689 research outputs found
C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems
This study focuses on some of the most metal-poor damped Lyman alpha
absorbers known in the spectra of high redshift QSOs, using new and archival
observations obtained with UV-sensitive echelle spectrographs on the Keck and
VLT telescopes. The weakness and simple velocity structure of the absorption
lines in these systems allows us to measure the abundances of several elements,
and in particular those of C, N, and O, a group that is difficult to study in
DLAs of more typical metallicities. We find that when the oxygen abundance is
less than about 1/100 of solar, the C/O ratio in high redshift DLAs and
sub-DLAs matches that of halo stars of similar metallicity and shows higher
values than expected from galactic chemical evolution models based on
conventional stellar yields. Furthermore, there are indications that at these
low metallicities the N/O ratio may also be above simple expectations and may
exhibit a minimum value, as proposed by Centurion and her collaborators in
2003. Both results can be interpreted as evidence for enhanced production of C
and N by massive stars in the first few episodes of star formation, in our
Galaxy and in the distant proto-galaxies seen as QSO absorbers. The higher
stellar yields implied may have an origin in stellar rotation which promotes
mixing in the stars' interiors, as considered in some recent model
calculations. We briefly discuss the relevance of these results to current
ideas on the origin of metals in the intergalactic medium and the universality
of the stellar initial mass function.Comment: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Cosmological Uses of Gamma-Ray Bursts
Studies of the cosmic gamma-ray bursts (GRBs) and their host galaxies are
starting to provide interesting or even unique new insights in observational
cosmology. GRBs represent a new way of identifying a population of star-forming
galaxies at cosmological redshifts. GRB hosts are broadly similar to the normal
field galaxy populations at comparable redshifts and magnitudes, and indicate
at most a mild luminosity evolution out to z ~ 1.5 - 2. GRB optical afterglows
seen in absorption provide a powerful new probe of the ISM in dense, central
regions of their host galaxies, complementary to the traditional studies using
QSO absorbers. Some GRB hosts are heavily obscured, and provide a new way to
select a population of cosmological sub-mm sources, and a novel constraint on
the total obscured fraction of star formation over the history of the universe.
Finally, detection of GRB afterglows at z > 6 may provide a unique way to probe
the primordial star formation, massive IMF, early IGM, and chemical enrichment
at the end of the cosmic reionization era.Comment: An invited review, to appear in: "Gamma-Ray Bursts in the Afterglow
Era: 3rd Workshop", ASPCS, in press; LaTeX file, 8 pages, 1 eps figure, style
files include
The Host Galaxy of GRB 990123
We present deep images of the field of gamma-ray burst (GRB) 990123 obtained
in a broad-band UV/visible bandpass with the Hubble Space Telescope, and deep
near-infrared images obtained with the Keck-I 10-m telescope. Both the HST and
Keck images show that the optical transient (OT) is clearly offset by 0.6
arcsec from an extended object, presumably the host galaxy. This galaxy is the
most likely source of the metallic-line absorption at z = 1.6004 seen in the
spectrum of the OT. With magnitudes V_{C} ~ 24.6 +/- 0.2 and K = 21.65 +/- 0.30
mag this corresponds to an L ~ 0.7 L_* galaxy, assuming that it is located at z
= 1.6. The estimated unobscured star formation rate is SFR ~ 6 M_sun/yr, which
is not unusually high for normal galaxies at comparable redshifts. The strength
of the observed metallic absorption lines is suggestive of a relatively high
metallicity of the gas, and thus of a chemically evolved system which may be
associated with a massive galaxy. It is also indicative of a high column
density of the gas, typical of damped Ly-alpha systems at high redshifts. We
conclude that this is the host galaxy of GRB 990123. No other obvious galaxies
are detected within the same projected radius from the OT. There is thus no
evidence for strong gravitational lensing magnification of this burst, and some
alternative explanation for its remarkable energetics may be required. The
observed offset of the OT from the center of its apparent host galaxy, 5.5 +/-
0.9 proper kpc (projected) in the galaxy's rest-frame, both refutes the
possibility that GRBs are related to galactic nuclear activity and supports
models of GRBs which involve the death and/or merger of massive stars. Further,
the HST image suggests an intimate connection of GRB 990123 and a star-forming
region.Comment: Updated references. 12 pages including 3 Postscript figures. Camera-
ready reproductions of the figures can be found at
http://astro.caltech.edu/~jsb/GRB/grb990123.htm
Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions.
The dentition is an extremely important organ in mammals with variation in timing and sequence of eruption, crown morphology, and tooth size enabling a range of behavioral, dietary, and functional adaptations across the class. Within this suite of variable mammalian dental phenotypes, relative sizes of teeth reflect variation in the underlying genetic and developmental mechanisms. Two ratios of postcanine tooth lengths capture the relative size of premolars to molars (premolar-molar module, PMM), and among the three molars (molar module component, MMC), and are known to be heritable, independent of body size, and to vary significantly across primates. Here, we explore how these dental traits vary across mammals more broadly, focusing on terrestrial taxa in the clade of Boreoeutheria (Euarchontoglires and Laurasiatheria). We measured the postcanine teeth of NÂ =Â 1,523 boreoeutherian mammals spanning six orders, 14 families, 36 genera, and 49 species to test hypotheses about associations between dental proportions and phylogenetic relatedness, diet, and life history in mammals. Boreoeutherian postcanine dental proportions sampled in this study carry conserved phylogenetic signal and are not associated with variation in diet. The incorporation of paleontological data provides further evidence that dental proportions may be slower to change than is dietary specialization. These results have implications for our understanding of dental variation and dietary adaptation in mammals
The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later
We have used the VLA FIRST survey and the APM catalog of the POSS-I plates as
the basis for constructing a new radio-selected sample of optically bright
quasars. This is the first radio-selected sample that is competitive in size
with current optically selected quasar surveys. Using only two basic criteria,
radio-optical positional coincidence and optical morphology, quasars and BL
Lacs can be identified with 60% selection efficiency; the efficiency increases
to 70% for objects fainter than magnitude 17. We show that a more sophisticated
selection scheme can predict with better than 85% reliability which candidates
will turn out to be quasars.
This paper presents the second installment of the FIRST Bright Quasar Survey
with a catalog of 636 quasars distributed over 2682 square degrees. The quasar
sample is characterized and all spectra are displayed. The FBQS detects both
radio-loud and radio-quiet quasars out to a redshift z>3. We find a large
population of objects of intermediate radio-loudness; there is no evidence in
our sample for a bimodal distribution of radio characteristics. The sample
includes ~29 broad absorption line quasars, both high and low ionization, and a
number of new objects with remarkable optical spectra.Comment: 41 pages plus 39 gifs which contain all quasar spectra. Accepted for
publication in the Astrophysical Journal Supplement Serie
The GRB Host Galaxies and Redshifts
Observations of GRB host galaxies and their environments in general can
provide valuable clues about the nature of progenitors. Bursts are associated
with faint, ~ 25 mag, galaxies at cosmological redshifts, ~ 1. The host
galaxies span a range of luminosities and morphologies, and appear to be
broadly typical for the normal, evolving, actively star-forming galaxy
populations at comparable redshifts and magnitudes, but may have somewhat
elevated SFR per unit luminosity. There are also spectroscopic hints of massive
star formation, from the ratios of [Ne III] and [O II] lines. The observed,
unobscured star formation rates are typically a few M_sun/yr, but a
considerable fraction of the total star formation in the hosts may be obscured
by dust. A census of detected optical afterglows provides a powerful new handle
on the obscured fraction of star formation in the universe; the current results
suggest that at most a half of the massive star formation was hidden by dust.Comment: Invited review, latex, 8 pages with 3 eps figures, style files
included. To appear in proc. "Gamma-Ray Bursts in the Afterglow Era: 2nd
Workshop", eds. N. Masetti et al., ESO Astrophysics Symposia, Berlin:
Springer Verlag, in press (2001
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