3,105 research outputs found
A reduced star catalog containing 537 named stars
Positional and color magnitude data for the 537 named stars, are given. A brief translation of the star names and their source language is also presented
A technique for computation of star magnitudes relative to an optical sensor
The theory and techniques used to compute star magnitudes relative to any optical detector (such as the Mariner Mars 1971 Canopus star tracker) are described. Results are given relative to various star detectors
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
The Dynamics and Light Curves of Beamed Gamma Ray Burst Afterglows
The energy requirements of gamma ray bursts have in past been poorly
constrained because of three major uncertainties: The distances to bursts, the
degree of burst beaming, and the efficiency of gamma ray production. The first
of these has been resolved, with both indirect evidence (the distribution of
bursts in flux and position) and direct evidence (redshifted absorption
features in the afterglow spectrum of GRB 970508) pointing to cosmological
distances. We now wish to address the second uncertainty. Afterglows allow a
statistical test of beaming, described in an earlier paper. In this paper, we
modify a standard fireball afterglow model to explore the effects of beaming on
burst remnant dynamics and afterglow emission. If the burst ejecta are beamed
into angle zeta, the burst remnant's evolution changes qualitatively once its
bulk Lorentz factor Gamma < 1/zeta: Before this, Gamma declines as a power law
of radius, while afterwards, it declines exponentially. This change results in
a broken power law light curve whose late-time decay is faster than expected
for a purely spherical geometry. These predictions disagree with afterglow
observations of GRB 970508. We explored several variations on our model, but
none seems able to change this result. We therefore suggest that this burst is
unlikely to have been highly beamed, and that its energy requirements were near
those of isotropic models. More recent afterglows may offer the first practical
applications for our beamed models.Comment: 18 pages, uses emulateapj.sty, four embedded postscript figures.
Submitted to The Astrophysical Journal, 199
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 990123: Reverse and Internal Shock Flashes and Late Afterglow
The prompt (t \siml 0.16 days) light curve and initial 9-th magnitude
optical flash from GRB 990123 can be attributed to a reverse external shock, or
possibly to internal shocks. We discuss the time decay laws and spectral slopes
expected under various dynamical regimes, and discuss the constraints imposed
on the model by the observations, arguing that they provide strongly suggestive
evidence for features beyond those in the simple standard model. The longer
term afterglow behavior is discussed in the context of the forward shock, and
it is argued that, if the steepening after three days is due to a jet geometry,
this is likely to be due to jet-edge effects, rather than sideways expansion.Comment: M.N.R.A.S., subm. 2/26/99; (preprint uses aaspp4.sty), 9 page
The Supreme Court and Public Prayer: The Need for Restrant. By Charles E. Rice.
Results from the school prayer decisions are but a portion of the analysis given by the author. His attitude is unequivocal: The school prayer decisions, handed down by the Supreme Court of the United States in 1962 and 1963, were wrongly decided. Therefore, his analysis is argument. Considering the burden, it is borne well
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