2,597 research outputs found
The Detectability of High Redshift Lyman Alpha Emission Lines Prior to the Reionization of the Universe
For a source of Ly alpha radiation embedded in a neutral intergalactic medium
(IGM) prior to the reionization epoch, the emission line is strongly suppressed
by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough
can extend to the red side of the emission line, and erase a significant
fraction of the total line flux. However, the transmitted fraction increases
with the size of the local cosmological HII region surrounding the source, and
therefore with the ionizing luminosity and age of the source. Motivated by the
recent discovery of a Ly alpha emitting galaxy at a redshift z=6.56 (Hu et al.
2002), possibly prior to the reionization of the IGM, we revisit the effects of
a neutral IGM on the Ly alpha emission line. We show that even for faint
sources with little ionizing continuum, the emission line can remain
observable. In particular, the line detected by Hu et al. is consistent with a
source embedded in a neutral IGM. We provide characterizations of the asymmetry
and total transmitted flux of the Ly alpha line as functions of the ionizing
emissivity of its source. A statistical sample of Ly alpha emitters extending
beyond the reionization redshift can be a useful probe of reionization.Comment: Submitted to ApJL, 4 figures include
`First Light' in the Universe; What Ended the "Dark Age"?
The universe would have been completely dark between the epoch of
recombination and the development of the first non-linear structure. But at
redshifts beyond 5 -- perhaps even beyond 20 -- stars formed within
`subgalaxies' and created the first heavy elements; these same systems
(together perhaps with `miniquasars') generated the UV radiation that ionized
the IGM, and maybe also the first significant magnetic fields. Although we can
already probe back to , these very first objects may be so faint
that their detection must await next-generation optical and infrared
telescopes. Observations in other wavebands may offer indirect clues to when
reionization occurred. Despite the rapid improvements in numerical simulations,
the processes of star formation and feedback are likely to remain a challenge
for the next decade.Comment: For ``Physics Reports'' special issue in memory of D.N. Schram
A Combinatorial Formula for Macdonald Polynomials
We prove a combinatorial formula for the Macdonald polynomial H_mu(x;q,t)
which had been conjectured by the first author. Corollaries to our main theorem
include the expansion of H_mu(x;q,t) in terms of LLT polynomials, a new proof
of the charge formula of Lascoux and Schutzenberger for Hall-Littlewood
polynomials, a new proof of Knop and Sahi's combinatorial formula for Jack
polynomials as well as a lifting of their formula to integral form Macdonald
polynomials, and a new combinatorial rule for the Kostka-Macdonald coefficients
K_{lambda,mu}(q,t) in the case that mu is a partition with parts less than or
equal to 2.Comment: 29 page
Probing the Reionization History Using the Spectra of High-Redshift Sources
We quantify and discuss the footprints of neutral hydrogen in the
intergalactic medium (IGM) on the spectra of high-redshift (z ~ 6) sources,
using mock spectra generated from hydrodynamical simulations of the IGM. We
show that it should be possible to extract relevant parameters, including the
mean neutral fraction in the IGM, and the radius of the local cosmological
Stromgren region, from the flux distribution in the observed spectra of distant
sources. We focus on quasars, but a similar analysis is applicable to galaxies
and gamma ray burst (GRB) afterglows. We explicitly include uncertainties in
the spectral shape of the assumed source template near the Lyman alpha line.
Our results suggest that a mean neutral hydrogen fraction, x(HI) of unity can
be statistically distinguished from x(HI)<0.01, by combining the spectra of
tens of bright (M = -27) quasars. Alternatively, the same distinction can be
achieved using the spectra of several hundred sources that are ~100 times
fainter. Furthermore, if the radius of the Stromgren sphere can be
independently constrained to within ~10 percent, this distinction can be
achieved using a single source. The information derived from such spectra will
help in settling the current debate as to what extent the universe was
reionized at redshifts near z=6.Comment: modified version, accepted to appear in ApJ, vol. 613, 20 September
200
Effects of the complex mass distribution of dark matter halos on weak lensing cluster surveys
Gravitational lensing effects arise from the light ray deflection by all of
the mass distribution along the line of sight. It is then expected that weak
lensing cluster surveys can provide us true mass-selected cluster samples. With
numerical simulations, we analyze the correspondence between peaks in the
lensing convergence -map and dark matter halos. Particularly we
emphasize the difference between the peak value expected from a dark
matter halo modeled as an isolated and spherical one, which exhibits a
one-to-one correspondence with the halo mass at a given redshift, and that of
the associated -peak from simulations. For halos with the same expected
, their corresponding peak signals in the -map present a wide
dispersion. At an angular smoothing scale of , our
study shows that for relatively large clusters, the complex mass distribution
of individual clusters is the main reason for the dispersion. The projection
effect of uncorrelated structures does not play significant roles. The
triaxiality of dark matter halos accounts for a large part of the dispersion,
especially for the tail at high side. Thus lensing-selected clusters
are not really mass-selected. (abridged)Comment: ApJ accepte
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