1,720,987 research outputs found
Gravitational waves throughout galaxy evolution: stellar BH mergers and heavy SMBH seeds.
Full text linkThe main goal of my thesis is to carefully characterize different astrophysical processes leading to gravitational wave (GW) emission, strongly relying on theoretical and observational astrophysical basis. From an observational point of view, current interferometers (Advanced Laser Interferometer Gravitational wave Observatory/Virgo (AdvLIGO/Virgo)) and future detectors (Einstein Telescope (ET), Cosmic Explorer (CE), Deci-hertz Interferometer Gravitational wave Observatory (DECIGO), Laser Interferometer Space Antenna (LISA)) will greatly enlarge the number of detected GW events. However, in order to extract meaningful information about various astrophysical phenomena and improve our knowledge on cosmology and fundamental physics from this large sample of observational data, a correct modelization of the impact of different astrophysical processes on GWs rates is necessary.
The marking feature of all the work is an accurate and deep study of the galactic environment, making use of classic theoretical arguments and recent observational results in the galaxy formation and evolution field. Galactic properties, such as star formation rate, gas and stellar density, metallicity, can have a profound impact on stellar and compact object evolution and on the ensuing GW emissions. In particular, throughout the thesis I focused on the study of 2 different channels of GW production: merging of isolated double compact object binaries of stellar origin (neutron stars and stellar black holes) and dynamical merging of stellar and, eventually, primordial black holes in the central regions of early-type galaxy progenitors.
In the context of double compact object merging binaries, given the relevance of gas-phase metallicity for all the stellar and binary evolution processes, the main effort of my work is in the characterization of a metallicity dependent cosmic star formation rate density. I compute this term in various ways, highlighting the impact of different galactic prescriptions, such as galaxy statistics and metallicity scaling relations. In particular I focus on the gas-phase metallicity, showing that the two main empirical scaling relations present in literature, the Mass Metallicity Relation and the Fundamental Metallicity Relation, hold substantially different results at high redshift ( > 2), with the Fundamental Metallicity Relation featuring relatively high metallicitites ∼ 0.4 − 0.5 Z⊙ and the Mass Metallicity Relation predicting a significant metallicity drop below 0.1 Z⊙. I discuss the reasons and possible biases originating this discrepancy, arguing in favor of the Fundamental Metallicity Relation or of a slowly declining Mass Metallicity Relation. I also present a chemical evolution model to deal with metallicity from a theoretical point of view and I find a pleasant agreement between the model and the Fundamental Metallicity Relation. Finally, I show the impact of these different astrophysical prescriptions on the merging rates and on the properties of compact objects binaries, such as their chirp mass or time delay distribution. I complete the work forecasting the ensuing GW detection rates with present and future detectors, as well as the expected lensed event rates and the stochastic GW background.
As for the dynamical merging channel, recent observations of the extremely star-forming and gas-dense environments in the central regions of early-type galaxy progenitors at z bigger than 1, inspired the idea for the proposal of a new mechanism for the growth of supermassive black hole seeds. This envisages the migration and merging of compact objects via gaseous dynamical friction toward the galactic center where a central black hole accumulates mass thanks to these continuous merging events. I show that, under reasonable assumptions, the process can build up central BH masses of order 10^4 − 10^5 M⊙ within some 10^7 yr, so effectively providing heavy seeds before standard (Eddington-like) disk accretion takes over to become the dominant process for further BH growth. Remarkably, such a mechanism may provide an explanation, alternative or complementary to other processes, for the buildup of billion solar masses black holes in quasar hosts at z bigger than 7, when the age of the Universe less than 0.8 Gyr constitutes a demanding constraint. This process naturally present a possibility to be tested via detections of the gravitational waves produced by mergers between the migrating compact objects and the growing central black hole. I also make predictions for the produced stochastic GW background which extends over a wide range of frequencies [10^(−6) Hz, 10 Hz], very different from the typical range originated by mergers of isolated binaries. I show that both the single events and the background could be revealed by future ground- and space-based interferometers as ET, DECIGO and LISA
Semi-empirical models of galaxy formation and evolution
No full textWe provide a review on semi-empirical models of galaxy formation and evolution. We present a brief census of the three main modeling approaches to galaxy evolution, namely hydrodynamical simulations, semi-analytic models, and semi-empirical models (SEMs). We focus on SEMs in their different flavors, i.e. interpretative, descriptive and hybrid, discussing the peculiarities and highlighting virtues and shortcomings for each of these variants. We dissect a simple and recent hybrid SEM from our team to highlight some technical aspects. We offer some outlook on the prospective developments of SEMs. Finally, we provide a short summary of this review
Modelling the host galaxies of binary compact object mergers with observational scaling relations
Full text linkThe merger rate density evolution of binary compact objects and the
properties of their host galaxies carry crucial information to understand the
sources of gravitational waves. Here, we present galaxyRate, a new code that
estimates the merger rate density of binary compact objects and the properties
of their host galaxies, based on observational scaling relations. We generate
our synthetic galaxies according to the galaxy stellar mass function. We
estimate the metallicity according to both the mass-metallicity relation (MZR)
and the fundamental metallicity relation (FMR). Also, we take into account
galaxy-galaxy mergers and the evolution of the galaxy properties from the
formation to the merger of the binary compact object. We find that the merger
rate density changes dramatically depending on the choice of the star-forming
galaxy main sequence, especially in the case of binary black holes (BBHs) and
black hole neutron star systems (BHNSs). The slope of the merger rate density
of BBHs and BHNSs is steeper if we assume the MZR with respect to the FMR,
because the latter predicts a shallower decrease of metallicity with redshift.
In contrast, binary neutron stars (BNSs) are only mildly affected by both the
galaxy main sequence and metallicity relation. Overall, BBHs and BHNSs tend to
form in low-mass metal-poor galaxies and merge in high-mass metal-rich
galaxies, while BNSs form and merge in massive galaxies. We predict that
passive galaxies host at least ~5-10%, ~15-25%, and ~15-35% of all BNS, BHNS
and BBH mergers in the local Universe.Comment: 21 pages, 22 figures (including appendices), 3 tables, published in
MNRA
Merging Rates of Compact Binaries in Galaxies: Perspectives for Gravitational Wave Detections
No full textWe investigate the merging rates of compact binaries in galaxies and the related detection rate of gravitational wave (GW) events with AdvLIGO/Virgo and with the Einstein Telescope. To this purpose, we rely on three basic ingredients: (i) the redshift-dependent galaxy statistics provided by the latest determination of the star formation rate functions from UV+far-IR/(sub)millimeter/radio data; (ii) star formation and chemical enrichment histories
for individual galaxies, modeled on the basis of observations; and (iii) compact remnant mass distribution and prescriptions for merging of compact binaries from stellar evolution simulations. We present results for the intrinsic birth rate of compact remnants, the merging rates of compact binaries, GW detection rates, and GW counts, attempting to differentiate the outcomes among black hole–black hole, neutron star–neutron star, and black hole–neutron star mergers and to estimate their occurrence in disk and spheroidal host galaxies. We compare our approach with the one based on cosmic star formation rate density and cosmic metallicity, exploited by many literature studies; the merging rates from the two approaches are in agreement within the overall astrophysical uncertainties. We also investigate the effects of galaxy-scale strong gravitational lensing of GW in enhancing the rate of detectable events toward high redshift. Finally, we discuss the contribution of undetected GW emission
from compact binary mergers to the stochastic background
Gravitational waves HI intensity mapping: cosmological and astrophysical applications
No full textTwo 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: (i) statistical inference of the observed redshift distribution of GWs; (ii) constraints on dynamical dark energy models as an example of cosmological studies; (iii) determination of the nature of the progenitors of merging binary black holes, distinguishing between primordial and astrophysical origin. Our results show that: (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; (ii) the constrains on the dynamical dark energy parameters are competitive with IM-only experiments, in a complementary way and potentially with less systematics; (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 GW × 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.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 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
Intensity and anisotropies of the stochastic gravitational wave background from merging compact binaries in galaxies
No full textWe 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 behavior 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
TOPSEM, TwO parameters semi empirical model: galaxy evolution and bulge/disk dicothomy from two-stage halo accretion
No full textIn recent years, increasing attention has been devoted to semi empirical, data-driven models to tackle some aspects of the complex and still largely debated topic of galaxy formation and evolution. We here present a new semi empirical model whose marking feature is simplicity: it relies on solely two assumptions, one initial condition and two free parameters. Galaxies are connected to evolving dark matter haloes through abundance matching between specific halo accretion rate (sHAR) and specific star formation rate (sSFR). Quenching is treated separately, in a fully empirical way, to marginalize over quiescent galaxies and test our assumption on the sSFR evolution without contaminations from passive objects. Our flexible and transparent model is able to reproduce the observed stellar mass functions up to z\sim 5, giving support to our hypothesis of a monotonic relation between sHAR and sSFR. We then exploit the model to test a hypothesis on morphological evolution of galaxies. We attempt to explain the bulge/disk bimodality in terms of the two halo accretion modes: fast and slow accretion. Specifically, we speculate that bulge/spheroidal components might form during the early phase of fast halo growth, while disks form during the later phase of slow accretion. We find excellent agreement with both the observational bulge and elliptical mass functions
The Black Hole Mass Function: From Stellar to Supermassive
Full text linkThe overall purpose of this thesis is to establish the black hole (BH) mass function, from stellar to supermassive. We divide this work into two: 1) the stellar black hole mass function covering the mass range ; 2) the supermassive BH mass function spanning ; the intermediate BH mass function will be a byproduct of the continuity equation approach used to track supermassive BH growth and allow for the stitching together of a complete BH mass function. We incorporate a wide range of disciplines including: galaxy evolution, stellar evolution, and black hole evolution. We seek to create a self-contained, self-consistent, model that spans from the present day to , which can be used to make estimations of future observational predictions specifically in regards to gravitational wave instruments (Laser Interferometer Space Antenna, Deci-hertz Interferometer, and Einstein Telescope) and electromagnetic detections (James Webb Space Telescope and \textit{Athena}).
Regarding the stellar BH mass function, we mainly consider the standard, and likely dominant, production channel of stellar mass BHs constituted by isolated single/binary star evolution. 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 as a function of the BH mass features a rather flat shape up to and then a log-normal decline for larger masses, while its overall normalisation 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 predominantly at the high-mass end. We estimate a local stellar BH relic mass density of Mpc, which exceeds by more than two orders of magnitude that in supermassive BHs; this translates into an energy density parameter , implying that the total mass in stellar BHs amounts to 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. We highlight that our results can provide a firm theoretical basis for a physically-motivated light seed distribution at high redshift, to be implemented in semi-analytic and numerical models of BH formation and evolution.
In terms of the supermassive BH mass function, we consider two main mechanisms to grow the central BH, that are expected to cooperate in the high-redshift star-forming progenitors of local massive galaxies. The first is the gaseous dynamical friction process, that can cause the migration toward the nuclear regions of stellar-mass BHs originated during the intense bursts of star formation in the gas-rich host progenitor galaxy, and the buildup of a central heavy BH seed within short timescales some yr. The second mechanism is the standard Eddington-type gas disk accretion onto the heavy BH seed, through which the central BH can become (super)massive within the typical star-formation duration Gyr of the host. We validate our semi-empirical approach by reproducing the observed redshift-dependent bolometric AGN luminosity functions and Eddington ratio distributions, and the relationship between the star-formation and the bolometric luminosity of the accreting central BH. We then derive the relic (super)massive BH mass function at different redshifts via a generalised continuity equation approach, and compare it with present observational estimates.
Finally, we reconstruct the overall BH mass function from the stellar to the (super)massive regime, over more than ten orders of magnitudes in BH mass. Overall we have found that the number of black holes within the observable Universe (a sphere of diameter around 90 billion light years) at present time is about 40 billion billions (i.e. about )
Shedding light on the star formation rate-halo accretion rate connection and halo quenching mechanism via DECODE, the Discrete statistical sEmi-empiriCal mODEl
No full textAims: the relative roles of the physical mechanisms involved in quenching galaxy star formation are still unclear. We tackle this fundamental problem with our cosmological semi-empirical model DECODE (Discrete statistical sEmi-empiriCal mODEl), designed to predict galaxy stellar mass assembly histories, from minimal input assumptions. Methods: specifically, in this work the star formation history of each galaxy is calculated along its progenitor dark matter halo by assigning at each redshift a star formation rate extracted from a monotonic star formation rate-halo accretion rate (SFR-HAR) relation derived from abundance matching between the (observed) SFR function and the (numerically predicted) HAR function, a relation that is also predicted by the TNG100 simulation. SFRs are integrated across cosmic time to build up the mass of galaxies, which may halt their star formation following input physical quenching recipes. Results: in this work we test the popular halo quenching scenario and we find that (1) the assumption of a monotonic relation between the SFR and HAR allows us to reproduce the number densities of the bulk of star-forming galaxies in the local Universe; (2) the halo quenching is sufficient to reproduce the statistics of the quenched galaxies and flat (steep) high-mass end of the stellar mass-halo mass relation (or SMF); and (3) to align with the observed steep (flat) low-mass end of the stellar mass-halo mass (or SMF) additional quenching processes in the least massive haloes are needed. Conclusions: DECODE is an invaluable tool and will pave the way to investigate the origin of newly observed high-redshift objects from the latest ongoing facilities such as JWST and Euclid.</p
Impact of Wolf-Rayet stellar winds on compact object formation
Full text linkopenWolf-Rayet (WR) stars are massive stars that have lost their hydrogen envelopes, exposing the underlying helium cores. In the Small Magellanic Cloud (SMC), 12 WR stars have been observed, 7 of which appear to be single.
To exist without a binary companion, these stars must have undergone a self-stripping process. Although such a mechanism is theoretically plausible, current models of line-driven stellar winds, combined with the relatively low metallicity of the SMC, fail to fully account for the presence of apparently isolated WR stars. This is primarily due to the reduced strength of stellar winds at low metallicity, which is generally insufficient to remove the hydrogen envelope.
In this thesis, I have implemented a new model for radiation-driven stellar winds within the MESA stellar evolution code. This model includes the possibility of activating optically thick winds when a star approaches the Eddington limit.
I demonstrate that rapidly rotating stars can enter the thick-wind regime, successfully removing their hydrogen envelopes and evolving into WR stars even at low metallicity. The resulting stellar evolutionary tracks were subsequently used as input for the population synthesis code SEVN, to evaluate the impact of the new wind model on binary star populations.
Finally, the output from SEVN was used to estimate the merger efficiency of binary black hole (BBH) systems in this population, with the goal of addressing the persistent overestimation of the BBH merger rate density by theoretical models, compared to that inferred by the LIGO-Virgo-Kagra (LVK) collaboration.
Multiple relevant results have been found. Firstly, optically thick winds are able to reduce the hardening of the binary, therefore lowering the merger efficiency. On the other hand, this effect significantly increases the time delay between formation and black hole merger, which on average becomes about 10 Gyr. This means that the mergers observed today through GWs originate from objects formed at cosmic noon or earlier.Wolf-Rayet (WR) stars are massive stars that have lost their hydrogen envelopes, exposing the underlying helium cores. In the Small Magellanic Cloud (SMC), 12 WR stars have been observed, 7 of which appear to be single.
To exist without a binary companion, these stars must have undergone a self-stripping process. Although such a mechanism is theoretically plausible, current models of line-driven stellar winds, combined with the relatively low metallicity of the SMC, fail to fully account for the presence of apparently isolated WR stars. This is primarily due to the reduced strength of stellar winds at low metallicity, which is generally insufficient to remove the hydrogen envelope.
In this thesis, I have implemented a new model for radiation-driven stellar winds within the MESA stellar evolution code. This model includes the possibility of activating optically thick winds when a star approaches the Eddington limit.
I demonstrate that rapidly rotating stars can enter the thick-wind regime, successfully removing their hydrogen envelopes and evolving into WR stars even at low metallicity. The resulting stellar evolutionary tracks were subsequently used as input for the population synthesis code SEVN, to evaluate the impact of the new wind model on binary star populations.
Finally, the output from SEVN was used to estimate the merger efficiency of binary black hole (BBH) systems in this population, with the goal of addressing the persistent overestimation of the BBH merger rate density by theoretical models, compared to that inferred by the LIGO-Virgo-Kagra (LVK) collaboration.
Multiple relevant results have been found. Firstly, optically thick winds are able to reduce the hardening of the binary, therefore lowering the merger efficiency. On the other hand, this effect significantly increases the time delay between formation and black hole merger, which on average becomes about 10 Gyr. This means that the mergers observed today through GWs originate from objects formed at cosmic noon or earlier
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