143 research outputs found
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
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
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
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
Super-Critical Growth of Massive Black Holes from Stellar-Mass Seeds
We consider super-critical accretion with angular momentum onto stellar-mass
black holes as a possible mechanism for growing billion-solar-mass holes from
light seeds at early times. We use the radiatively-inefficient "slim disk"
solution -- advective, optically thick flows that generalize the standard
geometrically thin disk model -- to show how mildly super-Eddington
intermittent accretion may significantly ease the problem of assembling the
first massive black holes when the Universe was less than 0.8 Gyr old. Because
of the low radiative efficiencies of slim disks around non-rotating as well as
rapidly rotating holes, the mass e-folding timescale in this regime is nearly
independent of the spin parameter. The conditions that may lead to
super-critical growth in the early Universe are briefly discussed.Comment: 5 pages, 2 figures, matches version accepted by The Astrophysical
Journal Letter
Interaction of massive black hole binaries with their stellar environment: III. Scattering of bound stars
We develop a formalism for studying the dynamics of massive black hole
binaries embedded in gravitationally-bound stellar cusps, and study the binary
orbital decay by three-body interactions, the impact of stellar slingshots on
the density profile of the inner cusp, and the properties of the ejected
hypervelocity stars (HVSs). We find that the scattering of bound stars shrinks
the binary orbit and increases its eccentricity more effectively than that of
unbound ambient stars. Binaries with initial eccentricities e>0.3 and/or
unequal-mass companions (M_2/M_1<0.1) can decay by three-body interactions to
the gravitational wave emission regime in less than a Hubble time. The stellar
cusp is significantly eroded, and cores as shallow as \rho\propto r^-0.7 may
develop from a pre-existing singular isothermal density profile. A population
of HVSs is ejected in the host galaxy halo, with a total mass ~M_2. We scale
our results to the scattering of stars bound to Sgr A*, the massive black hole
in the Galactic Center, by an inspiraling companion of intermediate mass.
Depending on binary mass ratio, eccentricity, and initial slope of the stellar
cusp, a core of radius ~0.1 pc typically forms in 1-10 Myr. On this timescale
about 500-2500 HVSs are expelled with speeds sufficiently large to escape the
gravitational potential of the Milky Way.Comment: 13 pages, 16 figures, 2 tables, some major revisions, accepted for
publication by Ap
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