1,996 research outputs found
A GBT Survey for HI 21 cm Absorption in the Disks and Halos of Low-Redshift Galaxies
We present an HI 21 cm absorption survey with the Green Bank Telescope (GBT)
of galaxy-quasar pairs selected by combining data from the Sloan Digital Sky
Survey (SDSS) and the Faint Images of the Radio Sky at Twenty-Centimeters
(FIRST) survey. Our sample consists of 23 sightlines through 15 low-redshift
foreground galaxy - background quasar pairs with impact parameters ranging from
1.7 kpc up to 86.7 kpc. We detected one absorber in the GBT survey from the
foreground dwarf galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc and
another possible absorber in our follow-up Very Large Array (VLA) imaging of
the nearby foreground galaxy, UGC 7408. Both of the absorbers are narrow (FWHM
of 3.6 and 4.8 km/s), have sub Damped Lyman alpha column densities, and most
likely originate in the disk gas of the foreground galaxies. We also detected H
I emission from three foreground galaxies, including UGC 7408. Although our
sample contains both blue and red galaxies, the two H I absorbers as well as
the H I emissions are associated with blue galaxies. We discuss the physical
conditions in the 21 cm absorbers and some drawbacks of the large GBT beam for
this type of survey.Comment: 15 pages, 6 figures, 3 table
Metal abundances at z<1.5: new measurements in sub-Damped Lyman-alpha Absorbers
Damped Lyman-alpha systems (DLAs) and sub-DLAs seen toward background quasars
provide the most detailed probes of elemental abundances. Somewhat
paradoxically these measurements are more difficult at lower redshifts due to
the atmospheric cut-off, and so a few years ago our group began a programme to
study abundances at z < 1.5 in quasar absorbers. In this paper, we present new
UVES observations of six additional quasar absorption line systems at z < 1.5,
five of which are sub-DLAs. We find solar or above solar metallicity, as
measured by the abundance of zinc, assumed not to be affected by dust, in two
sub-DLAs: one, towards Q0138-0005 with [Zn/H]=+0.28 +/- 0.16; the other towards
Q2335+1501 with [Zn/H]=+0.07 +/- 0.34. Relatively high metallicity was observed
in another system: Q0123-0058 with [Zn/H]=-0.45 +/- 0.20. Only for the one DLA
in our sample, in Q0449-1645, do we find a low metallicity, [Zn/H]=-0.96 +/-
0.08. We also note that in some of these systems large relative abundance
variations from component to component are observed in Si, Mn, Cr and Zn.Comment: 7 figures and 10 tables. Accepted for publication in MNRA
Community-engaged primary care medical education
BACKGROUND: Community-engaged medical education (CEME) requires medical schools to partner with local communities to help address community priorities, whilst enhancing the learning experiences of students. Current literature on CEME has focused on evaluating its effects on students; however, there remains a gap in exploring whether CEME initiatives can have a sustainable impact for communities. APPROACH: The Community Action Project (CAP) at Imperial College London, is an eight-week, community-engaged, quality improvement project for Year 3 medical students. Students initially consult with clinicians, patients and wider community stakeholders to understand local needs and assets, and identify a health priority to address. They then work with relevant stakeholders to design, implement and evaluate a project to help address their identified priority. EVALUATION: All CAPs (n = 264) completed in the 2019-2021 academic years were evaluated for evidence of several key areas, including community engagement and sustainability. 91% of projects evidenced a needs analysis, 71% demonstrated patient involvement in their development, and 64% demonstrated sustainable impacts from their projects. Analysis revealed the topics frequently addressed, and the formats used by students. Two CAPs are described in more detail to demonstrate their community impact. IMPLICATIONS: The CAP demonstrates how the principles of CEME (meaningful community engagement and social accountability) can lead to sustainable benefits for local communities through purposeful collaboration with patients and local communities. Strengths, limitations and future directions are highlighted
The Evolution of Damped Lyman-alpha Absorbers: Metallicities and Star Formation Rates
The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide
powerful probes of the evolution of metals, gas, and stars in galaxies. One
major obstacle in trying to understand the evolution of DLAs and sub-DLAs has
been the small number of metallicity measurements at z < 1.5, an epoch spanning
\~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope
and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5.
Combining our results with those at higher redshifts from the literature, we
find that the global mean metallicity of DLAs does not rise to the solar value
at low redshifts. These surprising results appear to contradict the near-solar
mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of
cosmic chemical evolution models based on the global star formation history.
Finally, we discuss direct constraints on the star formation rates (SFRs) in
the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and
other emission-line studies in the literature. A large fraction of the observed
heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially
below the global mean SFR, consistent with the low metallicities observed in
the spectroscopic studies.Comment: 6 pages,3 figures, To appear in "Probing Galaxies through Quasar
Absorption Lines", Proceedings IAU Colloquium 199, 2005, Eds. P. R. Williams,
C. Shu, and B. Menar
Complete homochirality induced by the nonlinear autocatalysis and recycling
A nonlinear autocatalysis of a chiral substance is shown to achieve
homochirality in a closed system, if the back-reaction is included. Asymmetry
in the concentration of two enantiomers or the enantiometric excess increases
due to the nonlinear autocatalysis. Furthermore, when the back-reaction is
taken into account, the reactant supplied by the decomposition of the
enantiomers is recycled to produce more and more the dominant one, and
eventually the homochirality is established.Comment: 4 pages, 2 figure
The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are A Major Reservoir of Galactic Metals
The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of
intergalactic gas, but its mass, heavy element enrichment, and relation to
galaxy properties are poorly constrained by observations. In a survey of the
outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the
Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of
ionized oxygen surrounding star-forming galaxies, but we find much less ionized
oxygen around galaxies with little or no star formation. This ionized CGM
contains a substantial mass of heavy elements and gas, perhaps far exceeding
the reservoirs of gas in the galaxies themselves. It is a basic component of
nearly all star-forming galaxies that is removed or transformed during the
quenching of star formation and the transition to passive evolution.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with the Cosmic Origins Spectrograph on HST, to be published in
Science, together with related papers by Tripp et al. and Lehner & Howk, in
the November 18, 2011 edition. This version has not undergone final
copyediting. Please see Science online for the final printed versio
The COS Absorption Survey of Baryon Harbors (CASBaH): Warm-hot Circumgalactic Gas Reservoirs Traced by Ne VIII Absorption
We survey the highly ionized circumgalactic media (CGM) of 29 blindly
selected galaxies at 0.49 < z_(gal) < 1.44 based on high-S/N ultraviolet
spectra of z > 1 QSOs and the galaxy database from the COS Absorption Survey of
Baryon Harbors (CASBaH). We detect the Ne VIII doublet in nine of the galaxies,
and for gas with N(Ne VIII) > 10^13.3 cm^-2 (> 10^13.5 cm^-2), we derive a Ne
VIII covering fraction f_c = 75 +15/-25% (44 +22/-20%) within impact parameter
(rho) < 200 kpc of M_* = 10^(9.5-11.5) Msol galaxies and f_c = 70 +16/-22% (f_c
= 42 +20/-17%) within rho < 1.5 virial radii. We estimate the mass in Ne
VIII-traced gas to be M_gas(Ne VIII) > 10^9.5 Msol (Z/Zsol)^-1, or 6-20% of the
expected baryonic mass if the Ne VIII absorbers have solar metallicity.
Ionizing Ne VII to Ne VIII requires 207 eV, and photons with this energy are
scarce in the CGM. However, for the median halo mass and redshift of our
sample, the virial temperature is close to the peak temperature for the Ne VIII
ion, and the Ne VIII-bearing gas is plausibly collisionally ionized near this
temperature. Moreover, we find that photoionized Ne VIII requires cool and
low-density clouds that would be highly underpressured (by approximately two
orders of magnitude) relative to the putative, ambient virialized medium,
complicating scenarios where such clouds could survive. Thus, more complex
(e.g., non-equilibrium) models may be required; this first statistical sample
of Ne VIII absorber/galaxy systems will provide stringent constraints for
future CGM studies.Comment: Published in ApJL, Volume 877, Issue 2, Article L2
The Role of Sub-damped Lyman-alpha Absorbers in the Cosmic Evolution of Metals
Observations of low mean metallicity of damped Lyman-alpha (DLA) quasar
absorbers at all redshifts studied appear to contradict the predictions for the
global mean interstellar metallicity in galaxies from cosmic chemical evolution
models. On the other hand, a number of metal-rich sub-DLA systems have been
identified recently, and the fraction of metal-rich sub-DLAs appears to be
considerably larger than that of metal-rich DLAs, especially at z < 1.5. In
view of this, here we investigate the evolution of metallicity in sub-DLAs. We
find that the mean Zn metallicity of the observed sub-DLAs may be higher than
that of the observed DLAs, especially at low redshifts, reaching a near-solar
level at z <~ 1. This trend does not appear to be an artifact of sample
selection, the use of Zn, the use of N_{HI}-weighting, or observational
sensitivity. While a bias against very low metallicity could be present in the
sub-DLA sample in some situations, this cannot explain the difference between
the DLA and sub-DLA metallicities at low z. The primary reason for the
difference between the DLAs and sub-DLAs appears to be the dearth of metal-rich
DLAs. We estimate the sub-DLA contribution to the total metal budget using
measures of their metallicity and comoving gas density. These calculations
suggest that at z <~ 1, the contribution of sub-DLAs to the total metal budget
may be several times that of DLAs. At higher redshifts also, there are
indications that the sub-DLAs may contribute significantly to the cosmic metal
budget.Comment: 9 pages, 2 figures, Accepted for Publication in the Astrophysical
Journa
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