670 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
Evidence for Cold Accretion: Primitive Gas Flowing onto a Galaxy at z~0.274
We present UV and optical observations from the Cosmic Origins Spectrograph
on the Hubble Space Telescope and Keck of a z= 0.27395 Lyman limit system (LLS)
seen in absorption against the QSO PG1630+377. We detect H I absorption with
log N(HI)=17.06\pm0.05 as well as Mg II, C III, Si III, and O VI in this
system. The column densities are readily explained if this is a multi-phase
system, with the intermediate and low ions arising in a very low metallicity
([Mg/ H] =-1.71 \pm 0.06) photoionized gas. We identify via Keck spectroscopy
and Large Binocular Telescope imaging a 0.3 L_* star-forming galaxy projected
37 kpc from the QSO at nearly identical redshift (z=0.27406, \Delta v = -26
\kms) with near solar metallicity ([O/ H]=-0.20 \pm 0.15). The presence of very
low metallicity gas in the proximity of a near-solar metallicity, sub-L_*
galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A
search of the literature reveals that such low metallicity LLSs are not
uncommon. We found that 50% (4/8) of the well-studied z < 1 LLSs have
metallicities similar to the present system and show sub-L_* galaxies with rho
< 100 kpc in those fields where redshifts have been surveyed. We argue that the
properties of these primitive LLSs and their host galaxies are consistent with
those of cold mode accretion streams seen in galaxy simulations.Comment: Accepted for publication in the Astrophysical Journa
A Bayesian Nonparametric Approach to Modeling Motion Patterns
The most difficult—and often most essential—
aspect of many interception and tracking tasks is constructing
motion models of the targets to be found. Experts can
often provide only partial information, and fitting parameters
for complex motion patterns can require large amounts
of training data. Specifying how to parameterize complex
motion patterns is in itself a difficult task.
In contrast, nonparametric models are very flexible and
generalize well with relatively little training data. We propose
modeling target motion patterns as a mixture of Gaussian
processes (GP) with a Dirichlet process (DP) prior over
mixture weights. The GP provides a flexible representation
for each individual motion pattern, while the DP assigns observed
trajectories to particular motion patterns. Both automatically
adjust the complexity of the motion model based
on the available data. Our approach outperforms several parametric
models on a helicopter-based car-tracking task on
data collected from the greater Boston area
New abundance determinations in z < 1.5 QSO absorbers: seven sub-DLAs and one DLA
We present chemical abundance measurements from high resolution observations
of 7 sub-damped Lyman-alpha absorbers and 1 damped Lyman-alpha system at z<1.5.
Three of these objects have high metallicity, with near or super-solar Zn
abundance. Grids of Cloudy models for each system were constructed to look for
possible ionization effects in these systems. For the systems in which we could
constrain the ionization parameter, we find that the ionization corrections as
predicted by the Cloudy models are generally small and within the typical error
bars (~0.15 dex), in general agreement with previous studies. The Al III to Al
II ratio for these and other absorbers from the literature are compared, and we
find that while the sub-DLAs have a larger scatter in the Al III to Al II
ratios than the DLAs, there appears to be little correlation between the ratio
and N(H I). The relationship between the metallicity and the velocity width of
the profile for these systems is investigated. We show that the sub-DLAs that
have been observed to date follow a similar trend as DLA absorbers, with the
more metal rich systems exhibiting large velocity widths. We also find that the
systems at the upper edge of this relationship with high metallicities and
large velocity widths are more likely to be sub-DLAs than DLA absorbers,
perhaps implying that the sub-DLA absorbers are more representative of massive
galaxies.Comment: 15 pages, 15 Figures, Accepted by MNRAS, updated references and
author
Using 21cm Absorption in Small Impact Parameter Galaxy-QSO Pairs to Probe Low-Redshift Damped and Sub-Damped Lyman-alpha System
To search for low-redshift damped Lyman-alpha (DLA) and sub-DLA quasar
absorbers, we have conducted a 21cm absorption survey of radio-loud quasars at
small impact parameters to foreground galaxies selected from the Sloan Digital
Sky Survey (SDSS). Here we present the first results from this survey based on
observations of SDSS J104257.58+074850.5 ( = 2.66521), a quasar at an
angular separation from a foreground galaxy ( = 0.03321) of 2.5" (1.7
kpc in projection). The foreground galaxy is a low-luminosity spiral with
on-going star formation (0.004 M yr kpc) and a
metallicity of dex. We detect 21cm absorption from the galaxy
with the Green Bank Telescope (GBT), the Very Large Array (VLA), and the Very
Long Baseline Array (VLBA). The absorption appears to be quiescent disk gas
corotating with the galaxy and we do not find any evidence for outflowing cold
neutral gas. The width of the main absorption line indicates that the gas is
cold, K, and the HI column is surprisingly low given the impact
parameter of 1.7 kpc; we find that N(HI) cm
(GBT) and N(HI) cm (VLBA). VLBA marginally
resolves the continuum source and the absorber, and a lower limit of 27.1
13.9 pc is derived for the size of the absorbing cloud. In turn, this
indicates a low density for a cold cloud, n(HI) 3.5 cm. We
hypothesize that this galaxy, which is relatively isolated, is becoming
depleted in HI because it is converting its interstellar matter into stars
without a replenishing source of gas, and we suggest future observations to
probe this and similar galaxies.Comment: 16 Pages including 12 Figures and 2 Table
AIDS-related mycoses: the way forward.
The contribution of fungal infections to the morbidity and mortality of HIV-infected individuals is largely unrecognized. A recent meeting highlighted several priorities that need to be urgently addressed, including improved epidemiological surveillance, increased availability of existing diagnostics and drugs, more training in the field of medical mycology, and better funding for research and provision of treatment, particularly in developing countries
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