1,382 research outputs found
The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System
As large optical quasar surveys for damped Lya become a reality and the study
of star forming gas in the early Universe achieves statistical robustness, it
is now vital to identify and quantify the sources of systematic error. Because
the nature of optically-selected quasar surveys makes them vulnerable to dust
obscuration, we have undertaken a radio-selected quasar survey for damped Lya
systems to address this bias. We present the definition and results of this
survey. We then combine our sample with the CORALS dataset to investigate the
HI column density distribution function f(N) of damped Lya systems toward
radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1
N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in
excellent agreement with that of optically-selected samples at low N(HI), an
important yet expected result given that obscuration should have negligible
effect at these gas columns. However, because of the relatively small size of
the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not
well constrained at large N(HI) and the first moment of the HI distribution
function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep
enough, however, that extrapolating it to higher column densities implies only
a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g
= 1.15 x 10^-3, agrees well with the results of optically-selected surveys.
While our results indicate that dust obscuration is likely not a major issue
for surveys of damped Lya systems, we estimate that a radio-selected sample of
approximately 100 damped Lya systems will be required to obtain the precision
necessary to absolutely confirm an absence of dust bias.Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 200
The Deuterium Abundance at z=0.701 towards QSO 1718+4807
We present constraints on the deuterium to hydrogen ratio (D/H) in the
metal-poor gas cloud at redshift towards QSO 1718+4807. We use new
Keck spectra in addition to Hubble Space Telescope (HST) and International
Ultraviolet Explorer (IUE) spectra. We use an improved redshift and a lower \HI
column density to model the absorption. The HST spectrum shows an asymmetric
Lyman- (\lya) feature which is produced by either \HI at a second
velocity, or a high abundance of D. Three models with a single simple H+D
component give (95%), a much
larger range than reported by Webb et al (1997a,b). A more sophisticated
velocity distribution, or a second component is necessary for lower D/H. With
two components, which could be a part of one absorbing structure, or separate
clouds in a galaxy halo, we find . We do not know if
this second component is present, but it is reasonable because 40 -- 100% of
absorption systems with similar redshifts and \HI column densities have more
than one component.Comment: 13 pages, 4 figures, to appear in the Astronomical Journal (Jan 1999
Elemental Abundances in Two High Column Density Damped Lyman Alpha Systems at z < 1.5
We present Keck/HIRES abundance measurements and metal-line kinematic
profiles of the damped Lyman alpha systems (DLAs) towards the quasars Q0933+733
(z_abs=1.479) and Q0948+433 (z_abs=1.233). These two DLAs have among the five
highest HI column densities at any redshift: N(HI)=4.2E21 cm^{-2}. The
metal-line data, presented here for the first time, reveal that these DLAs are
noteworthy for several other reasons as well. 1) The Q0933+733 DLA exhibits
simple kinematic structure with unusually narrow velocity widths as measured
from its unsaturated metal lines (delta v=16 km/s). At 2.6% solar, it has the
second lowest metallicity at z<2. 2) The Q0948+433 DLA has among the strongest
metal-line transitions of any known DLA. The saturated SiII1808 line implies a
high metallicity ([Si/H]> -1) and a significant alpha-enhancement. The strong
metal lines of this DLA have made possible the detection of TiII1910, CoII2012,
and MgI2026. 3) We find that the relative gas-phase abundances of both DLAs
follow the general trend seen at high redshift, e.g., enhanced Zn/Fe and Si/Fe,
and sub-solar Mn/Fe, indicating that there is little evolution in the
nucleosynthetic patterns of DLAs down to this epoch. 4) Their high HI column
densities imply that these DLAs dominate the column density-weighted cosmic
mean metallicity, , of the universe at z<1.5. Using the 15 DLAs with
measured metallicities in the redshift interval 0.4 =
-0.89^{+0.40}_{-0.33}, where the uncertainties are 95% confidence limits.Comment: 25 pages. Accepted for publication in the Astronomical Journa
UVUDF: Ultraviolet Imaging of the Hubble Ultradeep Field with Wide-field Camera 3
We present an overview of a 90-orbit Hubble Space Telescope treasury program
to obtain near ultraviolet imaging of the Hubble Ultra Deep Field using the
Wide Field Camera 3 UVIS detector with the F225W, F275W, and F336W filters.
This survey is designed to: (i) Investigate the episode of peak star formation
activity in galaxies at 1<z<2.5; (ii) Probe the evolution of massive galaxies
by resolving sub-galactic units (clumps); (iii) Examine the escape fraction of
ionizing radiation from galaxies at z~2-3; (iv) Greatly improve the reliability
of photometric redshift estimates; and (v) Measure the star formation rate
efficiency of neutral atomic-dominated hydrogen gas at z~1-3. In this overview
paper, we describe the survey details and data reduction challenges, including
both the necessity of specialized calibrations and the effects of charge
transfer inefficiency. We provide a stark demonstration of the effects of
charge transfer inefficiency on resultant data products, which when
uncorrected, result in uncertain photometry, elongation of morphology in the
readout direction, and loss of faint sources far from the readout. We agree
with the STScI recommendation that future UVIS observations that require very
sensitive measurements use the instrument's capability to add background light
through a "post-flash". Preliminary results on number counts of UV-selected
galaxies and morphology of galaxies at z~1 are presented. We find that the
number density of UV dropouts at redshifts 1.7, 2.1, and 2.7 is largely
consistent with the number predicted by published luminosity functions. We also
confirm that the image mosaics have sufficient sensitivity and resolution to
support the analysis of the evolution of star-forming clumps, reaching 28-29th
magnitude depth at 5 sigma in a 0.2 arcsecond radius aperture depending on
filter and observing epoch.Comment: Accepted A
The UCSD HIRES/KeckI Damped Lya Abundance Database: I. The Data
We present new chemical abundance measurements of 16 damped Lya systems at
z>1.5 and update our previous abundance analyses. The entire database presented
here was derived from HIRES observations on the Keck I telescope, reduced with
the same software package, and analysed with identical techniques. Altogether,
we present a large, homogeneous database of chemical abundance measurements for
protogalaxies in the early universe, ideal for studying a number of important
aspects of galaxy formation. In addition, we have established an online
directory for this database and will continuously update the results.Comment: 49 pages, 39 figures. Uses emulateapj.sty. Accepted to ApJS June 8,
2001. Visit http://kingpin.ucsd.edu/~hiresdl
The catalytic subunit of the system L1 amino acid transporter (S<i>lc7a5</i>) facilitates nutrient signalling in mouse skeletal muscle
The System L1-type amino acid transporter mediates transport of large neutral amino acids (LNAA) in many mammalian cell-types. LNAA such as leucine are required for full activation of the mTOR-S6K signalling pathway promoting protein synthesis and cell growth. The SLC7A5 (LAT1) catalytic subunit of high-affinity System L1 functions as a glycoprotein-associated heterodimer with the multifunctional protein SLC3A2 (CD98). We generated a floxed Slc7a5 mouse strain which, when crossed with mice expressing Cre driven by a global promoter, produced Slc7a5 heterozygous knockout (Slc7a5+/-) animals with no overt phenotype, although homozygous global knockout of Slc7a5 was embryonically lethal. Muscle-specific (MCK Cre-mediated) Slc7a5 knockout (MS-Slc7a5-KO) mice were used to study the role of intracellular LNAA delivery by the SLC7A5 transporter for mTOR-S6K pathway activation in skeletal muscle. Activation of muscle mTOR-S6K (Thr389 phosphorylation) in vivo by intraperitoneal leucine injection was blunted in homozygous MS-Slc7a5-KO mice relative to wild-type animals. Dietary intake and growth rate were similar for MS-Slc7a5-KO mice and wild-type littermates fed for 10 weeks (to age 120 days) with diets containing 10%, 20% or 30% of protein. In MS-Slc7a5-KO mice, Leu and Ile concentrations in gastrocnemius muscle were reduced by ∼40% as dietary protein content was reduced from 30 to 10%. These changes were associated with >50% decrease in S6K Thr389 phosphorylation in muscles from MS-Slc7a5-KO mice, indicating reduced mTOR-S6K pathway activation, despite no significant differences in lean tissue mass between groups on the same diet. MS-Slc7a5-KO mice on 30% protein diet exhibited mild insulin resistance (e.g. reduced glucose clearance, larger gonadal adipose depots) relative to control animals. Thus, SLC7A5 modulates LNAA-dependent muscle mTOR-S6K signalling in mice, although it appears non-essential (or is sufficiently compensated by e.g. SLC7A8 (LAT2)) for maintenance of normal muscle mass
B →Vℓ+ℓ− in the Standard Model from light-cone sum rules
We present , , , and
form factors from light-cone sum rules (LCSR) at
for twist-2 and 3 and for
twist-4 with updated hadronic input parameters. Three asymptotic light-cone
distribution amplitudes of twist- (and ) are determined, necessary for
the form factors to obey the equations of motion. It is argued that the latter
constrain the uncertainty of tensor-to-vector form factor ratios thereby
improving the prediction of zeros of helicity amplitudes of major importance
for angular observables. We provide easy-to-use fits to the
LCSR results, including the full error correlation matrix, in all modes at low
as well as combined fits to LCSR and lattice results covering the entire
kinematic range for , and . The error
correlation matrix avoids the problem of overestimating the uncertainty in
phenomenological applications. Using the new form factors and recent
computations of non-factorisable contributions we provide Standard Model
predictions for as well as and
at low dilepton invariant mass. Employing our form factor results we extract the CKM element
from the semileptonic decays and
find good agreement with other exclusive determinations.Comment: 64 pages, 7 figures, 15 tables. v3: Minor clarifications, numerics
unchanged. Matches version published in JHE
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