.GA↗

arXiv2025

SHORES II: Multi-frequency Characterisation of the Sub-mJy Radio Population in FIR-selected Fields

We present a new deep multi-frequency radio survey of two extragalactic fields observed with the Australia Telescope Compact Array (ATCA) as part of the SHORES project (Serendipitous H-ATLAS fields Observations of Radio Extragalactic Sources). The observations, centred at 2.1, 5.5, and 9 GHz, cover the central 0.5 deg$^2$ of two Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) fields down to rms sensitivities of 9-17 $\mu$Jy$/$beam at 2.1 GHz, 28-39 $\mu$Jy/beam at 5.5 GHz and 38-61 $\mu$Jy/beam at 9 GHz. This setup allows us to investigate the spectral energy distributions (SEDs) of faint radio sources and probe the nature of the sub-mJy population. We extract and validate a robust catalogue of 489 sources at 2.1 GHz, 101 of which are also detected at 5.5 GHz. We perform a multi-frequency analysis of the radio number counts and derive the spectral indices of sources in the deep fields. The spectral index distribution of our sources peaks around $\alpha \sim -0.7$, consistent with synchrotron emission from the faint radio population. The number counts at 2.1 GHz are consistent with previous deep surveys and theoretical models, and provide a lower limit on the star-forming galaxy population, which is expected to dominate the faint end. The 5.5 GHz data offer new, direct constraints on the sub-mJy radio sky at higher frequencies. By cross-matching with the H-ATLAS catalogue, we identify a sample of sources with far-infrared (FIR) counterparts and explore the far-infrared-radio correlation (FIRRC). The sources with $q_{FIR} \geq 1.69$ exhibit radio spectral indices typical of star-forming galaxies. Furthermore, we identify a population of radio-only sources with similar indices that may correspond to high-redshift SFGs, lacking counterparts in the FIR survey due to its limited resolution and sensitivity.

astro-ph.GA↗

arXiv2025

The ViSta method for stacking in the Fourier domain and its application to the dusty star-forming galaxies in the ALMA Science Archive

We present ViSta, a Visibility Stacking method to combine interferometric observations in the Fourier domain at radio to sub-millimeter wavelengths for galaxies. The goal of our method is to maximize the exploitation of available archival interferometric data. By stacking visibilities of galaxies with secure spectroscopic redshifts directly in the Fourier domain and transforming them into the rest-frame, we can enhance the stacked signal, suppress noise, and improve image reconstruction thanks to an extended coverage of the visibility domain. The ViSta method is highly flexible, allowing stacking of visibilities regardless of the array configuration or spectral setup. It is effective for both targeted sources and spurious detections offset from the phase center, whether unresolved or extended, within the field of view of the telescope. We validated the method using simulated interferometric datasets. For point-like sources, we can reconstruct the true emission with approximately 90% accuracy, obtaining similar results to classical image-plane stacking. In contrast, for faint and extended sources below the noise level, our method can provide a more accurate estimate of the signal compared to traditional image-based approaches. Finally, we applied ViSta to a sample of dusty star-forming galaxies (DSFGs) observed with the Atacama Large Millimeter/sub-millimeter Array (ALMA) to detect the CO(3-2) emission line. As for the simulated case, we demonstrated that our tool performs better than image-plane stacking when the signal from individual sources is no longer easily detectable, achieving higher SNR. Finally, we outline potential future applications of this stacking approach.

astro-ph.GA↗

arXiv2025

The role of black hole feedback on galaxy star formation and the degeneracy with halo quenching

The interplay between the accretion of supermassive black holes (SMBHs) and the stellar mass growth of the host galaxies is still a matter of hot debate. The accretion of the SMBHs is expected to release energy under the form of AGNs. This energy is believed to impact the star formation activity and contribute to the quenching of galaxies. Here, we address this key unsolved issue with our cosmological semi-empirical model DECODE. In DECODE, we grow galaxies with their SFR linked to halo accretion rate distributions via abundance matching. SMBHs are evolved following the stellar mass growth of their host galaxies by assigning an accretion rate at each redshift from the empirical Eddington ratio distributions and duty cycles. We test the assumption that galaxies permanently quench when their central SMBHs approach the limit imposed by the observed $M_{\rm BH} - \sigma_\star$ relation, as a proxy of SMBH disruptive feedback. We find that simply imposing the $M_{\rm BH} - \sigma_\star$ condition is sufficient to generate a fraction of quenched galaxies consistent with current data, including the newest ones from Euclid. In addition, our minimal, data-driven model, also predicts SMBH scaling relations consistent in slope and normalisation with those observed, and an $M_{\rm BH} - M_\star$ relation weakly evolving with redshift. The model also naturally generates SMBH accretion rates peaking within 1 Gyr of their host SFHs. We note that all the main predictions on galaxy quenched fractions and SMBH growth histories and scaling relations are degenerate with those expected in a halo quenching model. The comprehensive data-driven model presented in this work represents an invaluable tool to investigate SMBH demography across time and environments in an accurate, physically motivated manner, ideally suited to rapidly explore the implications from large surveys, such as Euclid and Rubin-LSST.

astro-ph.GA↗

arXiv2025

Cosmic magnification on high-redshift submillimeter galaxies

Weak lensing magnification probes the correlation between galaxies and the underlying matter field in a similar fashion to galaxy-galaxy lensing shear. Although it has long been sidelined in favor of the latter on the grounds of a poorer performance in terms of statistical significance, the provision of a large sample of high-redshift submillimeter galaxies by the \emph{Herschel} observatory has transformed the landscape of cosmic magnification due to their optimal physical properties for magnification analyses. This review aims to summarize the core principles and unique advantages of cosmic magnification on high-redshift submillimeter galaxies and discuss recent results applied for cosmological inference. The outlook and challenges of this observable are also outlined, with a focus on the ample scope for exploration and its potential to emerge as a competitive independent cosmological probe.

astro-ph.CO↗

arXiv2025

Cosmography via stellar archaeology of low-redshift early-type galaxies from SDSS

Cosmic chronometers offer a model-independent way to trace the expansion history of the Universe via the dating of passively evolving objects. This enables testing the validity of cosmological models without concrete assumptions of their energy content. The main goal of this work is to derive model-independent constraints on the Hubble parameter up to $z \sim 0.4$ using stellar ages from the fitting of Lick index absorption lines in passively evolving galaxies. Contrary to recent related works that rely on finite differences to obtain a discrete measurement of the expansion of the Universe at an average redshift, our goal is to perform a cosmographic fit of $H(z)$ in terms of the Hubble constant ($H_0$) and the deceleration ($q_0$) and jerk ($j_0$) parameters. We carefully select spectra of massive and passively evolving galaxies from the SDSS Legacy Survey. After applying a stacking procedure to ensure a high signal-to-noise ratio, the strength of Lick indices is fit using two stellar population models (TMJ and Knowles) to derive stellar population parameters. A cosmographic fit to the stellar ages is performed, which in turn enables the sampling of the Hubble parameter within the considered redshift range. The baseline result comes from using the TMJ-modelled ages, and it yields a value of $H_0 = 70.0^{+4.1}_{-7.6} \text{ km s}^{-1} \text{ Mpc}^{-1}$ for the Hubble constant, where uncertainties refer only to the statistical treatment of the data. The sampling of the Hubble parameter at $0.05 < z < 0.35$ is competitive with discreet model-independent measurements from the literature. We finally draw attention to an unexpected oscillating pattern in a number of critical indices with respect to redshift, which translates into a similar behaviour in the $t-z$ relations. These features have never been discussed before, although they are present in previous measurements.

astro-ph.CO↗

arXiv2025

Characterizing the roles of transitory obscured phases and inner torus in shaping the fractions of obscured AGN at cosmic noon

The origin of obscuration in Active Galactic Nuclei (AGN) is still a matter of contention. It is unclear whether obscured AGN are primarily due to line-of-sight effects, a transitory, dust-enshrouded phase in galaxy evolution, or a combination of both. The role of an inner torus around the central SMBH also remains unclear in pure Evolution models. We use cosmological semi-analytic models and semi-empirical prescriptions to explore obscuration effects in AGN at 1<z<3. We consider a realistic object-by-object modelling of AGN evolution including different light curves (LCs) composed of phases of varying levels of obscuration, mimicking the possible clearing effects of strong AGN feedback. Evolution models characterized by AGN LCs with relatively short pre-peak obscured phases followed by more extended optical/UV visible post-peak phases, struggle to reproduce the high fraction of obscured AGN at z~2-3 inferred from X-ray surveys. Evolution models characterised by LCs with sharp post-peak declines or persistent or multiple obscuration phases are more successful, although they still face challenges in reproducing the steady drop in the fractions of obscured AGN with increasing luminosity measured by some groups. Invoking a fine-tuning in the input LCs, with more luminous AGN defined by longer optical/UV visible windows, can improve the match to the decreasing fractions of obscured AGN with luminosity. Alternatively, a long-lived central torus-like component, with thickness decreasing with increasing AGN power, naturally boosts the luminosity-dependent fractions of obscured AGN, suggesting that small-scale orientation effects may still represent a key component even in Evolution models. We also find that in our models major mergers and starbursts, when considered in isolation, fall short in accounting for the large fractions of highly obscured faint AGN detected at cosmic noon.

astro-ph.GA↗

arXiv2025

Mysteries of Capotauro: investigating the puzzling nature of an extreme F356W-dropout

JWST has uncovered a diverse population of extreme near-infrared dropouts, including ultra high-redshift ($z>15$) galaxy candidates, dust-obscured galaxies challenging dust production theories, sources with strong Balmer breaks - possibly compact AGN in dense environments - and cold, sub-stellar Galactic objects. This work presents Capotauro, a F356W-dropout in the CEERS survey with F444W AB magnitude of $\sim27.68$ and a sharp $>3$ mag flux drop between $3.5{-}4.5\,\mu$m, undetected below $3.5\,\mu$m. We combine JWST/NIRCam, MIRI, and NIRSpec/MSA data with HST/ACS and WFC3 observations to perform a spectro-photometric analysis of Capotauro using multiple SED-fitting codes. Our setup tests $z\geq15$ as well as $z<10$ dusty, Balmer-break or strong-line galaxy solutions, and the possibility of Capotauro being a Milky Way sub-stellar object. Among extragalactic options, our analysis favors interpreting the sharp drop as a Lyman break at $z\sim32$, consistent with the epoch of formation of the first stars and black holes, with only $\sim0.5\%$ of the posterior volume at $z<25$. Lower-redshift solutions struggle to reproduce the extreme break, suggesting that if Capotauro lies at $z<10$, it must show a non-standard combination of strong dust attenuation and/or Balmer breaks, making it a peculiar interloper. Alternatively, its properties match a very cold (Y2-Y3 type) brown dwarf or a free-floating exoplanet with a record-breaking combination of low temperature and large distance ($T_{\mathrm{eff}}<300\,\mathrm{K}$, $d\gtrsim130\,\mathrm{pc}$, up to $\sim2\,\mathrm{kpc}$). While current data cannot determine its nature, Capotauro emerges as a remarkably unique object in all plausible scenarios, and a compelling target for follow-up.

astro-ph.GA↗

arXiv2025

Cosmology with a Non-minimally Coupled Dark Matter Fluid I. Background Evolution

We explore a cosmological model in which dark matter is non-minimally coupled to gravity at the fluid level. While typically subdominant compared to Standard Model forces, such couplings may dominate dark matter dynamics. We show that this interaction modifies the early-time Friedmann equations, driving a phase of accelerated expansion that can resolve the horizon and flatness problems without introducing additional fields. At even earlier times, the coupling to spatial curvature may give rise to a cosmological bounce, replacing the initial singularity of standard cosmology. These results suggest that non-minimally coupled dark matter could offer a unified framework for addressing both the singularity and fine-tuning problems.

gr-qc↗

arXiv2025

Semi-empirical Framework of Supermassive Black Hole Evolution: Highlighting a possible tension between Demographics and Gravitational Wave Background

The evolution of the supermassive Black Hole (BH) population across cosmic times remains a central unresolved issue in modern astrophysics, due to the many noticeable uncertainties in the involved physical processes that span a huge range of spatial, temporal and energy scales. Here we tackle the problem via a semi-empirical approach with minimal assumptions and data-driven inputs. This is based on a continuity plus Smoluchowski equation framework that allows to unitarily describe the two primary modes of BH growth: gas accretion and binary mergers. Key quantities related to the latter processes are incorporated through educated parameterizations, and then constrained in a Bayesian setup from joint observational estimates of the local BH mass function, of the large-scale BH clustering, and of the nano-Hz stochastic gravitational wave (GW) background measured from Pulsar Timimg Array (PTA) experiments. We find that the BH accretion-related parameters are strongly dependent on the local BH mass function determination: higher normalizations and flatter high-mass slopes in the latter imply lower radiative efficiencies and mean Eddington ratios with a stronger redshift evolution. Additionally, the binary BH merger rate is estimated to be a fraction $\lesssim 10^{-1}$ of the galaxy merger rate derived from galaxy pairs counts by \texttt{JWST}, and constrained not to exceed the latter at $\gtrsim 2\sigma$. Relatedly, we highlight hints of a possible tension between current constraints on BH demographics and the interpretation of the nano-Hz GW background as predominantly caused by binary BH mergers. Specifically, we bound the latter's contribution to $\lesssim 30-50\%$ at $\sim 3\sigma$, suggesting that additional astrophysical/cosmological sources are needed to explain the residual part of the signal measured by PTA experiments.

astro-ph.CO↗

arXiv2025

Combining (post-)Newtonian ideas with quasi-equilibrium (QE) sequence analysis for black hole-neutron star (BHNS) gravitational wave events

In this paper we present quasi equilibrium models of black hole-neutron star (BHNS) binaries with mass and spin values compatible with parameter estimates derived from gravitational radiation events GW200105 and GW200115, events consistent with the merger of BHNSs. Using the FUKA initial data framework, we determine the location of ISCO (Innermost Stable Circular Orbit) and radius of mass shedding. In most of the cases studied here the innermost stable orbit is located at larger separations. This is consistent with the fact that for those two events no electromagnetic counterparts have been observed since it is believed that the NS will enter into the plunge phase in a short time once the separation of the components of the binary will be smaller of ISCO. In analogy with classical binaries, we have associated to these QE sequences a Newtonian and Post Newtonian Roche Lobe analysis to verify whether the NS is filling its Roche Lobe before approaching the ISCO. For selected configurations explored here, the location of ISCO and of the orbit at which mass shedding occurs are at separation smaller than the last converged solution of our sequences. Our analysis shows that in such cases the neutron star is filling its Roche lobe suggesting that mass transfer might occur well before encountering the last stable orbit, and this should happen in catastrophic way in order to prevent any electromagnetic emissions. If this is the case, we are suggestion a third fate of the neutron star beyond the plunge or tidal disruption ones.

gr-qc↗