12 research outputs found
Temperature Anisotropies and Distortions Induced by Hot Intracluster Gas on the Cosmic Microwave Background
The power spectrum of temperature anisotropies induced by hot intracluster
gas on the cosmic background radiation is calculated. For low multipoles it
remains constant while at multipoles above it is exponentially damped.
The shape of the radiation power spectrum is almost independent of the average
intracluster gas density profile, gas evolution history or clusters virial
radii; but the amplitude depends strongly on those parameters and could be as
large as 20% that of intrinsic contribution. The exact value depends on the
global properties of the cluster population and the evolution of the
intracluster gas. The distortion on the Cosmic Microwave Background black body
spectra varies in a similar manner. The ratio of the temperature anisotropy to
the mean Comptonization parameters is shown to be almost independent of the
cluster model and, in first approximation, depends only on the number density
of clusters.Comment: 10 pages, Latex, 3 figures; to be published in Ap
The Search for Intergalactic Hydrogen Clouds in Voids
I present the results of a search for intergalactic hydrogen clouds in voids.
Clouds are detected by their HI LyA absorption lines in the HST spectra of
low-redshift AGN. The parameter with which the environments of clouds are
characterized is the tidal field, which places a lower limit on the cloud
mass-density which is dynamically stable against disruption. Galaxy redshift
catalogs are used to sum the tidal fields along the lines of sight, sorting
clouds according to tidal field upper, or lower limits. The analytical
methodology employed is designed to detect gas clouds whose expansion following
reionization is restrained by dark matter perturbations. End-products are the
cloud equivalent width distribution functions (EWDF) of catalogs formed by
sorting clouds according to various tidal field upper, or lower limits.
Cumulative EWDFs are steep in voids (S ~ -1.5 \pm 0.2), but flatter in high
tidal field zones (S ~ -0.5 \pm 0.1). Most probable cloud Doppler parameters
are ~30 km/s in voids and ~60 km/s in proximity to galaxies. In voids, the
cumulative line density at low EW (~ 15 mA) is ~ 500 per unit redshift. The
void filling factor is found to be 0.87 <= f_v <= 0.94. The void EWDF is
remarkably uniform over this volume, with a possible tendency for more massive
clouds to be in void centers. The size and nature of the void cloud population
suggested by this study is completely unanticipated by the results of published
3-D simulations, which predict that most clouds are in filamentary structures
around galaxy concentrations, and that very few observable absorbers would lie
in voids. Strategies for modeling this population are briefly discussed.Comment: 21 pages, 19 figures, apjemulate style, to appear in ApJ vol. 57
A Minihalo Model for the Lyman Limit Absorption Systems at High Redshift
We propose that a large fraction of the QSO Lyman limit absorption systems
(LLS) observed at high redshift (z > 3) originate from gas trapped in small
objects, such as minihalos, that form prior to reionization. In the absence of
a strong UV flux, the gas is predominantly neutral and may form clouds with HI
column density NHI > 10^18 cm^-2. Due to their high densities and high HI
column densities, these clouds are not destroyed by the onset of the UV
background at a later time. Thus, if not disrupted by other processes, such as
mergers into larger systems or `blow away' by supernovae, they will produce
LLS. We show that the observed number density of LLS at high redshifts can well
be reproduced by the survived `minihalos' in hierarchical clustering models
such as the standard cold dark matter model. The number density of LLS in such
a population increases with z even beyond the redshifts accessible to current
observations and dies off quickly at z < 2. This population is distinct from
other populations because the absorbing systems have small velocity widths and
a close to primordial chemical composition. The existence of such a population
requires that the reionization of the universe occurs late, at z < 20.Comment: 4 pages, 1 PostScript figure, accepted for publication in ApJ
Letters, 8 Dec 199
The Low Redshift Lyman Alpha Forest in Cold Dark Matter Cosmologies
We study the physical origin of the low-redshift Lyman alpha forest in
hydrodynamic simulations of four CDM cosmologies. Our main conclusions are
insensitive to the cosmological model but depend on our assumption that the UV
background declines at low redshift. We find that the expansion of the universe
drives rapid evolution of dN/dz (the number of absorbers per unit z) at z >
1.7, but that at lower redshift the fading of the UV background counters the
influence of expansion, leading to slow evolution. At every redshift, weaker
lines come primarily from moderate fluctuations of the diffuse, unshocked IGM,
and stronger lines originate in shocked or radiatively cooled gas of higher
overdensity. However, the neutral hydrogen column density associated with
structures of fixed overdensity drops as the universe expands, so an absorber
at z = 0 is dynamically analogous to an absorber with neutral hydrogen column
density 10 to 50 times higher at z = 2-3. We find no clear distinction between
lines arising in "galaxy halos" and lines arising in larger scale structures;
however, galaxies tend to lie near the dense regions of the IGM that produce
strong Lyman alpha lines. The simulations provide a unified physical picture
that accounts for the most distinctive observed properties of the low redshift
Lyman alpha forest: (1) a sharp transition in the evolution of dN/dz at z ~
1.7, (2) stronger evolution for absorbers of higher equivalent width, (3) a
correlation of increasing Lyman alpha equivalent width with decreasing galaxy
impact parameter, and (4) a tendency for stronger lines to arise in close
proximity to galaxies while weaker lines trace more diffuse large scale
structure. (Abridged)Comment: 57 pages, 18 figures, submitted to Ap
Observational Matter Power Spectrum and the Height of the Second Acoustic Peak
We show that the amplitude of the second acoustic peak in the newly released
BOOMERANG-98 and MAXIMA-I data is compatible with the standard primordial
nucleosynthesis and with the locally broken-scale-invariant matter power
spectrum suggested by recent measurements of the power spectrum in the range
Mpc. If the slope of matter density perturbations on large scales is
, the Hubble constant is , and r.m.s. mass
fluctuations at 8 Mpc are , then for a Universe
approximately 14 Gyr old our best fit within the nucleosynthesis bound
requires . Cluster
abundances further constraint the matter density to be .
The CMB data alone are not able to determine the detailed form of the matter
power spectrum in the range \hmpc where deviations from the
scale-invariant spectrum are expected to be most significant, but they do not
contradict the existence of the previously claimed peak at \hmpc,
and a depression at \hmpc.Comment: Few typos corrected. Matches published versio
Recovery of the Power Spectrum of Mass Fluctuations from Observations of the Lyman-alpha Forest
We present a method to recover the shape and amplitude of the power spectrum
of mass fluctuations, P(k), from observations of the high redshift \lya forest.
The method is motivated by the physical picture of the \lya forest that has
emerged from hydrodynamic cosmological simulations and related semi-analytic
models, which predicts a tight correlation between the \lya optical depth and
the underlying matter density. We monotonically map the QSO spectrum to a
Gaussian density field, measure its 3-d P(k), and normalize by evolving
cosmological simulations with this P(k) until they reproduce the observed power
spectrum of the QSO flux. Imposing the observed mean \lya opacity as a
constraint makes the derived P(k) normalization insensitive to the choice of
cosmological parameters, ionizing background spectrum, or reionization history.
Thus, in contrast to estimates of P(k) from galaxy clustering, there are no
uncertain "bias parameters" in the recovery of the mass power spectrum. We test
the full procedure on SPH simulations of 3 cosmological models and show that it
recovers their true mass power spectra on comoving scales ~1-10/h Mpc, the
upper scale being set by the size of the simulation boxes. The procedure works
even for noisy (S/N ~ 10), moderate resolution (~40 km/s pixels) spectra. We
present an illustrative application to Q1422+231; the recovered P(k) is
consistent with an \Omega=1, \sigma_8=0.5 CDM model. Application to large QSO
samples should yield the power spectrum of mass fluctuations on small scales at
z ~ 2-4. (Compressed)Comment: AASlatex, 40 pages w/ 16 embedded ps figures. Submitted to Ap
Large-scale structure in the Lyman-alpha forest II: analysis of a group of ten QSOs
The spatial distribution of Ly-alpha forest absorption systems towards ten
QSOs has been analysed to search for large-scale structure over the redshift
range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are
concentrated within a 1 deg^2 field. We have employed a technique based on the
first and second moments of the transmission probability density function which
is capable of identifying and assessing the significance of regions of over- or
underdense Ly-alpha absorption. We find evidence for large-scale structure in
the distribution of Ly-alpha forest absorption at the > 99 per cent confidence
level. In individual spectra we find overdense Ly-alpha absorption on scales of
up to 1200 km s^-1. There is also strong evidence for correlated absorption
across line of sight pairs separated by < 3 h^-1 proper Mpc (q_0 = 0.5). For
larger separations the cross-correlation signal becomes progressively less
significant.Comment: 15 pages, LaTeX, 6 Postscript figures, accepted for publication in
MNRA
HST STIS observations of four QSO pairs
We present HST STIS observations of four quasar pairs with redshifts 0.84200 km s(-1). This probably reflects the fact that the scale probed by our observations is not related to the real size of individual absorbers but rather to large scale correlation. Statistics are too small to conclude about any difference between pairs separated by either 2 or 3 arcmin. A damped Lyman-alpha system is detected at z(abs)=1.2412 toward LBQS 0019-0145A with log N(HI)similar to20.5. From the absence of ZnII absorption, we derive a metallicity relative to solar [Zn/H]< -1.75
Extension of Lyman-alpha Complexes from HST Observations of Four Pairs of Quasars
We present HST observations of four pairs of quasars with 2,3 arcmin angular separation and redshift z 0.9. We apply the Nearest-Neighbor method to the sample of observed Lyman-alpha lines with rest equivalent width greater than 0.3 Angstrum. According to Monte-Carlo simulations, we detect an excess of coincidences with velocity difference smaller than 500 km/s