Cosmological evolution of supermassive black holes and AGN: a synthesis model for accretion and feedback

The growth of supermassive black holes (SMBH) through accretion is accompanied by the release of enormous amounts of energy which can either be radiated away, as happens in quasars, advected into the black hole, or disposed of in kinetic form through powerful jets, as is observed, for example, in radio galaxies. Here, I will present new constraints on the evolution of the SMBH mass function and Eddington ratio distribution, obtained from a study of AGN luminosity functions aimed at accounting for both radiative and kinetic energy output of AGN in a systematic way. First, I discuss how a refined Soltan argument leads to joint constraints on the mass-weighted average spin of SMBH and of the total mass density of high redshift (z~5) and "wandering" black holes. Then, I will show how to describe the downsizing trend observed in the AGN population in terms of cosmological evolution of physical quantities (black hole mass, accretion rate, radiative and kinetic energy output). Finally, the redshift evolution of the AGN kinetic feedback will be briefly discussed and compared with the radiative output of the evolving SMBH population, thus providing a robust physical framework for phenomenological models of AGN feedback within structure formation.Comment: Proceedings of "The Central Kiloparsec: Active Galactic Nuclei and Their Hosts", Ierapetra, Crete, 4-6 June, 2008. To appear in Volume 79 of the Memorie della Societa' Astronomica Italiana. 8 pages, 4 figure

The fundamental plane of black hole activity and the census of the local black holes' population

Studying a sample of both strongly and weakly active galactic nuclei with measured masses and 5 GHz and 2-10 keV core luminosities, together with a few galactic black holes simultaneously observed in the radio and X-ray bands, Merloni, Heinz, & Di Matteo (2003) showed that the sources are correlated through a ``fundamental plane'' relationship in the three-dimensional (log L_R, log L_X, log M) space. Here I elaborate on how such a relationship can be used to infer directly mass and accretion rate of any black hole given its radio and X-ray fluxes, complementing the information obtained from optical/UV surveys. As an example, I show how the local X-ray and radio luminosity functions, coupled with the black hole mass function derived from the M-\sigma relation, provide us with an accretion rate function. It is found that the typical X-ray Eddington ratio of an active black hole at redshift zero is about 5 \times 10^{-4}.Comment: 4 pages, 2 figures. Proceedings of the Princeton meeting on "AGN Physics with the Sloan Digital Sky Survey" (July 2003), ed. G. T. Richards and P. B. Hall, ASP Conf. Series, in pres

A synthetic view of AGN evolution and supermassive black holes growth

I will describe the constraints available from a study of AGN evolution synthesis models on the growth of the supermassive black holes (SMBH) population in the two main 'modes' observed (kinetic- and radiatively-dominated, respectively). I will show how SMBH mass function evolves anti-hierarchically, i.e. the most massive holes grew earlier and faster than less massive ones, and I will also derive tight constraints on the average radiative efficiency of AGN. An outlook on the redshift evolution of the AGN kinetic luminosity function will also be discussed, thus providing a robust physical framework for phenomenological models of AGN feedback within structure formation. Finally, I will present new constraints on the evolution of the black hole-galaxy scaling relation at 1<z<2 derived by exploiting the full multi-wavelength coverage of the COSMOS survey on a complete sample of 90 type 1 AGN.Comment: 8 pages, 5 color figures, Proceedings of the conference "Accretion and ejection in AGN: a global view" (Como, 22-26 June 2009

Mass Functions of Supermassive Black Holes Across Cosmic Time

The black hole mass function of supermassive black holes describes the evolution of the distribution of black hole mass. It is one of the primary empirical tools available for mapping the growth of supermassive black holes and for constraining theoretical models of their evolution. In this review we discuss methods for estimating the black hole mass function, including their advantages and disadvantages. We also review the results of using these methods for estimating the mass function of both active and inactive black holes. In addition, we review current theoretical models for the growth of supermassive black holes that predict the black hole mass function. We conclude with a discussion of directions for future research which will lead to improvement in both empirical and theoretical determinations of the mass function of supermassive black holes.Comment: 40 pages, 7 figures, review paper accepted for the Advances in Astronomy Special Issue "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes

Quiescent times in gamma-ray bursts: I. An observed correlation between the durations of subsequent emission episodes

Although more than 2000 astronomical gamma-ray bursts (GRBs) have been detected, the precise progenitor responsible for these events is unknown. The temporal phenomenology observed in GRBs can significantly constrain the different models. Here we analyse the time histories of a sample of bright, long GRBs, searching for the ones exhibiting relatively long (more than 5 per cent of the total burst duration) quiescent times, defined as the intervals between adjacent episodes of emission during which the gamma-rays count rate drops to the background level. We find a quantitative relation between the duration of an emission episode and the quiescent time elapsed since the previous episode. We suggest here that the mechanism responsible for the extraction and the dissipation of energy has to take place in a meta-stable configuration, such that the longer the accumulation period, the higher is the stored energy available for the next emission episode.Comment: 5 pages, 3 figures, with final revision

Thunderclouds and accretion discs: a model for the spectral and temporal variability of Seyfert 1 galaxies

X-ray observations of Seyfert 1 galaxies offer the unique possibility of observing spectral variability on timescales comparable to the dynamical time of the inner accretion flow. They typically show highly variable lightcurves, with Power Density Spectra characterized by `red noise' and a break at low frequencies. Time resolved spectral analysis have established that spectral variability on the shortest timescales is important in all these sources, with the spectra getting softer at high fluxes. Here we present a model that is able to explain a number of the above mentioned properties in terms of magnetic flares shining above a standard accretion disc and producing the X-ray spectrum via inverse Compton scattering soft photons (both intrinsic and reprocessed thermal emission from the accretion disc and locally produced synchrotron radiation). We show that the fundamental heating event, likely caused by magnetic reconnection, must be compact, with typical size comparable to the accretion disc thickness and must be triggered at a height at least an order of magnitude larger than its size; the spatial and temporal distribution of flares are not random: the heating of the corona proceeds in correlated trains of events in an avalanche fashion. The amplitude of the avalanches obeys a power-law distribution and determines the size of the active regions where the spectrum is produced. With our model we simulate X-ray lightcurves that reproduce the main observational properties of the Power Density Spectra and of the X-ray continuum short-term variability of Seyfert 1 galaxies. By comparing them with observations of MGC--6-30-15, we are able to infer that the corona in this source must have a large optical depth (tau >1.5) and small average covering fraction.Comment: 12 pages, 8 figures, accepted for publication in MNRA

AGN spectral states from simultaneous UV and X-ray observations by XMM-Newton

The supermassive black holes in active galactic nuclei (AGN) and stellar-mass black holes in X-ray binaries (XRBs) are believed to work in a similar way. While XRBs evolve rapidly and several sources have undergone a few complete cycles from quiescence to an outburst and back, most AGN remain in the same state over periods of decades, due to their longer characteristic timescale proportional to their size. However, the study of the AGN spectral states is still possible with a large sample of sources. Multi-wavelength observations are needed for this purpose since the AGN thermal disc emission dominates in the ultraviolet energy range, while the up-scattered hot-corona emission is detected in X-rays. We compared simultaneous UV and X-ray measurements of AGN obtained by the XMM-Newton satellite. The non-thermal flux was constrained from the 2-12 keV X-ray luminosity, while the thermal disc component was estimated from the UV flux at 2900A. The hardness (ratio between the X-ray and UV plus X-ray luminosity) and the total luminosity were used to construct the AGN state diagrams. For sources with reliable mass measurements, the Eddington ratio was used instead of the total luminosity. The state diagrams show that the radio-loud sources have on average higher hardness, due to the lack of the thermal disc emission in the UV band, and have flatter intrinsic X-ray spectra. In contrast, the sources with high luminosity and low hardness are radio-quiet AGN. The hardness-Eddington ratio diagram reveals that the average radio-loudness is stronger for low-accreting sources, while it decreases when the accretion rate is close to the Eddington limit. Our results indicate that the general properties of AGN accretion states are similar to those of X-ray binaries. This suggests that the AGN radio dichotomy of radio-loud and radio-quiet sources can be explained by the evolution of the accretion states.Comment: 13 pages, 12 figures, accepted in A&

Linking the fate of massive black hole binaries to the active galactic nuclei luminosity function

Massive black hole binaries are naturally predicted in the context of the hierarchical model of structure formation. The binaries that manage to lose most of their angular momentum can coalesce to form a single remnant. In the last stages of this process, the holes undergo an extremely loud phase of gravitational wave emission, possibly detectable by current and future probes. The theoretical effort towards obtaining a coherent physical picture of the binary path down to coalescence is still underway. In this paper, for the first time, we take advantage of observational studies of active galactic nuclei evolution to constrain the efficiency of gas-driven binary decay. Under conservative assumptions we find that gas accretion toward the nuclear black holes can efficiently lead binaries of any mass forming at high redshift (> 2) to coalescence within the current time. The observed "downsizing" trend of the accreting black hole luminosity function further implies that the gas inflow is sufficient to drive light black holes down to coalescence, even if they bind in binaries at lower redshifts, down to z~0.5 for binaries of ~10 million solar masses, and z~0.2 for binaries of ~1 million solar masses. This has strong implications for the detection rates of coalescing black hole binaries of future space-based gravitational wave experiments.Comment: 6 pages, 3 figure, accepted for publication in MNRA

Jet-Disc coupling in the accreting black hole XTEJ1118+480

We interpret the rapid correlated UV/optical/ X-ray variability of XTEJ1118+480 as a signature of the coupling between the X-ray corona and a jet emitting synchrotron radiation in the optical band.We propose a scenario in which the jet and the X-ray corona are fed by the same energy reservoir where large amounts of accretion power are stored before being channelled into either the jet or the high energy radiation. This time dependent model reproduces the main features of the rapid multi-wavelength variability of XTEJ1118+480. A strong requirement of the model is that the total jet power should be at least a few times larger than the observed X-ray luminosity. This would be consistent with the overall low radiative efficiency of the source. We present independent arguments showing that the jet probably dominates the energetic output of all accreting black holes in the low-hard state.Comment: 8 pages, 1 figure, to appear in the proceedings of "From X-ray binaries to quasars: Black hole accretion on all mass scales, (Amsterdam, July 2004)", Eds. T. Maccarone, R. Fender, L. H