arXiv subjects

Marta Volonteri

Publications and source records attributed to Marta Volonteri.

At least 235 records · Page 13Linked to original sources

arXiv2008

Cosmological black hole spin evolution by mergers and accretion

Using recent results from numerical relativity simulations of black hole mergers, we revisit previous studies of cosmological black hole spin evolution. We show that mergers are very unlikely to yield large spins, unless alignment of the spins of the merging holes with the orbital angular momentum is very efficient. We analyze the spin evolution in three specific scenarios: (1) spin evolves only through mergers, (2) spin evolves through mergers and prolonged accretion episodes, (3) spin evolves through mergers and short-lived (chaotic) accretion episodes. We study how different diagnostics can distinguish between these evolutionary scenarios, assessing the discriminating power of gravitational-wave measurements and X-ray spectroscopy. Gravitational radiation can produce three different types of spin measurements, yielding respectively the spins of the two black holes in a binary inspiral prior to merger, the spin of the merger remnant (as encoded in the ringdown waves), and the spin of ``single'' black holes during the extreme mass-ratio inspiral (EMRI) of compact objects. The latter spin population is also accessible to iron-line measurements. We compute and compare the spin distributions relevant for these different observations. If iron-line measurements and gravitational-wave observations of EMRIs only yield dimensionless spins j=J/M^2>0.9, then prolonged accretion should be responsible for spin-up, and chaotic accretion scenarios would be very unlikely. If only a fraction of the whole population of low-redshift black holes spins rapidly, spin-alignment during binary mergers (rather than prolonged accretion) could be responsible for spin-ups.

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arXiv2008

Compact massive objects in Virgo galaxies: the black hole population

We investigate the distribution of massive black holes (MBHs) in the Virgo cluster. Observations suggest that AGN activity is widespread in massive galaxies (M>1e10 solar masses), while at lower galaxy masses star clusters are more abundant, which might imply a limited presence of central black holes in these galaxy-mass regimes. We explore if this possible threshold in MBH hosting, is linked to nature, nurture, or a mixture of both. The nature scenario arises naturally in hierarchical cosmologies, as MBH formation mechanisms typically are efficient in biased systems, which would later evolve into massive galaxies. Nurture, in the guise of MBH ejections following MBH mergers, provides an additional mechanism that is more effective for low mass, satellite galaxies. The combination of inefficient formation, and lower retention of MBHs, leads to the natural explanation of the distribution of compact massive ob jects in Virgo galaxies. If MBHs arrive to the correlation with the host mass and velocity dispersion during merger-triggered accretion episodes, sustained tidal stripping of the host galaxies creates a population of MBHs which lie above the expected scaling between the holes and their host mass, suggesting a possible environmental dependence.

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arXiv2007

Evolution of massive black holes

Supermassive black holes are nowadays believed to reside in most local galaxies. Accretion of gas and black hole mergers play a fundamental role in determining the two parameters defining a black hole: mass and spin. I briefly review here some of the physical processes that are conducive to the evolution of the massive black hole population. I'll discuss black hole formation processes that are likely to place at early cosmic epochs, and how massive black hole evolve in a hierarchical Universe. The mass of the black holes that we detect today in nearby galaxy has mostly been accumulated by accretion of gas. While black hole--black hole mergers do not contribute substantially to the final mass of massive black holes, they influence the occupancy of galaxy centers by black hole, owing to the chance of merging black holes being kicked from their dwellings due to the gravitational recoil. Similarly, accretion leaves a deeper imprint on the distribution of black hole spins than black hole mergers do. The differences in accretion histories for black holes hosted in elliptical or disc galaxies may reflect on different spin distributions.

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arXiv2007

Black-Hole Spin and Galactic Morphology

We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN--radio-loudness bimodality can be explained by the morphology-related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped disc influences the evolution of black hole spins and conclude that within the cosmological framework, where the most massive BHs have grown in mass via merger driven accretion, one indeed expects most supermassive black holes in elliptical galaxies to have on average higher spin than black holes in spiral galaxies, where random, small accretion episodes (e.g. tidally disrupted stars, accretion of molecular clouds) might have played a more important role.

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arXiv2007

Gravitational recoil: signatures on the massive black hole population

In the last stages of a black hole merger, the binary can experience a recoil due to asymmetric emission of gravitational radiation. Recent numerical relativity simulations suggest that the recoil velocity can be as high as a few thousands kilometers per second for particular configurations. We consider here the effect of a worst case scenario for orbital and phase configurations on the hierarchical evolution of the massive black hole (MBH) population. The orbital configuration and spin orientation in the plane is chosen to be the one yielding the highest possible kick. Masses and spin magnitudes are instead derived self-consistently from the MBH evolutionary models. If seeds form early, e.g. as remnants of the first stars, almost the totality of the first few generation of binaries are ejected. The fraction of lost binaries decreases at later times due to a combination of the binary mass ratio distribution becoming shallower, and the deepening of the hosts potential wells. If seeds form at later times, in more massive halos, the retention rate is much higher. We show that the gravitational recoil does not pose a threat to the evolution of the MBH population that we observe locally in either case, although high mass seeds seem to be favored. The gravitational recoil is instead a real hazard for (i) MBHs in biased halos at high-redshift, where mergers are more common, and the potential wells still relatively shallow. Similarly, it is very challenging to retain (ii) MBHs merging in star clusters.

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arXiv2007

The imprint of massive black hole formation models on the LISA data stream

The formation, merging, and accretion history of massive black holes along the hierarchical build--up of cosmic structures leaves a unique imprint on the background of gravitational waves at mHz frequencies. We study here, by means of dedicated simulations of black hole build--up, the possibility of constraining different models of black hole cosmic evolution using future gravitational wave space--borne missions, such as LISA. We consider two main scenarios for black hole formation, namely, one where seeds are light (~10^2 \msun, remnant of Population III stars), and one where seeds are heavy (>~10^4 \msun, direct collapse). In all the models we have investigated, massive black hole binary coalescences do not produce a stochastic GW background, but rather, a set of individual resolved events. Detection of several hundreds merging events in 3 year LISA mission will be the sign of a heavy seed scenario with efficient formation of black hole seeds in a large fraction of high redshift halos. On the other extreme, a low event rate, about few tens in 3 years, is peculiar of scenarios where either the seeds are light, and many coalescences do not fall into the LISA band, or seeds are massive, but rare. In this case a decisive diagnostic is provided by the shape of the mass distribution of detected events. Light binaries (m<10^4\msun) are predicted in a fairly large number in Population III remnants models, but are totally absent in direct collapse models. Finally, a further, helpful diagnostic of black hole formation models lies in the distribution of the mass ratios in binary coalescences. While heavy seed models predict that most of the detected events involve equal mass binaries, in the case of light seeds, mass ratios are equally distributed in the range 0.1-1.

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arXiv2007

Massive Black Holes: formation and evolution

Supermassive black holes are nowadays believed to reside in most local galaxies. Observations have revealed us vast information on the population of local and distant black holes, but the detailed physical properties of these dark massive objects are still to be proven. Accretion of gas and black hole mergers play a fundamental role in determining the two parameters defining a black hole: mass and spin. We briefly review here the basic properties of the population of supermassive black holes, focusing on the still mysterious formation of the first massive black holes, and their evolution from early times to now.

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arXiv2006

Constraints on the Accretion History of Massive Black Holes from Faint X-ray Counts

We investigate how hierarchical models for the co-evolution of the massive black hole (MBH) and AGN population can reproduce the observed faint X-ray counts. We find that the main variable influencing the theoretical predictions is the Eddington ratio of accreting sources. We compare three different models proposed for the evolution of AGN Eddington ratio, f_Edd: constant f_Edd=1, f_Edd decreasing with redshift, and f_Edd depending on the AGN luminosity, as suggested by simulations of galactic mergers including BHs and AGN feedback. We follow the full assembly of MBHs and host halos from early times to the present in a LambdaCDM cosmology. AGN activity is triggered by halo major mergers and MBHs accrete mass until they satisfy the observed correlation with velocity dispersion. We find that all three models can reproduce fairly well the total faint X-ray counts. The redshift distribution is however poorly matched in the first two models. The Eddington ratios suggested by merger simulations predicts no turn-off of the faint end of the AGN optical luminosity function at redshifts z>=1 down to very low luminosity.

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arXiv2006

Supermassive black hole mergers and cosmological structure formation

Massive black holes (MBHs), with masses in the range 10^3-10^8 Msolar, which merge with a companion black hole of similar mass are expected to be the most powerful source of gravitational radiation in the frequency range probed by LISA. MBH binaries can be detected by LISA up to z~5-15. Gravitational waves from MBH mergers can serve as a powerful tool to study the early evolution of the MBH population, and possibly the role played by MBHs in the joint evolution with their hosts. I review scenarios for the co-evolution of MBHs and cosmological structures, where MBH seeds form in pre-galactic structures. These black holes evolve then in a hierarchical fashion, following the merger hierarchy of their hosts. Accretion of gas, traced by quasar activity, plays a fundamental role in determining the two parameters defining a black hole, mass and spin. Gravitational waves, together with observations in electromagnetic bands, can help constrain the evolution of both MBH mass and spin.

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arXiv2006

Formation of Supermassive Black Holes by Direct Collapse in Pregalactic Halos

We describe a mechanism by which supermassive black holes can form directly in the nuclei of protogalaxies, without the need for seed black holes left over from early star formation. Self-gravitating gas in dark matter halos can lose angular momentum rapidly via runaway, global dynamical instabilities, the so-called "bars within bars" mechanism. This leads to the rapid buildup of a dense, self-gravitating core supported by gas pressure - surrounded by a radiation pressure-dominated envelope - which gradually contracts and is compressed further by subsequent infall. These conditions lead to such high temperatures in the central region that the gas cools catastrophically by thermal neutrino emission, leading to the formation and rapid growth of a central black hole. We estimate the initial mass and growth rate of the black hole for typical conditions in metal-free halos with T_vir ~ 10^4 K, which are the most likely to be susceptible to runaway infall. The initial black hole should have a mass of <~20 solar masses, but in principle could grow at a super-Eddington rate until it reaches ~ 10^4-10^6 solar masses. Rapid growth may be limited by feedback from the accretion process and/or disruption of the mass supply by star formation or halo mergers. Even if super-Eddington growth stops at \~10^3-10^4 solar masses, this process would give black holes ample time to attain quasar-size masses by a redshift of 6, and could also provide the seeds for all supermassive black holes seen in the present universe.

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arXiv2006

Evolution of supermassive black holes

Supermassive black holes (SMBHs) are nowadays believed to reside in most local galaxies, and the available data show an empirical correlation between bulge luminosity - or stellar velocity dispersion - and black hole mass, suggesting a single mechanism for assembling black holes and forming spheroids in galaxy halos. The evidence is therefore in favour of a co-evolution between galaxies, black holes and quasars. In cold dark matter cosmogonies, small-mass subgalactic systems form first to merge later into larger and larger structures. In this paradigm galaxy halos experience multiple mergers during their lifetime. If every galaxy with a bulge hosts a SMBH in its center, and a local galaxy has been made up by multiple mergers, then a black hole binary is a natural evolutionary stage. The evolution of the supermassive black hole population clearly has to be investigated taking into account both the cosmological framework and the dynamical evolution of SMBHs and their hosts. The seeds of SMBHs have to be looked for in the early Universe, as very luminous quasars are detected up to redshift higher than z=6. These black holes evolve then in a hierarchical fashion, following the merger hierarchy of their host halos. Accretion of gas, traced by quasar activity, plays a fundamental role in determining the two parameters defining a black hole: mass and spin. A particularly intriguing epoch is the initial phase of SMBH growth. It is very challenging to meet the observational constraints at z=6 if BHs are not fed at very high rates in their infancy.

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arXiv2005

Rapid growth of high redshift black holes

We discuss a model for the early assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark halo `merger tree'. Motivated by the observations of luminous quasars around redshift z=6 with SMBH masses of billion solar masses, we assess the possibility of an early phase of stable super-critical quasi-spherical accretion in the BHs hosted by metal free halos with virial temperature larger than 10000 K. We assume that the first `seed' black holes formed with intermediate masses following the collapse of the first generation of stars, in mini-halos collapsing at z=20-30 from high peaks of density fluctuations. In high redshift halos with virial temperature larger than 10000 K, conditions exist for the formation of a fat disc of gas at T_gas=5000-10000 K. Cooling via hydrogen atomic lines is in fact effective in these comparatively massive halos. The cooling and collapse of an initially spherical configuration of gas leads to a rotationally supported disc at the center of the halo if baryons preserve their specific angular momentum during collapse. The conditions for the formation of the gas disc and accretion onto a central black holes out of this supply of gas are investigated, as well as the feedback of the emission onto the host and onto the intergalactic medium. We find that even a short phase of supercritical accretion eases the requirements set by the z=6 quasars.

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arXiv2005

Dynamical evolution of intermediate mass black holes and their observable signatures in the nearby Universe

We investigate the consequences of a model of the assembly and growth of massive black holes from primordial seeds, remnants of the first generation of stars in a hierarchical structure formation scenario. Our model traces the build-up of MBHs from an early epoch, and follows the merger history of dark matter halos and their associated holes via Monte Carlo realizations of the merger hierarchy from early times to the present time. The sequence of minor and major mergers experienced by galactic halos in their hierarchical growth affects the merger history of MBHs embedded in their nuclei. So, if the formation route for the assembly of SMBHs dates back to the early universe, a large number of BH interactions is inevitable. Binary black holes coalescence timescales can be long enough for a third BH to fall in and interact with the central binary. These BH triple interactions lead typically to the final expulsion of one of the three bodies and to the recoil of the binary. Also, asymmetric emission of gravitational waves in the last stages of the black hole merging can give a recoil velocity to the centre of mass of the coalescing binary. This scenario leads to the prediction of a population of intermediate mass BHs (IMBHs) wandering in galaxy halos at the present epoch. We compute the luminosity distribution produced by these IMBHs accreting from their circumstellar medium. We find that in a Milky Way-sized galaxy they are unable to account for sources with luminosities > 1e39 erg/s unless they carry a baryonic remnant from which they are able to accrete for a long time. We also find that, for typical spiral galaxies, the bright end of the point source distribution correlates with the mass of the galaxy, and the most luminous sources are expected to be found in the disk.

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arXiv2004

The distribution and cosmic evolution of massive black hole spins

We study the expected distribution of massive black hole (MBH) spins and its evolution with cosmic time in the context of hierarchical galaxy formation theories. Our model uses Monte Carlo realizations of the merger hierarchy in a LCDM cosmology, coupled to semi-analytical recipes, to follow the merger history of dark matter halos, the dynamics of the MBHs they host, and their growth via gas accretion and binary coalescences. The coalescence of comparable mass holes increases the spin of MBHs, while the capture of smaller companions in randomly-oriented orbits acts to spin holes down. We find that, given the distribution of MBH binary mass ratios in hierarchical models, binary coalescences alone do not lead to a systematic spin-up or spin-down of MBHs with time: the spin distribution retains memory of its initial conditions. By contrast, because of the Bardeen-Petterson effect, gas accretion via a thin disk tends to spin holes up even if the direction of the spin axis changes randomly in time. In our models, accretion dominates over black hole captures and efficiently spins holes up. The spin distribution is heavily skewed towards fast-rotating Kerr holes, is already in place at early epochs, and does not change much below redshift 5. If accretion is via a thin disk, about 70% of all MBHs are maximally rotating and have radiative efficiencies approaching 30% (assuming a "standard'' spin-efficiency conversion). Even in the conservative case where accretion is via a geometrically thick disk, about 80% of all MBHs have spin parameters a/m > 0.8 and accretion efficiencies > 12%. Rapidly spinning holes with high radiative efficiencies may satisfy constraints based on comparing the local MBH mass density with the mass density inferred from luminous quasars (Soltan's argument).

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arXiv2004

Low-frequency gravitational radiation from coalescing massive black hole binaries in hierarchical cosmologies

We compute the expected gravitational wave signal from coalescing massive black hole (MBH) binaries at the center of galaxies in a hierarchical structure formation scenario in which seed holes of intermediate mass form far up in the dark halo merger tree. The merger history of DM halos and MBHs is followed from z=20 to the present in a LCDM cosmology. MBHs get incorporated through halo mergers into larger and larger structures, sink to the center owing to dynamical friction against the DM background, accrete cold material in the merger remnant, and form MBH binary systems. Stellar dynamical interactions cause the hardening of the binary at large separations, while gravitational wave emission takes over at small radii and leads to the final coalescence of the pair. The integrated emission from inspiraling MBH binaries results in a gravitational wave background (GWB). The characteristic strain spectrum has the standard h_c(f)\propto f^{-2/3} behavior only in the range 1E-9 1E-6 Hz, the strain amplitude is shaped by the convolution of last stable circular orbit emission. We discuss the observability of inspiraling MBH binaries by the planned LISA. Over a 3-year observing period LISA should resolve this GWB into discrete sources, detecting ~60 (~250) individual events above a S/N=5 (S/N=1) confidence level. (Abridged)

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arXiv2003

Early reionization by miniquasars

Motivated by the recent detection by WMAP of a large optical depth to Thomson scattering -- implying a very early reionization epoch -- we assess a scenario where the universe was reionized by `miniquasars' powered by intermediate-mass black holes (IMBHs), the remnants of the first generation of massive stars. Pregalactic IMBHs form within minihalos above the cosmological Jeans mass collapsing at z=24, get incorporated through mergers into larger and larger systems, sink to the center owing to dynamical friction, and accrete cold material. The merger history of dark halos and associated IMBHs is followed by Monte Carlo realizations of the merger hierarchy in a LCDM cosmology. While seed IMBHs that are as rare as the 3.5-sigma peaks of the primordial density field evolve largely in isolation, a significant number of black hole binary systems will form if IMBHs populate the more numerous 3-sigma peaks instead. In the case of rapid binary coalescence a fraction of IMBHs will be displaced from galaxy centers and ejected into the IGM by the `gravitational rocket' effect, rather than accrete and shine as miniquasars. We show that, under a number of plausible assumptions for the amount of gas accreted onto IMBHs and their emission spectrum, miniquasars powered by IMBHs may be responsible for cosmological reionization at z~15. Reionization by miniquasars with a hard spectrum may be more `economical' than stellar reionization, as soft X-rays escape more easily from the dense sites of star formation and travel further than EUV radiation. Energetic photons will make the diffuse IGM warm and weakly ionized prior to the epoch of reionization breakthrough, set an entropy floor, and reduce gas clumping. Future 21 cm observations may detect a preheated, weakly-ionized IGM in emission against the CMB. (abridged)

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arXiv2003

The formation of galaxy stellar cores by the hierarchical merging of supermassive black holes

We investigate a hierarchical structure formation scenario in which galaxy stellar cores are created from the binding energy liberated by shrinking supermassive black hole (SMBH) binaries. The binary orbital decay heats the surrounding stars, eroding a preexisting 1/r^2 stellar cusp. We follow the merger history of dark matter halos and associated SMBHs via cosmological Monte Carlo realizations of the merger hierarchy from early times to the present in a LCDM cosmology. Massive black holes get incorporated through a series of mergers into larger and larger halos, sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, and form a binary system. Stellar dynamical processes drive the binary to harden and eventually coalesce. A simple scheme is applied in which the loss cone is constantly refilled and a constant density core forms due to the ejection of stellar mass. We find that a model in which the effect of the hierarchy of SMBH interactions is cumulative and cores are preserved during galaxy mergers produces at the present epoch a correlation between the `mass deficit' (the mass needed to bring a flat inner density profile to a 1/r^2 cusp) and the mass of the nuclear SMBH, with a normalization and slope comparable to the observed relation. Models in which the mass displaced by the SMBH binary is replenished after every major galaxy merger appear instead to underestimate the mass deficit observed in `core' galaxies.

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arXiv2002

The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation

We assess models for the assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark halo `merger tree'. We assume that the first `seed' black holes (BHs) formed in (mini)halos collapsing at z=20 from high-sigma density fluctuations. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. The merger history of dark matter halos and associated BHs is followed by cosmological Monte Carlo realizations of the merger hierarchy. A simple model, where quasar activity is driven by major mergers and SMBHs accrete at the Eddington rate a mass that scales with the fifth power of the velocity dispersion, is shown to reproduce the optical LF of quasars in the redshift range 1<z<4. Binary and triple BH interactions are followed in our merger tree. The assumptions underlying our scenario lead to the prediction of a population of massive BHs wandering in galaxy halos and the intergalactic medium at the present epoch, and contributing <10% to the total BH mass density. At all epochs the fraction of binary SMBHs in galaxy nuclei is of order 10%, while the fraction of binary quasars is less than 0.3%

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