1,104 research outputs found
Long-Term Acute Care Hospitals and Georgia Medicaid: Utilization, Outcomes, and Cost
Because most research on long-term acute care hospitals has focused on Medicare, the objective of this research is to describe the Georgia Medicaid population who received care at a long-term acute care hospital, the type and volume of services provided by these long-term acute care hospitals, and the costs and outcomes of these services. For those with select respiratory conditions, we descriptively compare costs and outcomes to those of patients who received care for the same services in acute care hospitals
Linking the Metallicity Distribution of Galactic Halo Stars to the Enrichment History of the Universe
We compare the metallicity distribution of Galactic Halo stars with 3D
realizations of hierarchical galaxy formation. Outflows from dwarf galaxies
enrich the intergalactic medium inhomogeneously, at a rate depending on the
local galaxy density. Consequently, the first stars created in small
early-forming galaxies are less metal-rich that the first stars formed in more
massive galaxies which typically form later. As most halo stars are likely to
originate in accreted dwarfs, while disk stars formed out of outflow-enriched
gas, this scenario naturally generates a ``metallicity floor'' for old disk
stars, which we find to be roughly coincident with the higher end of our
predicted metallicity distribution of halo stars, in agreement with
observations. The broad and centrally peaked distribution of halo star
metallicities is well reproduced in our models, with a natural dispersion
depending on the exact accretion history. Our modeling includes the important
``baryonic stripping'' effect of early outflows, which brush away the tenuously
held gas in neighboring pre-virialized density perturbations. This stripping
process does not significantly modify the predicted shape of the halo star
metal distribution but inhibits star-formation and hence the number of accreted
stars, helping to reproduce the observed total Galactic halo luminosity and
also the lack of low-luminosity local dwarf galaxies relative to N-body
predictions.Comment: 5 pages, 1 figure, ApJ Letters, accepte
Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History
We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy
IC 1613. The photometry is the deepest to date for an isolated dwarf irregular
galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three
different methods to derive a star formation history (SFH). All three find an
enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar
age-metallicity relationships (AMR). A comparison of the newly observed outer
field with an earlier studied central field of IC 1613 shows that the SFR in
the outer field has been significantly depressed during the last Gyr. This
implies that the optical scale length of the galaxy has been decreasing with
time and that comparison of galaxies at intermediate redshift with present day
galaxies should take this effect into account. We find strong similarities
between IC 1613 and the more distant Milky Way dSph companions in that all are
dominated by star formation at intermediate ages. In particular, the SFH and
AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr
galaxies cannot be distinguished from dSphs by their intermediate age stellar
populations. This type of a SFH may also be evidence for slower or suppressed
early star formation in dwarf galaxies due to photoionization after the
reionization of the universe by background radiation. Assuming that IC 1613 is
typical of a dIrr evolving in isolation, since most of the star formation
occurs at intermediate ages, these dwarf systems cannot be responsible for the
fast chemical enrichment of the IGM which is seen at high redshift. There is no
evidence for any large amplitude bursts of star formation in IC 1613, and we
find it highly unlikely that analogs of IC 1613 have contributed to the excess
of faint blue galaxies in existing galaxy redshift surveys.Comment: 32 pages, including 1 table and 17 figures, accepted for publication
in the Astrophysical Journal October 10, 2003 issu
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Structural basis of nSH2 regulation and lipid binding in PI3Kα
We report two crystal structures of the wild-type phosphatidylinositol 3-kinase α (PI3Kα) heterodimer refined to 2.9 Å and 3.4 Å resolution: the first as the free enzyme, the second in complex with the lipid substrate, diC4-PIP2, respectively. The first structure shows key interactions of the N-terminal SH2 domain (nSH2) and iSH2 with the activation loop that suggest a mechanism by which the enzyme is inhibited in its basal state. In the second structure, the lipid substrate binds in a positively charged pocket adjacent to the ATP-binding site, bordered by the P-loop, the activation loop and the iSH2 domain. An additional lipid-binding site was identified at the interface of the ABD, iSH2 and kinase domains. The ability of PI3Kα to bind an additional PIP2 molecule was confirmed in vitro by fluorescence quenching experiments. The crystal structures reveal key differences in the way the nSH2 domain interacts with wild-type p110α and with the oncogenic mutant p110αH1047R. Increased buried surface area and two unique salt-bridges observed only in the wild-type structure suggest tighter inhibition in the wild-type PI3Kα than in the oncogenic mutant. These differences may be partially responsible for the increased basal lipid kinase activity and increased membrane binding of the oncogenic mutant
Leo A: A Late-Blooming Survivor of the Epoch of Reionization in the Local Group
As part of a major program to use isolated Local Group dwarf galaxies as
near-field probes of cosmology, we have obtained deep images of the dwarf
irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble
Space Telescope. From these images we have constructed a color-magnitude
diagram (CMD) reaching apparent [absolute] magnitudes of (M475, M814) > (29.0
[+4.4], 27.9 [+3.4]), the deepest ever achieved for any irregular galaxy beyond
the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of
time over the entire history of the galaxy. We find that over 90% of all the
star formation that ever occurred in Leo A happened more recently than 8 Gyr
ago. The CMD shows only a very small amount of star formation in the first few
billion years after the Big Bang; a possible burst at the oldest ages cannot be
claimed with high confidence. The peak SFR occurred ~1.5-4 Gyr ago, at a level
5-10 times the current value. Our modelling indicates that Leo A has
experienced very little metallicity evolution; the mean inferred metallicity is
consistent with measurements of the present-day gas-phase oxygen abundance. We
cannot exclude a scenario in which ALL of the ancient star formation occurred
prior to the end of the era of reionization, but it seems unlikely that the
lack of star formation prior to ~8 Gyr ago was due to early loss or exhaustion
of the in situ gas reservoir.Comment: Accepted for publication in the Astrophysical Journal Letters; 5
pages, 3 figures (2 in color); uses emulateapj.st
Deep HST Imaging of Sextans A I. The Spatially Resolved Recent Star Formation History
We have measured stellar photometry from deep Cycle 7 Hubble Space
Telescope/WFPC2 imaging of the dwarf irregular galaxy Sextans A. The imaging
was taken in three filters: F555W (; 8 orbits), F814W (; 16 orbits), and
F656N (H; 1 orbit). Combining these data with Cycle 5 WFPC2
observations provides nearly complete coverage of the optically visible portion
of the galaxy. The Cycle 7 observations are nearly 2 magnitudes more sensitive
than the Cycle 5 observations, which provides unambiguous separation of the
faint blue helium burning stars (BHeB stars) from contaminant populations. The
depth of the photometry allows us to compare recent star formation histories
recovered from both the main sequence (MS) stars and the BHeB stars for the
last 300 Myr. The excellent agreement between these independent star formation
rate (SFR) calculations is a resounding confirmation for the legitimacy of
using the BHeB stars to calculate the recent SFR. Using the BHeB stars we have
calculated the global star formation history over the past 700 Myr. The history
calculated from the Cycle 7 data is remarkably identical to that calculated
from the Cycle 5 data, implying that both halves of the galaxy formed stars in
concert. We have also calculated the spatially resolved star formation history,
combining the fields from the Cycle 5 and Cycle 7 data. Our interpretation of
the pattern of star formation is that it is an orderly stochastic process.Comment: 27 pages, 14 figures, 2 mpeg movies, accepted in the Astronomical
Journa
Star formation at the edge of the Local Group: a rising star formation history in the isolated galaxy WLM
We present the star formation history (SFH) of the isolated (D~970 kpc) Local
Group dwarf galaxy WLM measured from color-magnitude diagrams constructed from
deep Hubble Space Telescope imaging. Our observations include a central (0.5
) and outer field (0.7 ) that reach below the oldest main sequence
turnoff. WLM has no early dominant episode of star formation: 20% of its
stellar mass formed by ~12.5 Gyr ago (z~5). It also has an SFR that rises to
the present with 50% of the stellar mass within the most recent 5 Gyr (z<0.7).
There is evidence of a strong age gradient: the mean age of the outer field is
5 Gyr older than the inner field despite being only 0.4 kpc apart. Some models
suggest such steep gradients are associated with strong stellar feedback and
dark matter core creation. The SFHs of real isolated dwarf galaxies and those
from the the Feedback In Realistic Environment suite are in good agreement for
, but in worse agreement at lower
masses (). These differences may be
explainable by systematics in the models (e.g., reionization model) and/or
observations (HST field placement). We suggest that a coordinated effort to get
deep CMDs between HST/JWST (crowded central fields) and WFIRST (wide-area halo
coverage) is the optimal path for measuring global SFHs of isolated dwarf
galaxies.Comment: 13 pages, 13 Figures, 4 Tables. Re-submitted to MNRAS after
addressing the referee's comment
The ACS Nearby Galaxy Survey Treasury
The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to
establish a legacy of uniform multi-color photometry of resolved stars for a
volume-limited sample of nearby galaxies (D<4 Mpc). The survey volume
encompasses 69 galaxies in diverse environments, including close pairs, small &
large groups, filaments, and truly isolated regions. The galaxies include a
nearly complete range of morphological types spanning a factor of ~10^4 in
luminosity and star formation rate. The survey data consists of images taken
with ACS on HST, supplemented with archival data and new WFPC2 imaging taken
after the failure of ACS. Survey images include wide field tilings covering the
full radial extent of each galaxy, and single deep pointings in uncrowded
regions of the most massive galaxies in the volume. The new wide field imaging
in ANGST reaches median 50% completenesses of m_F475W=28.0 mag, m_F606W=27.3
mag, and m_F814W=27.3 mag, several magnitudes below the tip of the red giant
branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the
structure in the red clump. The resulting photometric catalogs are publicly
accessible and contain over 34 million photometric measurements of >14 million
stars. In this paper we present the details of the sample selection, imaging,
data reduction, and the resulting photometric catalogs, along with an analysis
of the photometric uncertainties (systematic and random), for both the ACS and
WFPC2 imaging. We also present uniformly derived relative distances measured
from the apparent magnitude of the TRGB.Comment: 54 pages, including 24 pages of figures and 16 pages of tables.
Project website and data available at http://www.nearbygalaxies.org/ . Data
is also available through MAST. Scheduled to appear in the Astrophysical
Journal Supplements. (Replaced to fix several figures that were damaged
during compression
The Milky Way's dark matter halo appears to be lopsided
The atomic hydrogen gas (HI) disk in the outer region (beyond ~10 kpc from
the centre) of Milky Way can provide valuable information about the structure
of the dark matter halo. The recent 3-D thickness map of the outer HI disk from
the all sky 21-cm line LAB survey, gives us a unique opportunity to investigate
the structure of the dark matter halo of Milky Way in great detail. A striking
feature of this new survey is the North-South asymmetry in the thickness map of
the atomic hydrogen gas. Assuming vertical hydrostatic equilibrium under the
total potential of the Galaxy, we derive the model thickness map of the HI gas.
We show that simple axisymmetric halo models, such as softened isothermal halo
(producing a flat rotation curve with V_c ~ 220 km/s) or any halo with density
falling faster than the isothermal one, are not able to explain the observed
radial variation of the gas thickness. We also show that such axisymmetric
halos along with different HI velocity dispersion in the two halves, cannot
explain the observed asymmetry in the thickness map. Amongst the
non-axisymmetric models, it is shown that a purely lopsided (m=1, first
harmonic) dark matter halo with reasonable HI velocity dispersion fails to
explain the North-South asymmetry satisfactorily. However, we show that by
superposing a second harmonic (m=2) out of phase onto a purely lopsided halo
e.g. our best fit and more acceptable model A (with parameters
\epsilon_{h}^{1}=0.2, \epsilon_{h}^{2}=0.18 and \sigma_{HI}=8.5 km/s) can
provide an excellent fit to the observation and reproduce the North-South
asymmetry naturally. The emerging picture of the asymmetric dark matter halo is
supported by the \Lambda CDM halos formed in the cosmological N-body
simulation.Comment: 37 pages, 8 figures, accepted for publication in Ap
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