5,702 research outputs found
A possible gravitational lens in the Hubble Deep Field South
We model an apparent gravitational lens system HDFS 2232509-603243 in the Hubble Deep Field South. The system consists of a blue V=25 mag arc separated by 0.9 arcsec from a red V=22 mag elliptical galaxy. A mass distribution which follows the observed light distribution with a constant mass-to-light ratio can fit the arc component positions if external shear is added. A good fit is also obtained with simple parameterized models, and all the models predict a forth image fainter than the detection limit. The inferred mass-to-light ratio is roughly 15 in solar units if the lens is at redshift 0.6. Prospects for obtaining spectroscopic redshifts of the elliptical galaxy and the arc are good
The faint-galaxy hosts of gamma-ray bursts
The observed redshifts and magnitudes of the host galaxies of gamma-ray
bursts (GRBs) are compared with the predictions of three basic GRB models, in
which the comoving rate density of GRBs is (1) proportional to the cosmic star
formation rate density, (2) proportional to the total integrated stellar
density and (3) constant. All three models make the assumption that at every
epoch the probability of a GRB occuring in a galaxy is proportional to that
galaxy's broad-band luminosity. No assumption is made that GRBs are standard
candles or even that their luminosity function is narrow. All three rate
density models are consistent with the observed GRB host galaxies to date,
although model (2) is slightly disfavored relative to the others. Models (1)
and (3) make very similar predictions for host galaxy magnitude and redshift
distributions; these models will be probably not be distinguished without
measurements of host-galaxy star-formation rates. The fraction of host galaxies
fainter than 28 mag may constrain the faint end of the galaxy luminosity
function at high redshift, or, if the fraction is observed to be low, may
suggest that the bursters are expelled from low-luminosity hosts. In all
models, the probability of finding a z<0.008 GRB among a sample of 11 GRBs is
less than 10^(-4), strongly suggesting that GRB 980425, if associated with
supernova 1998bw, represents a distinct class of GRBs.Comment: 7 pages, ApJ in press, revised to incorporate yet more new and
revised observational result
A maximum-likelihood method for improving faint source flux and color estimates
Flux estimates for faint sources or transients are systematically biased high
because there are far more truly faint sources than bright. Corrections which
account for this effect are presented as a function of signal-to-noise ratio
and the (true) slope of the faint-source number-flux relation. The corrections
depend on the source being originally identified in the image in which it is
being photometered. If a source has been identified in other data, the
corrections are different; a prescription for calculating the corrections is
presented. Implications of these corrections for analyses of surveys are
discussed; the most important is that sources identified at signal-to-noise
ratios of four or less are practically useless.Comment: 9 pp., accepted for publication in PAS
Testing use of mitochondrial COI sequences for the identification and phylogenetic analysis of New Zealand caddisflies (Trichoptera)
We tested the hypothesis that cytochrome c oxidase subunit 1 (COI) sequences would successfully discriminate recognised species of New Zealand caddisflies. We further examined whether phylogenetic analyses, based on the COI locus, could recover currently recognised superfamilies and suborders. COI sequences were obtained from 105 individuals representing 61 species and all 16 families of Trichoptera known from New Zealand. No sequence sharing was observed between members of different species, and congeneric species showed from 2.3 to 19.5% divergence. Sequence divergence among members of a species was typically low (mean = 0.7%; range 0.0â8.5%), but two species showed intraspecific divergences in excess of 2%. Phylogenetic reconstructions based on COI were largely congruent with previous conclusions based on morphology, although the sequence data did not support placement of the purse-cased caddisflies (Hydroptilidae) within the uncased caddisflies, and, in particular, the Rhyacophiloidea. We conclude that sequence variation in the COI gene locus is an effective tool for the identification of New Zealand caddisfly species, and can provide preliminary phylogenetic inferences. Further research is needed to ascertain the significance of the few instances of high intra-specific divergence and to determine if any instances of sequence sharing will be detected with larger sample sizes
Early-type galaxies in the SDSS. II. Correlations between observables
A magnitude limited sample of nearly 9000 early-type galaxies, in the
redshift range 0.01 < z < 0.3, was selected from the Sloan Digital Sky Survey
using morphological and spectral criteria. The sample was used to study how
early-type galaxy observables, including luminosity L, effective radius R_o,
surface brightness I_o, color, and velocity dispersion sigma, are correlated
with one another. Measurement biases are understood with mock catalogs which
reproduce all of the observed scaling relations and their dependences on
fitting technique. At any given redshift, the intrinsic distribution of
luminosities, sizes and velocity dispersions in our sample are all
approximately Gaussian. A maximum likelihood analysis shows that sigma ~
L^{0.25\pm 0.012}, R_o ~ L^{0.63\pm 0.025}, and R_o ~ I^{-0.75\pm 0.02} in the
r* band. In addition, the mass-to-light ratio within the effective radius
scales as M_o/L ~ L^{0.14\pm 0.02} or M_o/L ~ M_o^{0.22\pm 0.05}, and galaxies
with larger effective masses have smaller effective densities: Delta_o ~
M_o^{-0.52\pm 0.03}. These relations are approximately the same in the g*, i*
and z* bands. Relative to the population at the median redshift in the sample,
galaxies at lower and higher redshifts have evolved only little, with more
evolution in the bluer bands. The luminosity function is consistent with weak
passive luminosity evolution and a formation time of about 9 Gyrs ago.Comment: 29 pages, 11 figures. Accepted by AJ (scheduled for April 2003). This
paper is part II of a revised version of astro-ph/011034
Field testing of World Health Organization (WHO) 2003 recommendations for initiating anti-retroviral therapy (ART) where CD4 is not available revealed low sensitivity among stage I and II patients; a combination of Automated Total Lymphocyte Count (TLC) and Haemoglobin (Hgb) can ration the referral for CD4 testing.
2006 AIDS Conference, Toront
Probing Dark Matter Substructure in Lens Galaxies
We investigate the effects of numerous dark matter subhalos in a galaxy-sized
halo on the events of strong lensing, to assess their presence as expected from
the cold dark matter scenario. Lens galaxies are represented by a smooth
ellipsoid in an external shear field and additional cold dark matter subhalos
taken from Monte Carlo realizations which accord with recent N-body results. We
also consider other possible perturbers, globular clusters and luminous dwarf
satellites, for comparison. We then apply the models to the particular lens
systems with four images, B1422+231 and PG1115+080, for which smooth lens
models are unable to reproduce both the positions of the images and their radio
flux ratios or dust-free optical flux ratios simultaneously. We show that the
perturbations by both globular clusters and dwarf satellites are too small to
change the flux ratios, whereas cold dark matter subhalos are most likely
perturbers to reproduce the observed flux ratios in a statistically significant
manner. This result suggests us the presence of numerous subhalos in lens
galaxies, which is consistent with the results of cosmological N-body
simulations.Comment: 19 pages, including 5 figures, ApJ in pres
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