190 research outputs found
The 21-cm Background from the Cosmic Dark Ages: Minihalos and the Intergalactic Medium before Reionization
The H atoms inside minihalos (i.e. halos with virial temperatures T_vir <
10^4 K, in the mass range roughly from 10^4 M_sun to 10^8 M_sun) during the
cosmic dark ages in a LCDM universe produce a redshifted background of
collisionally-pumped 21-cm line radiation which can be seen in emission
relative to the cosmic microwave background (CMB). Previously, we used
semi-analytical calculations of the 21-cm signal from individual halos of
different mass and redshift and the evolving mass function of minihalos to
predict the mean brightness temperature of this 21-cm background and its
angular fluctuations. Here we use high-resolution cosmological N-body and
hydrodynamic simulations of structure formation at high redshift (z > 8) to
compute the mean brightness temperature of this background from both minihalos
and the intergalactic medium (IGM) prior to the onset of Ly-alpha radiative
pumping. We find that the 21-cm signal from gas in collapsed, virialized
minihalos dominates over that from the diffuse shocked gas in the IGM.Comment: 8 pages, 5 figures. To appear in proceedings of UC Irvine May 2005
workshop on "First Light & Reionization", eds. E. Barton & A. Cooray, New
Astronomy Reviews, in pres
Nonlinear Bias of Cosmological Halo Formation in the Early Universe
We present estimates of the nonlinear bias of cosmological halo formation,
spanning a wide range in the halo mass from to , based upon both a suite of high-resolution cosmological
N-body simulations and theoretical predictions. The halo bias is expressed in
terms of the mean bias and stochasticity as a function of local overdensity
(), under different filtering scales, which is realized as the density
of individual cells in uniform grids. The sampled overdensities span a range
wide enough to provide the fully nonlinear bias effect on the formation of
haloes. A strong correlation between and halo population overdensity
is found, along with sizable stochasticity. We find that the
empirical mean halo bias matches, with good accuracy, the prediction by the
peak-background split method based on the excursion set formalism, as long as
the empirical, globally-averaged halo mass function is used. Consequently, this
bias formalism is insensitive to uncertainties caused by varying halo
identification schemes, and can be applied generically. We also find that the
probability distribution function of biased halo numbers has wider distribution
than the pure Poisson shot noise, which is attributed to the sub-cell scale
halo correlation. We explicitly calculate this correlation function and show
that both overdense and underdense regions have positive correlation, leading
to stochasticity larger than the Poisson shot noise in the range of haloes and
halo-collapse epochs we study.Comment: 18 pages, 8 figures, in press for publication in MNRAS; supplementary
material (additional 16 figures) separately supplied (supplement.pdf) as a
part of source file
Scale-dependent bias induced by local non-Gaussianity: a comparison to N-body simulations
We investigate the effect of primordial non-Gaussianity of the local fNL type on the auto- and cross-power spectra of dark matter haloes using simulations of the Λ cold dark matter cosmology. We perform a series of large N-body simulations of both positive and negative fNL, spanning the range between 10 and 100. Theoretical models predict a scale-dependent bias correction Δb(k, fNL) that depends on the linear halo bias b(M). We measure the power spectra for a range of halo mass and redshifts covering the relevant range of existing galaxy and quasar populations. We show that auto- and cross-correlation analyses of bias are consistent with each other. We find that for low wavenumbers with k 1.5. We show that a scale-independent bias correction improves the comparison between theory and simulations on smaller scales, where the scale-dependent effect rapidly becomes negligible. The current limits on fNL from Slosar et al. come mostly from very large scales k < 0.01 h Mpc−1 and, therefore, remain valid. For the halo samples with b(M) < 1.5 − 2, we find that the scale-dependent bias from non-Gaussianity actually exceeds the theoretical predictions. Our results are consistent with the bias correction scaling linearly with fN
Simulating the Impact of X-ray Heating during the Cosmic Dawn
Upcoming observations of the 21-cm signal from the Epoch of Reionization will
soon provide the first direct detection of this era. This signal is influenced
by many astrophysical effects, including long range X-ray heating of the
intergalactic gas. During the preceding Cosmic Dawn era the impact of this
heating on the 21-cm signal is particularly prominent, especially before spin
temperature saturation. We present the largest-volume (349\,Mpc
comoving=244~Mpc) full numerical radiative transfer simulations to date
of this epoch that include the effects of helium and multi-frequency heating,
both with and without X-ray sources. We show that X-ray sources contribute
significantly to early heating of the neutral intergalactic medium and, hence,
to the corresponding 21-cm signal. The inclusion of hard, energetic radiation
yields an earlier, extended transition from absorption to emission compared to
the stellar-only case. The presence of X-ray sources decreases the absolute
value of the mean 21-cm differential brightness temperature. These hard sources
also significantly increase the 21-cm fluctuations compared the common
assumption of temperature saturation. The 21-cm differential brightness
temperature power spectrum is initially boosted on large scales, before
decreasing on all scales. Compared to the case of the cold, unheated
intergalactic medium, the signal has lower rms fluctuations and increased
non-Gaussianity, as measured by the skewness and kurtosis of the 21-cm
probability distribution functions. Images of the 21-cm signal with resolution
around 11~arcmin still show fluctuations well above the expected noise for deep
integrations with the SKA1-Low, indicating that direct imaging of the X-ray
heating epoch could be feasible.Comment: 13 pages, 8 figure
Implications of WMAP 3 Year Data for the Sources of Reionization
New results on the anisotropy of the cosmic microwave background (CMB) and
its polarization based on the first 3 years of data from the Wilkinson
Microwave Anisotropy Probe (WMAP) have revised the electron scattering optical
depth downward from tau_es=0.17+0.08-0.07 to tau_es=0.09+/-0.03. This implies a
shift of the effective reionization redshift from z_r~17 to z_r~11. Previous
attempts to explain the high redshift of reionization inferred from the WMAP 1
year data have led to widespread speculation that the sources of reionization
must have been much more efficient than those associated with the star
formation observed at low redshift. This is consistent, for example, with the
suggestion that early star formation involved massive, Population III stars
that early on produced most of the ionizing radiation escaping from halos. It
is therefore tempting to interpret the new WMAP results as implying that we can
now relax those previous high demands on the efficiency of the sources of
reionization and perhaps even turn the argument around as evidence against such
high efficiency. We show that this is not the case, however. The new WMAP
results also find that the primordial density fluctuation power spectrum has a
lower amplitude, sigma_8, and departs substantially from the scale-invariant
spectrum. We show that these effects combine to cancel the impact of the later
reionization implied by the new value of tau_es on the required ionizing
efficiency per collapsed baryon. The delay of reionization is surprisingly well
matched by a comparable delay (by a factor of ~1.4 in scale factor) in the
formation of the halos responsible for reionization.Comment: 4 pages, 3 figures, Published in ApJ Letters, revised to match
published versio
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