51 research outputs found
High-Redshift Starbursting Dwarf Galaxies Revealed by GRB Afterglows
We present a study of 15 long-duration gamma-ray burst (GRB) host galaxies at
z>2. The GRBs are selected with available early-time afterglow spectra in order
to compare interstellar medium (ISM) absorption-line properties with stellar
properties of the host galaxies. In addition to five previously studied hosts,
we consider new detections for the host galaxies of GRB050820 and GRB060206 and
place 2-sigma upper limits to the luminosities of the remaining unidentified
hosts. We examine the nature of the host galaxy population and find that (1)
the UV luminosity distribution of GRB host galaxies is consistent with
expectations from a UV luminosity weighted random galaxy population with a
median luminosity of =0.1 L*; (2) there exists a moderate correlation
between UV luminosity and SiII 1526 absorption width, which together with the
observed large line widths of W(1526)>1.5 Ang for a large fraction of the
objects suggests a galactic outflow driven velocity field in the host galaxies;
(3) there is tentative evidence for a trend of declining ISM metallicity with
decreasing galaxy luminosity in the star-forming galaxy population at z=2-4;
(4) the interstellar UV radiation field is found ~ 35-350 times higher in GRB
hosts than the Galactic mean value; and (5) additional galaxies are found at <
2" from the GRB host in all fields with known presence of strong MgII
absorbers, but no additional faint galaxies are found at < 2" in fields without
strong MgII absorbers. Our study confirms that the GRB host galaxies (with
known optical afterglows) are representative of unobscured star-forming
galaxies at z>2, and demonstrates that high spatial resolution images are
necessary for an accurate identification of GRB host galaxies in the presence
of strong intervening absorbers.Comment: 24 emulateapj pages, 24 figures, ApJ in press; full-resolution
version available at http://lambda.uchicago.edu/public/tmp/ghost.pd
Ubiquitous outflows in DEEP2 spectra of star-forming galaxies at z=1.4
Galactic winds are a prime suspect for the metal enrichment of the
intergalactic medium and may have a strong influence on the chemical evolution
of galaxies and the nature of QSO absorption line systems. We use a sample of
1406 galaxy spectra at z~1.4 from the DEEP2 redshift survey to show that
blueshifted Mg II 2796, 2803 A absorption is ubiquitous in starforming galaxies
at this epoch. This is the first detection of frequent outflowing galactic
winds at z~1. The presence and depth of absorption are independent of AGN
spectral signatures or galaxy morphology; major mergers are not a prerequisite
for driving a galactic wind from massive galaxies. Outflows are found in
coadded spectra of galaxies spanning a range of 30x in stellar mass and 10x in
star formation rate (SFR), calibrated from K-band and from MIPS IR fluxes. The
outflows have column densities of order N_H ~ 10^20 cm^-2 and characteristic
velocities of ~ 300-500 km/sec, with absorption seen out to 1000 km/sec in the
most massive, highest SFR galaxies. The velocities suggest that the outflowing
gas can escape into the IGM and that massive galaxies can produce
cosmologically and chemically significant outflows. Both the Mg II equivalent
width and the outflow velocity are larger for galaxies of higher stellar mass
and SFR, with V_wind ~ SFR^0.3, similar to the scaling in low redshift
IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy
population at z~1 indicates that galactic winds occur in the progenitors of
massive spirals as well as those of ellipticals. The increase of outflow
velocity with mass and SFR constrains theoretical models of galaxy evolution
that include feedback from galactic winds, and may favor momentum-driven models
for the wind physics.Comment: Accepted by ApJ. 25 pages, 17 figures. Revised to add discussions of
intervening absorbers and AGN-driven outflows; conclusions unchange
The Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph
with unprecedented sensitivity that was installed into the Hubble Space
Telescope (HST) in May 2009, during HST Servicing Mission 4 (STS-125). We
present the design philosophy and summarize the key characteristics of the
instrument that will be of interest to potential observers. For faint targets,
with flux F_lambda ~ 1.0E10-14 ergs/s/cm2/Angstrom, COS can achieve comparable
signal to noise (when compared to STIS echelle modes) in 1-2% of the observing
time. This has led to a significant increase in the total data volume and data
quality available to the community. For example, in the first 20 months of
science operation (September 2009 - June 2011) the cumulative redshift
pathlength of extragalactic sight lines sampled by COS is 9 times that sampled
at moderate resolution in 19 previous years of Hubble observations. COS
programs have observed 214 distinct lines of sight suitable for study of the
intergalactic medium as of June 2011. COS has measured, for the first time with
high reliability, broad Lya absorbers and Ne VIII in the intergalactic medium,
and observed the HeII reionization epoch along multiple sightlines. COS has
detected the first CO emission and absorption in the UV spectra of low-mass
circumstellar disks at the epoch of giant planet formation, and detected
multiple ionization states of metals in extra-solar planetary atmospheres. In
the coming years, COS will continue its census of intergalactic gas, probe
galactic and cosmic structure, and explore physics in our solar system and
Galaxy.Comment: 17 pages, 15 figure
Genome-Scale Reconstruction and Analysis of the Pseudomonas putida KT2440 Metabolic Network Facilitates Applications in Biotechnology
A cornerstone of biotechnology is the use of microorganisms for the efficient
production of chemicals and the elimination of harmful waste.
Pseudomonas putida is an archetype of such microbes due to
its metabolic versatility, stress resistance, amenability to genetic
modifications, and vast potential for environmental and industrial applications.
To address both the elucidation of the metabolic wiring in P.
putida and its uses in biocatalysis, in particular for the production
of non-growth-related biochemicals, we developed and present here a genome-scale
constraint-based model of the metabolism of P. putida KT2440.
Network reconstruction and flux balance analysis (FBA) enabled definition of the
structure of the metabolic network, identification of knowledge gaps, and
pin-pointing of essential metabolic functions, facilitating thereby the
refinement of gene annotations. FBA and flux variability analysis were used to
analyze the properties, potential, and limits of the model. These analyses
allowed identification, under various conditions, of key features of metabolism
such as growth yield, resource distribution, network robustness, and gene
essentiality. The model was validated with data from continuous cell cultures,
high-throughput phenotyping data, 13C-measurement of internal flux
distributions, and specifically generated knock-out mutants. Auxotrophy was
correctly predicted in 75% of the cases. These systematic analyses
revealed that the metabolic network structure is the main factor determining the
accuracy of predictions, whereas biomass composition has negligible influence.
Finally, we drew on the model to devise metabolic engineering strategies to
improve production of polyhydroxyalkanoates, a class of biotechnologically
useful compounds whose synthesis is not coupled to cell survival. The solidly
validated model yields valuable insights into genotype–phenotype
relationships and provides a sound framework to explore this versatile bacterium
and to capitalize on its vast biotechnological potential
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