1,557 research outputs found
On the compared accuracy and reliability of spectroscopic and photometric redshift measurements
We present a comparison between the spectroscopic catalog of the HDF recently
published by Cohen et al and the redshifts measured by our group for those
objects using photometric techniques. We perform this comparison in order to
characterize the errors associated to the photometric redshift technique. The
spectroscopic sample includes over 140 objects in the HDF proper, representing
the deepest, cleanest, most complete spectroscopic catalog ever compiled. We
study each object for which our redshift and the one measured by Cohen et al
disagree. In most cases the photometric evidence is strong enough to call for a
careful review of the spectroscopic values, as they seem to be in error. It is
possible to characterize the systematic errors associated to our technique,
which when combined with the photometric errors allow us to obtain complete
information on the redshift of each galaxy and its associated confidence
interval, regardless of apparent magnitude. One of the main conclusions of this
study is that, to date, all the redshifts from our published catalog that have
been checked have been shown to be correct (within the stated confidence
limits). This implies that our galaxy template set is a fair representation of
the galaxy population at all redshifts (0<z<6) and magnitudes (R<24) explored
to date. On the other hand, spectroscopy of faint sources is subject to unknown
and uncharacterized systematic errors, that will in turn be transmitted to any
photometric redshift technique which uses spectroscopic samples as calibration.
Our analysis proves that photometric redshift techniques can and must be used
to extend the range of applicability of the spectroscopic redshift
measurements. (Abridged)Comment: Accepted for publication in ApJSS. Minor changes in presentation and
discussion. No changes to main results. ApJ preprint format, 27 pages, 16
embedded figure
Early afterglows as probes for the reionization epoch
The nature of Gamma-Ray Burst progenitors is still a debated issue, but
consensus is growing on the association of GRBs with massive stars.
Furthermore, current models for the reionization of the universe consider
massive Pop--III stars as the sources of the ionizing photons. There could then
be a natural link between GRBs and reionization. The reionization epoch can be
measured through prompt IR spectral observations of high redshift GRBs. For
this, GRBs are better than quasars: they produce a smaller HII region even if
they are much brighter than quasars (but for a much shorter time) and then,
contrary to quasars, they do not modify the absorption properties of the
surrounding IGM.Comment: 3 pages, to appear in the proceedings of the the 2nd Workshop on
Gamma-Ray Bursts in the Afterglow Era, Rome, Oct. 200
The Origin of \lya Absorption Systems at ---Implications from the Hubble Deep Field
The Hubble Deep Field images have provided us with a unique chance to relate
statistical properties of high-redshift galaxies to statistical properties of
\lya absorption systems. Combining an {\em empirical} measure of the galaxy
surface density versus redshift with an {\em empirical} measure of the gaseous
extent of galaxies, we predict the number density of \lya absorption systems
that originate in extended gaseous envelopes of galaxies versus redshift. We
show that at least 50% and as much as 100% of observed \lya absorption systems
of W\apg0.32 \AA can be explained by extended gaseous envelops of galaxies.
Therefore, we conclude that known galaxies of known gaseous extent must produce
a significant fraction and perhaps all of \lya absorption systems over a large
redshift range.Comment: 12 pages, 3 figures, accepted for publication in the Astrophysical
Journal, April 10, 2000 issu
The Star Formation Rate Intensity Distribution Function--Implications for the Cosmic Star Formation Rate History of the Universe
We address the effects of cosmological surface brightness dimming on
observations of faint galaxies by examining the distribution of "unobscured"
star formation rate intensities versus redshift. We use the star formation rate
intensity distribution function to assess the ultraviolet luminosity density
versus redshift, based on our photometry and photometric redshift measurements
of faint galaxies in the HDF and the HDF--S WFPC2 and NICMOS fields. We find
that (1) previous measurements have missed a dominant fraction of the
ultraviolet luminosity density of the universe at high redshifts by neglecting
cosmological surface brightness dimming effects, which are important at
redshifts larger than z = 2, (2) the incidence of the highest intensity star
forming regions increases monotonically with redshift, and (3) the ultraviolet
luminosity density plausibly increases monotonically with redshift through the
highest redshifts observed. By measuring the spectrum of the luminosity density
versus redshift, we also find that (4) previous measurements of the ultraviolet
luminosity density at redshifts z < 2 must be reduced by a factor 2 to allow
for the spectrum of the luminosity density between rest-frame wavelengths 1500
and 2800 A. And by comparing with observations of high-redshift damped
Lyman-alpha absorption systems detected toward background QSOs, we further find
that (5) the distribution of star formation rate intensities matches the
distribution of neutral hydrogen column densities at redshifts z = 2 through 5,
which establishes a quantitative connection between high-redshift galaxies and
high column density gas and suggests that high-redshift damped Lyman-alpha
absorption systems trace lower star formation rate intensity regions of the
same galaxies detected in star light in the HDF and HDF--S.Comment: 28 pages, 9 figures; accepted for publication in the Astrophysical
Journa
Photometry and Photometric Redshifts of Faint Galaxies in the Hubble Deep Field South NICMOS Field
We present a catalog of photometry and photometric redshifts of 335 faint
objects in the HDF-S NICMOS field. The analysis is based on (1) infrared images
obtained with the Hubble Space Telescope (HST) using the Near Infrared Camera
and Multi-Object Spectrograph (NICMOS) with the F110W, F160W, and F222M
filters, (2) an optical image obtained with HST using the Space Telescope
Imaging Spectrograph (STIS) with no filter, and (3) optical images obtained
with the European Southern Observatory (ESO) Very Large Telescope (VLT) with U,
B, V, R, and I filters. The primary utility of the catalog of photometric
redshifts is as a survey of faint galaxies detected in the NICMOS F160W and
F222M images. The sensitivity of the survey varies significantly with position,
reaching a limiting depth of AB(16,000) ~ 28.7 and covering 1.01 arcmin^2 to
AB(16,000) = 27 and 1.05 arcmin^2 to AB(16,000) = 26.5. The catalog of
photometric redshifts identifies 21 galaxies (or 6% of the total) of redshift z
> 5, 8 galaxies (or 2% of the total) of redshift z > 10, and 11 galaxies (or 3%
of the total) of best-fit spectral type E/S0, of which 5 galaxies (or 1% of the
total) are of redshift z > 1.Comment: 33 pages, 10 figures, accepted for publication in the Astrophysical
Journal, August 1, 2000 issu
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