203 research outputs found
Was the Universe Reionized by Massive Population-III Stars?
The WMAP satellite has measured a large optical depth to electron scattering
after cosmological recombination of 0.17+-0.04, implying significant
reionization of the primordial gas only ~200 million years after the big bang.
However, the most recent overlap of intergalactic HII regions must have occured
at z<9 based on the Lyman-alpha forest constraint on the thermal history of the
intergalactic medium. Here we argue that a first generation of metal-free stars
with a heavy (rather than Salpeter) mass function is therefore required to
account for much of the inferred optical depth. This conclusion holds if
feedback regulates star formation in early dwarf galaxies as observed in
present-day dwarfs.Comment: 4 pages, 1 figure, replaced to match version accepted by ApJ Letter
Reionization of Hydrogen and Helium by Early Stars and Quasars
We compute the reionization histories of hydrogen and helium due to the
ionizing radiation fields produced by stars and quasars. For the quasars we use
a model based on halo-merger rates that reproduces all known properties of the
quasar luminosity function at high redshifts. The less constrained properties
of the ionizing radiation produced by stars are modeled with two free
parameters: (i) a transition redshift, z_tran, above which the stellar
population is dominated by massive, zero-metallicity stars and below which it
is dominated by a Scalo mass function; (ii) the product of the escape fraction
of stellar ionizing photons from their host galaxies and the star-formation
efficiency, f_esc f_*. We constrain the allowed range of these free parameters
at high redshifts based on the lack of the HI Gunn-Peterson trough at z<6 and
the upper limit on the total intergalactic optical depth for electron
scattering, tau_es<0.18, from recent cosmic microwave background (CMB)
experiments. We find that quasars ionize helium by a redshift z~4, but cannot
reionize hydrogen by themselves before z~6. A major fraction of the allowed
combinations of f_esc f_* and z_tran lead to an early peak in the ionized
fraction due to metal-free stars at high redshifts. This sometimes results in
two reionization epochs, namely an early HII or HeIII overlap phase followed by
recombination and a second overlap phase. Even if early overlap is not
achieved, the peak in the visibility function for scattering of the CMB often
coincides with the early ionization phase rather than with the actual
reionization epoch. Consequently, tau_es does not correspond directly to the
reionization redshift. We generically find values of tau_es>7%, that should be
detectable by the MAP satellite.Comment: 33 pages, 10 figures, Accepted for publication in Ap
Detection of Gravitational Waves from the Coalescence of Population-III Remnants with Advanced LIGO
The comoving mass density of massive black hole (MBH) remnants from
pre-galactic star formation could have been similar in magnitude to the
mass-density of supermassive black holes (SMBHs) in the present-day universe.
We show that the fraction of MBHs that coalesce during the assembly of SMBHs
can be extracted from the rate of ring-down gravitational waves that are
detectable by Advanced LIGO. Based on the SMBH formation history inferred from
the evolution of the quasar luminosity function, we show that an observed event
rate of 1 per year will constrain the SMBH mass fraction that was contributed
by MBHs coalescence down to a level of ~10^-6 for 20 solar mass MBH remnants
(or ~10^-4 for 260 solar mass remnants).Comment: 4 pages, 2 figures. Submitted to ApJ Letter
Gravitational Lensing of the SDSS High-Redshift Quasars
We predict the effects of gravitational lensing on the color-selected
flux-limited samples of z~4.3 and z>5.8 quasars, recently published by the
Sloan Digital Sky Survey (SDSS). Our main findings are: (i) The lensing
probability should be 1-2 orders of magnitude higher than for conventional
surveys. The expected fraction of multiply-imaged quasars is highly sensitive
to redshift and the uncertain slope of the bright end of the luminosity
function, beta_h. For beta_h=2.58 (3.43) we find that at z~4.3 and i*<20.0 the
fraction is ~4% (13%) while at z~6 and z*<20.2 the fraction is ~7% (30%). (ii)
The distribution of magnifications is heavily skewed; sources having the
redshift and luminosity of the SDSS z>5.8 quasars acquire median magnifications
of med(mu_obs)~1.1-1.3 and mean magnifications of ~5-50. Estimates of
the quasar luminosity density at high redshift must therefore filter out
gravitationally-lensed sources. (iii) The flux in the Gunn-Peterson trough of
the highest redshift (z=6.28) quasar is known to be f_lambda<3 10^-19
erg/sec/cm^2/Angstrom. Should this quasar be multiply imaged, we estimate a 40%
chance that light from the lens galaxy would have contaminated the same part of
the quasar spectrum with a higher flux. Hence, spectroscopic studies of the
epoch of reionization need to account for the possibility that a lens galaxy,
which boosts the quasar flux, also contaminates the Gunn-Peterson trough. (iv)
Microlensing by stars should result in ~1/3 of multiply imaged quasars in the
z>5.8 catalog varying by more than 0.5 magnitudes over the next decade. The
median equivalent width would be lowered by ~20% with respect to the intrinsic
value due to differential magnification of the continuum and emission-line
regions.Comment: 27 pages, 10 figures. Expansion on the discussion in
astro-ph/0203116. Replaced with version accepted for publication in Ap
Lyman Alpha Constraints on Very Low Luminosity AGN
Recent surveys have detected Lya emission from z=4.5-6.5 at luminosities as
low as 10^41 erg/s. There is good evidence that low numbers of AGN are among
observed faint Lya emitters. Combining these observations with an empirical
relation between the intrinsic Lya and B-band luminosities of AGN, we obtain an
upper limit on the number density of AGN with absolute magnitudes M_B=[-16,-19]
at z=4.5-6.5. These AGN are up to two orders of magnitude fainter than those
discovered in the Chandra Deep Field, resulting in the faintest observational
constraints to date at these redshifts. At z=4.5, the powerlaw slope of the
very faint end of the luminosity function of AGN is shallower than the slope
observed at lower redshifts, beta <1.6, at the 98% confidence level. In fact,
we find marginal evidence that the luminosity function rises with luminosity,
corresponding to a powerlaw slope beta <0, at magnitudes fainter than M_B~-20
(75% confidence level). These results suggest either that accretion onto lower
mass black holes is less efficient than onto their more massive counterparts,
or that the number of black holes powering AGN with M_B >-20 is lower than
expected from the M_BH-sigma relation by one-two orders of magnitude.
Extrapolating from reverberation-mapping studies suggests that these black
holes would have M_BH=10^6-10^7 Msun. To facilitate the identification of AGN
among observed Lya emitters, we derive observational properties of faint AGN in
the Lya line, as well as in the X-ray and optical bands.Comment: 12 pages, 5 figures. MNRAS in Pres
Redshifted 21cm Observations of High Redshift Quasar Proximity Zones
The introduction of low-frequency radio arrays is expected to revolutionize
the study of the reionization epoch. Observation of the contrast in redshifted
21cm emission between a large HII region and the surrounding neutral
inter-galactic medium (IGM) will be the simplest and most easily interpreted
signature. However the highest redshift quasars known are thought to reside in
an ionized IGM. Using a semi-analytic model we describe the redshifted 21cm
signal from the IGM surrounding quasars discovered using the i-drop out
technique (i.e. quasars at z~6). We argue that while quasars at z<6.5 seem to
reside in the post overlap IGM, they will still provide valuable probes of the
late stages of the overlap era because the light-travel time across a quasar
proximity zone should be comparable to the duration of overlap. For redshifted
21cm observations within a 32MHz bandpass, we find that the subtraction of a
spectrally smooth foreground will not remove spectral features due to the
proximity zone. These features could be used to measure the neutral hydrogen
content of the IGM during the late stages of reionization. The density of
quasars at z~6 is now well constrained. We use the measured quasar luminosity
function to estimate the prospects for discovery of high redshift quasars in
fields that will be observed by the Murchison Widefield Array.Comment: 14 pages, 9 figures. Accepted for publication in MNRA
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