1,652 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
An Overdensity of Lyman-alpha Emitters at Redshift z=5.7 near the Hubble Ultra Deep Field
We have identified an obvious and strong large scale structure at redshift
z=5.75 in a wide (31 by 33 arcminute) field, narrowband survey of the Chandra
Deep Field South region. This structure is traced by 17 candidate Lyman alpha
emitters, among which 12 are found in an 823nm filter (corresponding to Lyman
alpha at z=5.77 +- 0.03) and 5 in an 815nm image (z=5.70 +- 0.03). The Lyman
alpha emitters in both redshift bins are concentrated in one quadrant of the
field. The Hubble Ultra Deep Field, Chandra Deep Field South, and GOODS-South
fields all lie near the edge of this overdense region. Our results are
consistent with reports of an overdensity in the UDF region at z=5.9. This
structure is the highest redshift overdensity found so far.Comment: 12 pages, AASTeX. Submitted to ApJ Letters, and revised in response
to referee's comment
Starburst Intensity Limit of Galaxies at z~5-6
The peak star formation intensity in starburst galaxies does not vary
significantly from the local universe to redshift z~6. We arrive at this
conclusion through new surface brightness measurements of 47 starburst galaxies
at z~5-6, doubling the redshift range for such observations. These galaxies are
spectroscopically confirmed in the Hubble Ultra Deep Field (HUDF) through the
GRism ACS program for Extragalactic Science (GRAPES) project. The starburst
intensity limit for galaxies at z~5-6 agree with those at z~3-4 and z~0 to
within a factor of a few, after correcting for cosmological surface brightness
dimming and for dust. The most natural interpretation of this constancy over
cosmic time is that the same physical mechanisms limit starburst intensity at
all redshifts up to z~6 (be they galactic winds, gravitational instability, or
something else). We do see two trends with redshift: First, the UV spectral
slope of galaxies at z~5-6 is bluer than that of z~3 galaxies, suggesting an
increase in dust content over time. Second, the galaxy sizes from z~3 to z~6
scale approximately as the Hubble parameter 1/H(z). Thus, galaxies at z~6 are
high redshift starbursts, much like their local analogs except for slightly
bluer colors, smaller physical sizes, and correspondingly lower overall
luminosities. If we now assume a constant maximum star formation intensity, the
differences in observed surface brightness between z~0 and z~6 are consistent
with standard expanding cosmology and strongly inconsistent with tired light
model.Comment: Accepted for publication in ApJ (23 pages, 5 figures). Minor changes
to tex
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
GRB Energetics and the GRB Hubble Diagram: Promises and Limitations
We present a complete sample of 29 GRBs for which it has been possible to
determine temporal breaks (or limits) from their afterglow light curves. We
interpret these breaks within the framework of the uniform conical jet model,
incorporating realistic estimates of the ambient density and propagating error
estimates on the measured quantities. In agreement with our previous analysis
of a smaller sample, the derived jet opening angles of those 16 bursts with
redshifts result in a narrow clustering of geometrically-corrected gamma-ray
energies about E_gamma = 1.33e51 erg; the burst-to-burst variance about this
value is a factor of 2.2. Despite this rather small scatter, we demonstrate in
a series of GRB Hubble diagrams, that the current sample cannot place
meaningful constraints upon the fundamental parameters of the Universe. Indeed
for GRBs to ever be useful in cosmographic measurements we argue the necessity
of two directions. First, GRB Hubble diagrams should be based upon fundamental
physical quantities such as energy, rather than empirically-derived and
physically ill-understood distance indicators. Second, a more homogeneous set
should be constructed by culling sub-classes from the larger sample. These
sub-classes, though now first recognizable by deviant energies, ultimately must
be identifiable by properties other than those directly related to energy. We
identify a new sub-class of GRBs (``f-GRBs'') which appear both underluminous
by factors of at least 10 and exhibit a rapid fading at early times. About
10-20% of observed long-duration bursts appear to be f-GRBs.Comment: Accepted to the Astrophysical Journal (20 May 2003). 19 pages, 3
Postscript figure
A Physical Model of Lyman Alpha Emitters
We present a simple physical model for populating dark matter halos with
Lyman Alpha Emiiters(LAEs) and predict the physical properties of LAEs at
z~3-7. The central tenet of this model is that the Ly-alpha luminosity is
proportional to the star formation rate (SFR) which is directly related to the
halo mass accretion rate. The only free parameter in our model is then the
star-formation efficiency (SFE). An efficiency of 2.5% provides the best-fit to
the Ly-alpha luminosity function (LF) at redshift z=3.1, and we use this SFE to
construct Ly-alpha LFs at other redshifts. Our model reproduce the Ly-alpha
LFs, stellar ages, SFR ~1-10; Msun/yr, stellar masses ~ 10^7-10^8 Msun and the
clustering properties of LAEs at z~3-7. We find the spatial correlation lengths
ro ~ 3-6 Mpc/h, in agreement with the observations. Finally, we estimate the
field-to-field variation ~ 30% for current volume and flux limited surveys,
again consistent with observations. Our results suggest that the star
formation, and hence Ly-alpha emission in LAEs is powered by the accretion of
new material, and that the physical properties of LAEs do not evolve
significantly over a wide range of redshifts. Relating the accreted mass,
rather than the total mass of halos, to the Ly-alpha luminosity of LAEs
naturally gives rise to the duty cycle of LAEs.Comment: Published in Ap
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