2,201 research outputs found
Report of the ultraviolet and visible sensors panel
In order to meet the science objectives of the Astrotech 21 mission set the Ultraviolet (UV) and Visible Sensors Panel made a number of recommendations. In the UV wavelength range of 0.01 to 0.3 micro-m the focus is on the need for large format high quantum efficiency, radiation hard 'solar-blind' detectors. Options recommended for support include Si and non-Si charge coupled devices (CCDs) as well as photocathodes with improved microchannel plate readouts. For the 0.3 to 0.9 micro-m range, it was felt that Si CCDs offer the best option for high quantum efficiencies at these wavelengths. In the 0.9 to 2.5 micro-m the panel recommended support for the investigation of monolithic arrays. Finally, the panel noted that the implementation of very large arrays will require new data transmission, data recording, and data handling technologies
The Morphology - Density Relation in z ~ 1 Clusters
We measure the morphology--density relation (MDR) and morphology-radius
relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed
with the Advanced Camera for Surveys on board the Hubble Space Telescope.
Simulations and independent comparisons of ourvisually derived morphologies
indicate that ACS allows one to distinguish between E, S0, and spiral
morphologies down to zmag = 24, corresponding to L/L* = 0.21 and 0.30 at z =
0.83 and z = 1.24, respectively. We adopt density and radius estimation methods
that match those used at lower redshift in order to study the evolution of the
MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that
observed at z ~ 0, consistent with recent work -- specifically, the growth in
the bulge-dominated galaxy fraction, f_E+SO, with increasing density proceeds
less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and density <= 500
galaxies/Mpc^2, we find = 0.72 +/- 0.10. At z ~ 0, an E+S0 population
fraction of this magnitude occurs at densities about 5 times smaller. The
evolution in the MDR is confined to densities >= 40 galaxies/Mpc^2 and appears
to be primarily due to a deficit of S0 galaxies and an excess of Spiral+Irr
galaxies relative to the local galaxy population. The Elliptical fraction -
density relation exhibits no significant evolution between z = 1 and z = 0. We
find mild evidence to suggest that the MDR is dependent on the bolometric X-ray
luminosity of the intracluster medium. Implications for the evolution of the
disk galaxy population in dense regions are discussed in the context of these
observations.Comment: 30 pages, 18 figures. Accepted for publication in ApJ. Full
resolution versions of figs 2,3,6,8 are available at
http://www.stsci.edu/~postman/mdr_figure
Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456
The principal aim of this project is to determine the structural parameters
of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our
work forms part of an ongoing programme to constrain the internal
characteristics of hot B subdwarfs with the long-term goal of differentiating
between the various formation scenarios proposed for these objects. First
comparisons of asteroseismic values with evolutionary theory look promising,
however it is clear that more targets are needed for meaningful statistics to
be derived. The observational pulsation periods of PG 0911+456 were extracted
from rapid time-series photometry using standard Fourier analysis techniques.
Supplemented by spectroscopic estimates of the star's mean atmospheric
parameters, they were used as a basis for the "forward modelling" approach in
asteroseismology. The latter culminates in the identification of one or more
"optimal" models that can accurately reproduce the observed period spectrum.
This naturally leads to an identification of the oscillations detected in terms
of degree l and radial order k, and infers the structural parameters of the
target. From the photometry it was possible to extract 7 independent pulsation
periods in the 150-200 s range with amplitudes between 0.05 and 0.8 % of the
star's mean brightness. An asteroseismic search of parameter space identified
several models that matched the observed properties of PG 0911+456 well, one of
which was isolated as the "optimal" model on the basis of spectroscopic and
mode identification considerations. All the observed pulsations are identified
with low-order acoustic modes with degree indices l=0,1,2 and 4, and match the
computed periods with a dispersion of only ~0.26 %.Comment: accepted for publication in A&A, 14 pages, 13 figure
Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214
We present the first Advanced Camera for Surveys (ACS) observations of young
star clusters in the colliding/merging galaxy UGC 10214. The observations were
made as part of the Early Release Observation (ERO) program for the newly
installed ACS during service mission SM3B for the Hubble Space Telescope (HST).
Many young star clusters can be identified in the tails of UGC 10214, with ages
ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the
star clusters found in the tail of UGC 10214 can only be explained if strong
emission lines are included with a young stellar population. This has been
confirmed by our Keck spectroscopy of some of these bright blue stellar knots.
The most luminous and largest of these blue knots has an absolute magnitude of
M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star
cluster, would qualify as a super star cluster (SSC). Alternatively, it could
be a superposition of multiple scaled OB associations or clusters. With an
estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus
the young stellar knot is unbound and will not evolve into a normal globular
cluster. The bright blue clusters and associations are much younger than the
dynamical age of the tail, providing strong evidence that star formation occurs
in the tail long after it was ejected. UGC 10214 provides a nearby example of
processes that contributed to the formation of halos and intra-cluster media in
the distant and younger Universe.Comment: 6 pages with embedded figures, ApJ in pres
Estimating the Predictive Value of Negative SARS-CoV-2 Results: A Prospective Study
© 2020 by The Society for Healthcare Epidemiology of America. All rights reserved. We performed a prospective study of 501 patients, regardless of symptoms, admitted to the hospital, to estimate the predictive value of a negative Nasopharyngeal swab for SARS-CoV-2. At a positivity rate of 10.2%, the estimated Negative Predictive Value (NPV) was 97.2% and NPV rose as prevalence decreased during the study
Discovery of Globular Clusters in the Proto-Spiral NGC2915: Implications for Hierarchical Galaxy Evolution
We have discovered three globular clusters beyond the Holmberg radius in
Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark
matter dominated blue compact dwarf galaxy NGC2915. The clusters, all of which
start to resolve into stars, have M_{V606} = -8.9 to -9.8 mag, significantly
brighter than the peak of the luminosity function of Milky Way globular
clusters. Their colors suggest a metallicity [Fe/H] ~ -1.9 dex, typical of
metal-poor Galactic globular clusters. The specific frequency of clusters is at
a minimum normal, compared to spiral galaxies. However, since only a small
portion of the system has been surveyed it is more likely that the luminosity
and mass normalized cluster content is higher, like that seen in elliptical
galaxies and galaxy clusters. This suggests that NGC2915 resembles a key phase
in the early hierarchical assembly of galaxies - the epoch when much of the old
stellar population has formed, but little of the stellar disk. Depending on the
subsequent interaction history, such systems could go on to build-up larger
elliptical galaxies, evolve into normal spirals, or in rare circumstances
remain suspended in their development to become systems like NGC2915.Comment: ApJ Letters accepted; 6 pages, 2 figures, 3 table
Internal Color Properties of Resolved Spheroids in the Deep HST/ACS field of UGC 10214
(Abridged) We study the internal color properties of a morphologically
selected sample of spheroidal galaxies taken from HST/ACS ERO program of UGC
10214 (``The Tadpole''). By taking advantage of the unprecedented high
resolution of the ACS in this very deep dataset we are able to characterize
spheroids at sub-arcseconds scales. Using the V_606W and I_814W bands, we
construct V-I color maps and extract color gradients for a sample of spheroids
at I_814W < 24 mag. We investigate the existence of a population of
morphologically classified spheroids which show extreme variation in their
internal color properties similar to the ones reported in the HDFs. These are
displayed as blue cores and inverse color gradients with respect to those
accounted from metallicity variations. Following the same analysis we find a
similar fraction of early-type systems (~30%-40%) that show non-homologous
internal colors, suggestive of recent star formation activity. We present two
statistics to quantify the internal color variation in galaxies and for tracing
blue cores, from which we estimate the fraction of non-homogeneous to
homogeneous internal colors as a function of redshift up to z<1.2. We find that
it can be described as about constant as a function of redshift, with a small
increase with redshift for the fraction of spheroids that present strong color
dispersions. The implications of a constant fraction at all redshifts suggests
the existence of a relatively permanent population of evolving spheroids up to
z~1. We discuss the implications of this in the context of spheroidal
formation.Comment: Fixed URL for high resolution version. 13 Pages, 10 Figures. Accepted
for Publication in ApJ. Sep 1st issue. Higher resolution version and complete
table3B at http://acs.pha.jhu.edu/~felipe/e-prints/Tadpol
The Luminosity Function of Early-Type Galaxies at z~0.75
We measure the luminosity function of morphologically selected E/S0 galaxies
from to using deep high resolution Advanced Camera for Surveys
imaging data. Our analysis covers an area of 48\Box\arcmin (8 the
area of the HDF-N) and extends 2 magnitudes deeper ( mag) than was
possible in the Deep Groth Strip Survey (DGSS). At , we find
and , and at
, we find . These luminosity
functions are similar in both shape and number density to the luminosity
function using morphological selection (e.g., DGSS), but are much steeper than
the luminosity functions of samples selected using morphological proxies like
the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The
difference is due to the `blue', , E/S0 galaxies, which make up to
of the sample at all magnitudes and an increasing proportion of faint
galaxies. We thereby demonstrate the need for {\it both morphological and
structural information} to constrain the evolution of galaxies.
We find that the `blue' E/S0 galaxies have the same average sizes and Sersic
parameters as the `red', , E/S0 galaxies at brighter luminosities
(), but are increasingly different at fainter magnitudes where
`blue' galaxies are both smaller and have lower Sersic parameters. Fits of the
colors to stellar population models suggest that most E/S0 galaxies have short
star-formation time scales ( Gyr), and that galaxies have formed at an
increasing rate from until after which there has been a
gradual decline.Comment: 39 pages, 21 figures, accepted in A
Evolution of the Color-Magnitude Relation in High-Redshift Clusters: Blue Early-Type Galaxies and Red Pairs in RDCS J0910+5422
The color-magnitude relation has been determined for the RDCS J0910+5422
cluster of galaxies at redshift z = 1.106. Cluster members were selected from
HST ACS images, combined with ground--based near--IR imaging and optical
spectroscopy. The observed early--type color--magnitude relation (CMR) in
(i_775 -z_850) versus z_850 shows intrinsic scatters in color of 0.042 +/-
0.010 mag and 0.044 +/- 0.020 mag for ellipticals and S0s, respectively. From
the scatter about the CMR, a mean luminosity--weighted age t > 3.3 Gyr (z > 3)
is derived for the elliptical galaxies.
Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in
(i_775 - z_850) by 0.07 +/- 0.02 mag, with respect to the ellipticals. The
ellipticity distribution as a function of color indicates that the face-on S0s
in this particular cluster have likely been classified as elliptical. Thus, if
anything, the offset in color between the elliptical and S0 populations may be
even more significant.
The color offset between S0 and E corresponds to an age difference of ~1 Gyr,
for a single-burst solar metallicity model. A solar metallicity model with an
exponential decay in star formation will reproduce the offset for an age of 3.5
Gyr, i.e. the S0s have evolved gradually from star forming progenitors.
The early--type population in this cluster appears to be still forming. The
blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct
progenitors of the more evolved S0s that follow the same red sequence as
ellipticals in other clusters.
Thirteen red galaxy pairs are observed and the galaxies associated in pairs
constitute ~40% of the CMR galaxies in this cluster.Comment: ApJ, in pres
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