Radiative transfer in a clumpy universe: IV. New synthesis models of the cosmic UV/X-ray background

We present improved synthesis models of the evolving spectrum of the UV/X-ray diffuse background, updating and extending our previous results. Five new main components are added to our radiative transfer code CUBA: (1) the sawtooth modulation of the background intensity from resonant line absorption in the Lyman series of cosmic hydrogen and helium; (2) the X-ray emission from obscured and unobscured quasars; (3) a piecewise parameterization of the distribution in redshift and column density of intergalactic absorbers that fits recent measurements of the mean free path of 1 ryd photons; (4) an accurate treatment of the photoionization structure of absorbers; and (5) the UV emission from star-forming galaxies at all redshifts. We provide tables of the predicted HI and HeII photoionization and photoheating rates for use, e.g., in cosmological hydrodynamics simulations of the Lya forest, and a new metallicity-dependent calibration to the UV luminosity density-star formation rate density relation. A "minimal cosmic reionization model" is also presented in which the galaxy UV emissivity traces recent determinations of the cosmic history of star formation, the luminosity-weighted escape fraction of hydrogen-ionizing radiation increases rapidly with lookback time, the clumping factor of the high-redshift intergalactic medium evolves following the results of recent hydrodynamic simulations, and Population III stars and miniquasars make a negligible contribution to the metagalactic flux. The model provides a good fit to the hydrogen-ionization rates inferred from flux decrement and proximity effect measurements, predicts that cosmological HII (HeIII) regions overlap at redshift 6.7 (2.8), and yields an optical depth to Thomson scattering that is in agreement with WMAP results. (Abridged)Comment: 28 pages, 17 figures, accepted for publication in The Astrophysical Journa

Cosmic Reionization after Planck: Could Quasars Do It All?

We assess a model of late cosmic reionization in which the ionizing background radiation arises entirely from high redshift quasars and other active galactic nuclei (AGNs). The low optical depth to Thomson scattering reported by the Planck Collaboration pushes the redshift of instantaneous reionization down to z=8.8^{+1.7}_{-1.4} and greatly reduces the need for significant Lyman-continuum emission at very early times. We show that, if recent claims of a numerous population of faint AGNs at z=4-6 are upheld, and the high inferred AGN comoving emissivity at these epochs persists to higher, z~10, redshifts, then active galaxies may drive the reionization of hydrogen and helium with little contribution from normal star-forming galaxies. We discuss an AGN-dominated scenario that satisfies a number of observational constraints: the HI photoionization rate is relatively flat over the range 2<z<5, hydrogen gets fully reionized by z=5.7, and the integrated Thomson scattering optical depth is tau=0.056, in agreement with measurements based on the Lya opacity of the intergalactic medium (IGM) and cosmic microwave background (CMB) polarization. It is a prediction of the model that helium gets doubly reionized before redshift 4, the heat input from helium reionization dominates the thermal balance of the IGM after hydrogen reionization, and z>5 AGNs provide a significant fraction of the unresolved X-ray background at 2 keV. Singly- and doubly-ionized helium contribute about 13% to tau, and the HeIII volume fraction is already 50% when hydrogen becomes fully reionized.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical Journal Letter

High redshift AGNs and HI reionisation: limits from the unresolved X-ray background

The rapidly declining population of bright quasars at z~3 appears to make an increasingly small contribution to the ionising background at the HI Lyman limit. It is then generally though that massive stars in (pre-)galactic systems may provide the additional ionising flux needed to complete HI reionisation by z>6. A galaxy dominated background, however, may require that the escape fraction of Lyman continuum radiation from high redshift galaxies is as high as 10%, a value somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we question the latter hypothesis, and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z=0, can provide a fraction of the HII filling factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies >10%.Comment: A&A Letter, accepted (4 pages, 2 figures

The Early Growth of the First Black Holes

With detections of quasars powered by increasingly massive black holes (BHs) at increasingly early times in cosmic history over the past decade, there has been correspondingly rapid progress made on the theory of early BH formation and growth. Here we review the emerging picture of how the first massive BHs formed from the primordial gas and then grew to supermassive scales. We discuss the initial conditions for the formation of the progenitors of these seed BHs, the factors dictating the initial masses with which they form, and their initial stages of growth via accretion, which may occur at super-Eddington rates. Finally, we briefly discuss how these results connect to large-scale simulations of the growth of supermassive BHs over the course of the first billion years following the Big Bang.Comment: 13 pages, 9 figures, invited review accepted for publication in PAS

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.Comment: MNRAS in press, 7 pages, 8 figure

Massive black hole and gas dynamics in galaxy nuclei mergers. I. Numerical implementation

Numerical effects are known to plague adaptive mesh refinement (AMR) codes when treating massive particles, e.g. representing massive black holes (MBHs). In an evolving background, they can experience strong, spurious perturbations and then follow unphysical orbits. We study by means of numerical simulations the dynamical evolution of a pair MBHs in the rapidly and violently evolving gaseous and stellar background that follows a galaxy major merger. We confirm that spurious numerical effects alter the MBH orbits in AMR simulations, and show that numerical issues are ultimately due to a drop in the spatial resolution during the simulation, drastically reducing the accuracy in the gravitational force computation. We therefore propose a new refinement criterion suited for massive particles, able to solve in a fast and precise way for their orbits in highly dynamical backgrounds. The new refinement criterion we designed enforces the region around each massive particle to remain at the maximum resolution allowed, independently upon the local gas density. Such maximally-resolved regions then follow the MBHs along their orbits, and effectively avoids all spurious effects caused by resolution changes. Our suite of high resolution, adaptive mesh-refinement hydrodynamic simulations, including different prescriptions for the sub-grid gas physics, shows that the new refinement implementation has the advantage of not altering the physical evolution of the MBHs, accounting for all the non trivial physical processes taking place in violent dynamical scenarios, such as the final stages of a galaxy major merger.Comment: 11 pages, 11 figures, 1 table, it matches the published versio

LISA double black holes: Dynamics in gaseous nuclear discs

We study the inspiral of double black holes, with masses in the LISA window of detectability, orbiting inside a massive circum-nuclear disc. Using high-resolution SPH simulations, we follow the black hole dynamics in the early phase when gas-dynamical friction acts on the black holes individually, and continue our simulation until they form a close binary. We find that in the early sinking the black holes lose memory of their initial orbital eccentricity if they co-rotate with the gaseous disc, forming a binary with a low eccentricity, consistent with zero within our numerical resolution limit. The cause of circularization resides in the rotation present in the gaseous background where dynamical friction operates. Circularization may hinder gravitational waves from taking over and leading the binary to coalescence. In the case of counter-rotating orbits the initial eccentricity does not decrease, and the black holes may bind forming an eccentric binary. When dynamical friction has subsided, for equal mass black holes and regardless their initial eccentricity, angular momentum loss, driven by the gravitational torque exerted on the binary by surrounding gas, is nevertheless observable down to the smallest scale probed. In the case of unequal masses, dynamical friction remains efficient down to our resolution limit, and there is no sign of formation of any ellipsoidal gas distribution that may further harden the binary. During inspiral, gravitational capture of gas by the black holes occurs mainly along circular orbits: eccentric orbits imply high relative velocities and weak gravitational focusing. Thus, AGN activity may be excited during the black hole pairing process and double active nuclei may form when circularization is completed, on distance-scales of tens of pcs.Comment: Minor changes, accepted to MNRAS (11 pags, 14 figs). Movies (.avi) are available at http://pitto.mib.infn.it/~haardt/MOVIES

A Model for the X-Ray and UV Emission from Seyfert Galaxies and Galactic Black Holes

We propose that the X--ray emission from radio quiet AGN and galactic black holes is due to Comptonization of soft thermal photons emitted by the underlying accretion disk in localized structures (blobs). The power per unit area produced by the blobs, impinging on the disk, can easily dominate the radiation internally produced by the disk. In this case the electron temperature and the high energy spectrum can be determined in a similar way as in the previously studied homogeneous model (Haardt \& Maraschi 1991). However in the present model: a) the emitted spectrum is largely independent of the {\it fraction} of gravitational power dissipated in the blobs; b) the X--ray spectrum can be harder depending on a form factor of the blobs; c) the UV (or soft X--ray for galactic objects) luminosity that is not intercepted by the blobs can be larger than the X--ray luminosity. In the framework of a simplified accretion disk $\alpha-\Omega$ dynamo model, we make order of magnitude estimates of the number of active blobs, their size, luminosity and hence their compactness, finding values in agreement with what is observed. The expected UV to X--ray spectra and correlations of X--ray and UV light curves are discussed.Comment: 10 pages. Plain Tex. Accepted in ApJ Letter