206 research outputs found
Reionization of the Intergalactic Medium and the Damping Wing of the Gunn-Peterson Trough
Observations of high-redshift quasars show that the IGM must have been
reionized at some redshift . If a source of radiation could be observed at
the rest-frame Lya wavelength, at a sufficiently high redshift where some of
the IGM in the line-of-sight was not yet reionized, the Gunn-Peterson trough
should be present. Longward of the Lya wavelength, a damping wing should be
observed caused by the neutral IGM whose absorption profile can be predicted.
Measuring the shape of this damping wing would provide irrefutable evidence of
the observation of the IGM before reionization, and a determination of the
density of the neutral IGM. This measurement might be hindered by the possible
presence of a dense absorption system associated with the source.
Shortward of the \lya wavelength, absorption should be seen from the patchy
structure of the IGM in the process of reionization. We show that a complete
Gunn-Peterson trough is most likely to continue to be observed through the
epoch where the IGM is partially ionized. The damping wings of the neutral
patches should overlap if the proper pathlength through an ionized region is
less than 1 h^{-1} Mpc; even in larger ionized regions, the characteristic
background intensity should be low enough to yield a very high optical depth
due to the residual neutral fraction, although occasionally some flux may be
transmitted through large, underdense voids within an ionized region. The case
of the HeII reionization is also discussed, and we argue that helium was
already doubly ionized by z=3 throughout the IGM.
The recently discovered afterglows of gamma-ray bursts might soon be observed
at very high redshifts. Their featureless continuum spectrum and high
luminosities make them ideal sources for studying absorption by the IGM.Comment: submitted to Ap
Soft X-ray Absorption by High-Redshift Intergalactic Helium
The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has
been measured with HST in four quasars over the last few years, over the
redshift range 2.4 < z < 3.2. These observations have indicated that the HeII
reionization may not have been completed until z\simeq 2.8, and that large
fluctuations in the intensity of the HeII-ionizing background were present
before this epoch. The detailed history of HeII reionization at higher
redshifts is, however, model-dependent and difficult to determine from these
observations, because the IGM can be completely optically thick to Lya photons
when only a small fraction of the helium remains as HeII. In addition, finding
quasars in which the HeII Lya absorption can be observed becomes increasingly
difficult at higher redshift, owing to the large abundance of hydrogen Lyman
limit systems.
It is pointed out here that HeII in the IGM should also cause detectable
continuum absorption in the soft X-rays. The spectrum of a high-redshift source
seen behind the IGM when most of the helium was HeII should recover from the
HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic
absorption will generally be stronger, but not by a large factor; the
intergalactic HeII absorption can be detected as an excess over the expected
Galactic absorption from the 21cm HI column density. In principle, this method
allows a direct determination of the fraction of helium that was singly ionized
as a function of redshift, if the measurement is done on a large sample of
high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter
Intrinsic Properties of the <z>=2.7 Lyman Alpha Forest from Keck Spectra of QSO HS 1946+7658
We present the highest quality Lyman Alpha forest spectra published to date,
from the QSO HS 1946+7658. The distribution of H I column densities is a power
law of slope -1.5 from Log N = 12.1 - 14. This power law can extend to N = 0,
because lines weaker than Log N = 12.1 do not have a large H I optical depth.
Low column lines with Log N > 9 could account for all observed He II
absorption, but lines with Log N > 12 alone are unlikely to do so. The b
distribution between 20 and 60 km/sec is a Gaussian with a mean of 23 km/sec
(less than reported in past at this z), and a sigma b of 14 km/sec. We report
no evolution in the Lyman alpha forest (except the number of lines), because Lu
et al. (1997) found the same N and b distributions at = 3.7. We see lines
with 14 80 km/sec that cannot be accounted for by noise
or blending effects. We discover that the lower cutoff in the b distribution
varies with N, from b = 14 km/sec at Log N = 12.5 to b = 22 km/sec at Log N =
14.0, but otherwise b and N are not correlated. We see no Lyman Alpha line
clustering above 50 \kms, in disagreement with previous results from lower
signal to noise data, but we do see a 3 sigma clustering signal at 25 - 50
km/sec among lines with Log N > 13.6Comment: (Minor changes including new identifications for two weak lines) 46
pages including 16 Figures, Latex Table 1 Available at
http://nately.ucsd.edu/~david . To appear in Ap
A Measurement of the Temperature-Density Relation in the Intergalactic Medium Using a New Lyman-alpha Absorption Line Fitting Method
The evolution of the temperature in the intergalactic medium is related to
the reionization of hydrogen and helium, and has important consequences for our
understanding of the Lya forest and of galaxy formation in gravitational models
of large-scale structure. We measure the temperature-density relation of
intergalactic gas from Lya forest observations of eight quasar spectra with
high resolution and signal-to-noise ratio, using a new line fitting technique
to obtain a lower cutoff of the distribution of line widths from which the
temperature is derived. We carefully test the accuracy of this technique to
recover the gas temperature with a hydrodynamic simulation. The temperature at
redshift z=(3.9, 3.0, 2.4) is best determined at densities slightly above the
mean: T_star=(20200\pm2700, 20200\pm1300, 22600\pm1900)K (statistical error
bars) for gas density (in units of the mean density) Delta_star=(1.42\pm0.08,
1.37\pm0.11, 1.66\pm0.11). The power-law index of the temperature-density
relation, defined by T=T_star(Delta/Delta_star)^{gamma-1}, is gamma-1=
(0.43\pm0.45, 0.29\pm0.30, 0.52\pm0.14) for the same three redshifts. The
temperature at the fixed over-density Delta=1.4 is T_1.4=(20100\pm2800,
20300\pm1400, 20700\pm1900)K. These temperatures are higher than expected for
photoionized gas in ionization equilibrium with a cosmic background, and can be
explained by a gradual additional heating due to on-going HeII reionization.
The measurement of the temperature reduces one source of uncertainty in the
lower limit to the baryon density implied by the observed mean flux decrement.
We find that the temperature cannot be reliably measured for under-dense gas,
because the velocities due to expansion always dominate the widths of the
corresponding weak lines.Comment: submitted to Ap
Halo-Galaxy Lensing: A Full Sky Approach
The halo-galaxy lensing correlation function or the average tangential shear
profile over sampled halos is a very powerful means of measuring the halo
masses, the mass profile, and the halo-mass correlation function of very large
separations in the linear regime. We reformulate the halo-galaxy lensing
correlation in harmonic space. We find that, counter-intuitively, errors in the
conventionally used flat-sky approximation remain at a % level even at very
small angles. The errors increase at larger angles and for lensing halos at
lower redshifts: the effect is at a few % level at the baryonic acoustic
oscillation scales for lensing halos of , and comparable with the
effect of primordial non-Gaussianity with at large
separations. Our results allow to readily estimate/correct for the full-sky
effect on a high-precision measurement of the average shear profile available
from upcoming wide-area lensing surveys.Comment: 12 pages, 4 figure
Large-scale Correlation of Mass and Galaxies with the Lyman-alpha Forest Transmitted Flux
We present predictions of the correlation between the Lyman-alpha forest
absorption in quasar spectra and the mass within \sim 5 Mpc/h (comoving) of the
line of sight, using fully hydrodynamic and hydro-PM numerical simulations of
the cold dark matter model supported by present observations. The observed
correlation based on galaxies and the Lya forest can be directly compared to
our theoretical results, assuming that galaxies are linearly biased on large
scales. Specifically, we predict the average value of the mass fluctuation,
, conditioned to a fixed value of the Lya forest transmitted flux
delta_F, after they have been smoothed over a 10 Mpc/h cube and line of sight
interval, respectively. We find that /sigma_m as a function of
delta_F/sigma_F has a slope of 0.6 at this smoothing scale, where sigma_m and
sigma_F are the rms dispersions (this slope should decrease with the smoothing
scale). We show that this value is largely insensitive to the cosmological
model and other Lya forest parameters. Comparison of our predictions to
observations should provide a fundamental test of our ideas on the nature of
the Lya forest and the distribution of galaxies, and can yield a measurement of
the bias factor of any type of galaxies that are observed in the vicinity of
Lya forest lines of sight.Comment: Submitted to ApJ, 41 page
The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyman-alpha Forest
A sample of eight quasars observed at high resolution and signal-to-noise is
used to determine the probability distribution function (PDF), the power
spectrum, and the correlation function of the transmitted flux in the \lya
forest, in three redshift bins centered at z=2.41, 3.00, and 3.89. All the
results are presented in tabular form, with full error covariance matrices to
allow for comparisons with any numerical simulations and with other data sets.
The observations are compared with a numerical simulation of the \lya forest of
a Lambda-CDM model with Omega=0.4, known to agree with other large-scale
structure observational constraints. There is excellent agreement for the PDF,
if the mean transmitted flux is adjusted to match the observations. A small
difference between the observed and predicted PDF is found at high fluxes and
low redshift, which may be due to the uncertain effects of fitting the spectral
continuum. Using the numerical simulation, we show how the flux power spectrum
can be used to recover the initial power spectrum of density fluctuations. From
our sample of eight quasars, we measure the amplitude of the mass power
spectrum to correspond to a linear variance per unit ln(k) of
at k=0.04(km/s)^{-1} and z=3, and the slope of
the power spectrum near the same k to be (statistical error
bars). The results are statistically consistent with Croft et. al. (1999),
although our value for the rms fluctuation is lower by a factor 0.75. For the
Lambda-CDM model we use, the implied primordial slope is , and
the normalization is .Comment: submitted to Ap
Small-Angle Scattering of X-Rays from Extragalactic Sources by Dust in Intervening Galaxies
Gamma-ray bursts are now known to be a cosmological population of objects,
which are often accompanied by X-ray and optical afterglows. The total energy
emitted in the afterglow can be similar to the energy radiated in the gamma-ray
burst itself. If a galaxy containing a large column density of dust is near the
line of sight to a gamma-ray burst, small-angle scattering of the X-rays due to
diffraction by the dust grains will give rise to an X-ray echo of the
afterglow. A measurement of the angular size of the echo at a certain time
after the afterglow is observed yields a combination of the angular diameter
distances to the scattering galaxy and the gamma-ray burst that can be used to
constrain cosmological models in the same way as a time delay in a
gravitational lens. The scattering galaxy will generally cause gravitational
lensing as well, and this should modify the shape of the X-ray echo from a
circular ring.
The main difficulty in detecting this phenomenon is the very low flux
expected for the echo. The flux can be increased when the gamma-ray burst is
highly magnified by gravitational lensing, or when the deflecting galaxy is at
low redshift. X-ray echos of continuous (but variable) sources, such as
quasars, may also be detectable with high-resolution instruments and would
allow similar measurements.Comment: To be published in Ap
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