965 research outputs found
Detecting the redshifted 21cm forest during reionization
The 21cm forest -- HI absorption features in the spectra of high-redshift
radio sources -- can potentially provide a unique probe of the largely neutral
intergalactic medium (IGM) during the epoch of reionization. We present
simulations of the 21cm forest due to the large scale structure of the
reionization-era IGM, including a prescription for x-ray heating and the
percolation of photoionization bubbles. We show that, if detected with future
instruments such as the Square Kilometer Array (SKA), the 21cm forest can
provide a significant constraint on the thermal history of the IGM. Detection
will be aided by consideration of the sudden increase in signal variance at the
onset of 21cm absorption. If radio foregrounds and the intrinsic source spectra
are well understood, the flux decrement over wide bandwidths can also improve
detection prospects. Our analysis accounts for the possibility of narrow
absorption lines from intervening dense regions, but, unlike previous studies,
our results do not depend on their properties. Assuming x-ray heating
corresponding to a local stellar population, we estimate that a statistically
significant detection of 21cm absorption could be made by SKA in less than a
year of observing against a Cygnus A-type source at , as opposed to
nearly a decade for a significant detection of the detailed forest features. We
discuss observational challenges due to uncertainties regarding the abundance
of background sources and the strength of the 21cm absorption signal.Comment: Submitted to MNRAS. Revised version includes updated and extended
calculations, some corrections and added reference
Mapping the Cosmic Web with Ly-alpha Emission
We use a high-resolution cosmological simulation to predict the distribution
of HI Ly-alpha emission from the low-redshift (z<0.5) intergalactic medium
(IGM). Our simulation can be used to reliably compute the emission from
optically thin regions of the IGM but not that of self-shielded gas. We
therefore consider several models that bracket the expected emission from
self-shielded regions. Most galaxies are surrounded by extended (>10^2 kpc)
``coronae'' of optically thin gas with Ly-alpha surface brightness close to the
expected background. Most of these regions contain smaller cores of dense, cool
gas. Unless self-shielded gas is able to cool to T<10^4.1 K, these cores are
much brighter than the background. The Ly-alpha coronae represent ``cooling
flows'' of IGM gas accreting onto galaxies. We also estimate the number of
Ly-alpha photons produced through the reprocessing of stellar ionizing
radiation in the interstellar medium of galaxies; while this mechanism is
responsible for the brightest Ly-alpha emission, it occurs on small physical
scales and can be separated using high-resolution observations. In all cases,
we find that Ly-alpha emitters are numerous (with a space density ~0.1 h^3
Mpc^-3) and closely trace the filamentary structure of the IGM, providing a new
way to map gas inside the cosmic web.Comment: 4 pages, 3 figures, accepted by ApJ
Hubble Diagram of Gamma-Rays Bursts calibrated with Gurzadyan-Xue Cosmology
Gamma-ray bursts (GRBs) being the most luminous among known cosmic objects
carry an essential potential for cosmological studies if properly used as
standard candles. In this paper we test with GRBs the cosmological predictions
of the Gurzadyan-Xue (GX) model of dark energy, a novel theory that predicts,
without any free parameters, the current vacuum fluctuation energy density
close to the value inferred from the SNIa observations. We also compare the GX
results with those predicted by the concordance scenario -CDM.
According to the statistical approach by Schaefer (2007), the use of several
empirical relations obtained from GRBs observables, after a consistent
calibration for a specific model, enables one to probe current cosmological
models. Based on this recently introduced method, we use the 69 GRBs sample
collected by Schaefer (2007); and the most recently released SWIFT satellite
data (Sakamoto et al. 2007) together with the 41 GRBs sample collected by
Rizzuto et al. (2007), which has the more firmly determined redshifts. Both
data samples span a distance scale up to redshift about 7. We show that the GX
models are compatible with the Hubble diagram of the Schaefer (2007) 69 GRBs
sample. Such adjustment is almost identical to the one for the concordance
-CDM.Comment: 9 pages, 17 figures, 11 tables; Astr. & Astrophys. (in press
Modification of the 21-cm power spectrum by X-rays during the epoch of reionisation
We incorporate a contribution to reionization from X-rays within analytic and
semi-numerical simulations of the 21-cm signal arising from neutral hydrogen
during the epoch of reionization. We explore the impact that X-ray ionizations
have on the power spectrum (PS) of 21-cm fluctuations by varying both the
average X-ray MFP and the fractional contribution of X-rays to reionization. In
general, prior to the epoch when the intergalactic medium is dominated by
ionized regions (H {\sevensize II} regions), X-ray-induced ionization enhances
fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray
heating does not strongly supress galaxy formation. Conversely, at later times
when \H2 regions dominate, small-scale fluctuations in the 21-cm signal are
suppressed by X-ray ionization. Our modelling also shows that the modification
of the 21-cm signal due to the presence of X-rays is sensitive to the relative
scales of the X-ray MFP, and the characteristic size of \H2 regions. We
therefore find that X-rays imprint an epoch and scale-dependent signature on
the 21-cm PS, whose prominence depends on fractional X-ray contribution. The
degree of X-ray heating of the IGM also determines the extent to which these
features can be discerned. We show that the MWA will have sufficient
sensitivity to detect this modification of the PS, so long as the X-ray photon
MFP falls within the range of scales over which the array is most sensitive
( Mpc). In cases in which this MFP takes a much smaller value,
an array with larger collecting area would be required.Comment: 15 pages, 6 figures, Accepted for publication in MNRAS X-ray heating
contribution now adde
The Impact of HI in Galaxies on 21-cm Intensity Fluctuations During the Reionisation Epoch
We investigate the impact of neutral hydrogen (HI) in galaxies on the
statistics of 21-cm fluctuations using analytic and semi-numerical modelling.
Following the reionisation of hydrogen the HI content of the Universe is
dominated by damped absorption systems (DLAs), with a cosmic density in HI that
is observed to be constant at a level equal to ~2% of the cosmic baryon density
from z~1 to z~5. We show that extrapolation of this constant fraction into the
reionisation epoch results in a reduction of 10-20% in the amplitude of 21-cm
fluctuations over a range of spatial scales. The assumption of a different
percentage during the reionisation era results in a proportional change in the
21-cm fluctuation amplitude. We find that consideration of HI in galaxies/DLAs
reduces the prominence of the HII region induced shoulder in the 21-cm power
spectrum (PS), and hence modifies the scale dependence of 21-cm fluctuations.
We also estimate the 21cm-galaxy cross PS, and show that the cross PS changes
sign on scales corresponding to the HII regions. From consideration of the
sensitivity for forthcoming low-frequency arrays we find that the effects of HI
in galaxies/DLAs on the statistics of 21-cm fluctuations will be significant
with respect to the precision of a PS or cross PS measurement. In addition,
since overdense regions are reionised first we demonstrate that the
cross-correlation between galaxies and 21-cm emission changes sign at the end
of the reionisation era, providing an alternative avenue to pinpoint the end of
reionisation. The sum of our analysis indicates that the HI content of the
galaxies that reionise the universe will need to be considered in detailed
modelling of the 21-cm intensity PS in order to correctly interpret
measurements from forthcoming low-frequency arrays.Comment: 11 pages, 6 figures. Submitted to MNRA
Current models of the observable consequences of cosmic reionization and their detectability
A number of large current experiments aim to detect the signatures of the cosmic reionization at redshifts z > 6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale cosmic microwave background (CMB) anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by Wilkinson Microwave Anisotropy Probe three-year results. We find that the optimal frequency range for observing the `global step of the 21-cm emission is 120150 MHz, while statistical studies should aim at 140160 MHz, observable by GMRT. Some strongly non-Gaussian brightness features should be detectable at frequencies up to ~190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic SunyaevZel'dovich effect from reionized patches peaks at tens of K at arcminute scales and has an rms of ~1 K, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology
Descending from on high: Lyman series cascades and spin-kinetic temperature coupling in the 21 cm line
We examine the effect of Lyman continuum photons on the 21 cm background in
the high-redshift universe. The brightness temperature of this transition is
determined by the spin temperature T_s, which describes the relative
populations of the singlet and triplet hyperfine states. Once the first
luminous sources appear, T_s is set by the Wouthuysen-Field effect, in which
Lyman-series photons mix the hyperfine levels. Here we consider coupling
through n>2 Lyman photons. We first show that coupling (and heating) from
scattering of Lyman-n photons is negligible, because they rapidly cascade to
lower-energy photons. These cascades can result in either a Lyman-alpha photon
-- which will then affect T_s according to the usual Wouthuysen-Field mechanism
-- or photons from the 2s -> 1s continuum, which escape without scattering. We
show that a proper treatment of the cascades delays the onset of strong
Wouthuysen-Field coupling and affects the power spectrum of brightness
fluctuations when the overall coupling is still relatively weak (i.e., around
the time of the first stars). Cascades damp fluctuations on small scales
because only ~ 1/3 of Lyn photons cascade through Lyman-alpha, but they do not
affect the large-scale power because that arises from those photons that
redshift directly into the Lyman-alpha transition. We also comment on the
utility of Lyman-n transitions in providing "standard rulers" with which to
study the high-redshift universe.Comment: Accepted by MNRAS. 10 pages, 8 figures. Minor revisions + corrected
normalisation of figure
Reionization and the large-scale 21 cm-cosmic microwave background cross correlation
Of the many probes of reionization, the 21 cm line and the cosmic microwave
background (CMB) are among the most effective. We examine how the
cross-correlation of the 21 cm brightness and the CMB Doppler fluctuations on
large angular scales can be used to study this epoch. We employ a new model of
the growth of large scale fluctuations of the ionized fraction as reionization
proceeds. We take into account the peculiar velocity field of baryons and show
that its effect on the cross correlation can be interpreted as a mixing of
Fourier modes. We find that the cross-correlation signal is strongly peaked
toward the end of reionization and that the sign of the correlation should be
positive because of the inhomogeneity inherent to reionization. The signal
peaks at degree scales (l~100) and comes almost entirely from large physical
scales (k~0.01 Mpc). Since many of the foregrounds and noise that plague low
frequency radio observations will not correlate with CMB measurements, the
cross correlation might appear to provide a robust diagnostic of the
cosmological origin of the 21 cm radiation around the epoch of reionization.
Unfortunately, we show that these signals are actually only weakly correlated
and that cosmic variance dominates the error budget of any attempted detection.
We conclude that the detection of a cross-correlation peak at degree-size
angular scales is unlikely even with ideal experiments.Comment: 15 pages, 4 figures, submitted to MNRA
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