Marta Volonteri

The Chandra COSMOS Survey, I: Overview and Point Source Catalog

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.5 deg^2 of the COSMOS field (centered at 10 ^h , +02 ^o ) with an effective exposure of ~160 ks, and an outer 0.4 deg^2 area with an effective exposure of ~80 ks. The limiting source detection depths are 1.9 × 10^(–16) erg cm^(–2) s^(–1) in the soft (0.5-2 keV) band, 7.3 × 10^(–16) erg cm^(–2) s^(–1) in the hard (2-10 keV) band, and 5.7 × 10^(–16) erg cm^(–2) s^(–1) in the full (0.5-10 keV) band. Here we describe the strategy, design, and execution of the C-COSMOS survey, and present the catalog of 1761 point sources detected at a probability of being spurious of <2 × 10^(–5) (1655 in the full, 1340 in the soft, and 1017 in the hard bands). By using a grid of 36 heavily (~50%) overlapping pointing positions with the ACIS-I imager, a remarkably uniform (±12%) exposure across the inner 0.5 deg^2 field was obtained, leading to a sharply defined lower flux limit. The widely different point-spread functions obtained in each exposure at each point in the field required a novel source detection method, because of the overlapping tiling strategy, which is described in a companion paper. This method produced reliable sources down to a 7-12 counts, as verified by the resulting logN-logS curve, with subarcsecond positions, enabling optical and infrared identifications of virtually all sources, as reported in a second companion paper. The full catalog is described here in detail and is available online.

Formation of supermassive black holes

Evidence shows that massive black holes reside in most local galaxies. Studies have also established a number of relations between the MBH mass and properties of the host galaxy such as bulge mass and velocity dispersion. These results suggest that central MBHs, while much less massive than the host (~0.1%), are linked to the evolution of galactic structure. In hierarchical cosmologies, a single big galaxy today can be traced back to the stage when it was split up in hundreds of smaller components. Did MBH seeds form with the same efficiency in small proto-galaxies, or did their formation had to await the buildup of substantial galaxies with deeper potential wells? I briefly review here some of the physical processes that are conducive to the evolution of the massive black hole population. I will discuss black hole formation processes for ‘seed’ black holes that are likely to place at early cosmic epochs, and possible observational tests of these scenarios.

Low-frequency gravitational-wave science with eLISA/NGO

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultra-compact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISAs high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

Dancing in the dark: galactic properties trace spin swings along the cosmic web

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the large-scale cosmic filaments above redshift one. The analysis of more than 150 000 galaxies with morphological diversity in a 100h −1 Mpc comoving box size shows that the spin of low-mass, rotationdominated, blue, star-forming galaxies is preferentially aligned with their neighbouring filaments. High-mass, dispersion-dominated, red, quiescent galaxies tend to have a spin perpendicular to nearby filaments. The reorientation of the spin of massive galaxies is provided by galaxy mergers which are significant in the mass build up of high-mass galaxies. We find that the stellar mass transition from alignment to misalignment happens around 3×10 10 M⊙. This is consistent with earlier findings of a dark matter mass transition for the orientation of the spin of halos (5 × 10 11 M⊙ at the same redshift from Codis et al. 2012). With these numerical evidence, we advocate a scenario in which galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the physical and morphological properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as Active Galactic Nuclei is mandatory to quench in situ star formation in massive galaxies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping galaxy spins misaligned with cosmic filaments. It also promotes diversity amongst galaxy properties.

The evolution of massive black hole seeds

We investigate the evolution of high-redshift seed black hole masses at late times and their observational signatures. The massive black hole seeds studied here form at extremely high redshifts from the direct collapse of pre-galactic gas discs. Populating dark matter haloes with seeds formed in this way, we follow the mass assembly of these black holes to the present time using a Monte Carlo merger tree. Using this machinery, we predict the black hole mass function at high redshifts and at the present time, the integrated mass density of black holes and the luminosity function of accreting black holes as a function of redshift. These predictions are made for a set of three seed models with varying black hole formation efficiency. Given the accuracy of present observational constraints, all three models can be adequately fitted. Discrimination between the models appears predominantly at the low-mass end of the present-day black hole mass function which is not observationally well constrained. However, all our models predict that low surface brightness, bulgeless galaxies with large discs are least likely to be sites for the formation of massive seed black holes at high redshifts. The efficiency of seed formation at high redshifts has a direct influence on the black hole occupation fraction in galaxies at z = 0. This effect is more pronounced for low-mass galaxies. This is the key discriminant between the models studied here and the Population III remnant seed model. We find that there exist a population of low-mass galaxies that do not host nuclear black holes. Our prediction of the shape of the M BH -σ relation at the low-mass end is in agreement with the recent observational determination from the census of low-mass galaxies in the Virgo cluster.

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 cosmologicalblackholespinevolution.Weshowthatmergersareveryunlikelytoyieldlargespins,unlessalignment 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, and (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 ring-down waves), and the spin of ‘‘single’’ black holes during the extrememass-ratioinspiral(EMRI)of compactobjects.Thelatterspinpopulationisalsoaccessibletoiron-linemeasurements. 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 ¼ 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. Subject headingg black hole physics — cosmology: theory — galaxies: evolution — gravitational waves Online material: color figures

Quasars at z = 6: The Survival of the Fittest

The Sloan Digital Sky Survey has detected luminous quasars at very high redshift, z > 6. Follow-up observations indicate that at least some of these quasars are powered by supermassive black holes (SMBHs), with masses in excess of 109 M☉. SMBHs, therefore, seem to have already existed when the universe was less than 1 Gyr old and the bulk of galaxy formation had yet to take place. Here we investigate the extent to which accretion and dynamical processes influence the early growth of SMBHs. We assess the impact of (1) black hole mergers, (2) the influence of the merger efficiency, and (3) the negative contribution due to dynamical effects, which can kick black holes out of their host halos (gravitational recoil). We find that if accretion is always limited by the Eddington rate via a thin disk, the maximum allowed radiative efficiency (or spin) to reproduce the luminosity function at z = 6 is = 0.12 (or = 0.8), against the adverse effect of the gravitational recoil. Dynamical effects unquestionably cannot be neglected in studies of high-redshift SMBHs. If black holes can accrete at a supercritical rate during an early phase, reproducing the observed SMBH mass values is not an issue, even in the case that the recoil velocity is in the upper limit range, as the mass ratios of merging binaries are skewed toward low values, where the gravitational recoil effect is very mild. We propose that SMBH growth at early times is very selective, and efficient only for black holes hosted in high density peak halos.

Formation of the First Nuclear Clusters and Massive Black Holes at High Redshift

We present a model for the formation of massive black holes (~1000 M ☉) due to stellar-dynamical processes in the first stellar clusters formed at early cosmic times (z ~ 10-20). These black holes are likely candidates as seeds for the supermassive black holes detected in quasars and nearby quiescent galaxies. The high redshift black hole seeds form as a result of multiple successive instabilities that occur in low metallicity (Z ~ 10–5 Z ☉) protogalaxies. We focus on relatively massive halos at high redshift (T vir > 104 K, z 10) after the very first stars in the universe have completed their evolution. This set of assumptions ensures that (1) atomic hydrogen cooling can contribute to the gas cooling process, (2) a UV field has been created by the first stars, and (3) the gas inside the halo has been mildly polluted by the first metals. The second condition implies that at low density H 2 is dissociated and does not contribute to cooling. The third condition sets a minimum threshold density for fragmentation, so that stars form efficiently only in the very inner core of the protogalaxy. Within this core, very compact stellar clusters form. The typical star cluster masses are of order 105 M ☉ and the typical half mass radii ~1 pc. A large fraction of these very dense clusters undergoes core collapse before stars are able to complete stellar evolution. Runaway star-star collisions eventually lead to the formation of a very massive star, leaving behind a massive black hole remnant. Clusters unstable to runaway collisions are always the first, less massive ones that form. As the metallicity of the universe increases, the critical density for fragmentation decreases and stars start to form in the entire protogalactic disk so that (1) accretion of gas in the center is no longer efficient and (2) the core collapse timescale increases. Typically, a fraction ~0.05 of protogalaxies at z ~ 10-20 form black hole seeds, with masses ~1000-2000 M ☉, leading to a mass density in seeds of a few 102 M ☉/Mpc–3. This density allows enough room for black hole growth by accretion during the quasar epoch.

Gravitational waves from resolvable massive black hole binary systems and observations with Pulsar Timing Arrays

Massive black holes are key components of the assembly and evolution of cosmic structures, and a number of surveys are currently on going or planned to probe the demographics of these objects and to gain insight into the relevant physical processes. Pulsar Timing Arrays (PTAs) currently provide the only means to observe gravitational radiation from massive black hole binary systems with masses 10 7 M� . The whole cosmic population produces a stochastic background that could be detectable with upcoming PTAs. Sources sufficiently close and/or massive generate gravitational radiation that significantly exceeds the level of the background and could be individually resolved. We consider a wide range of massive black hole binary assembly scenarios, investigate the distribution of the main physical parameters of the sources, such as masses and redshift, and explore the consequences for PTAs observations. Depending on the specific massive black hole population model, we estimate that on average at least one resolvable source produces timing residuals in the range ∼5–50 ns. PTAs, and in particular the future Square Kilometre Array, can plausibly detect these unique systems, although the events are likely to be rare. These observations would naturally complement on the high-mass end of the massive black hole distribution function future surveys carried out by the Laser Interferometer Space Antenna.

Black Hole Spin and Galactic Morphology

We investigate the conjecture by Sikora, Stawarz, and Lasota that the observed active galactic nuclei (AGNs) radio loudness bimodality can be explained by the morphology-related bimodality of black hole spin distribution in the centers of galaxies: central black holes (BHs) in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disk galaxies. We study how accretion from a warped disk 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.