280 research outputs found
The Snapshot Hubble U-Band Cluster Survey (SHUCS) II. Star Cluster Population of NGC 2997
We study the star cluster population of NGC 2997, a giant spiral galaxy
located at 9.5 Mpc and targeted by the Snapshot Hubble U-band Cluster Survey
(SHUCS). Combining our U-band imaging from SHUCS with archival BVI imaging from
HST, we select a high confidence sample of clusters in the circumnuclear ring
and disk through a combination of automatic detection procedures and visual
inspection. The cluster luminosity functions in all four filters can be
approximated by power-laws with indices of to . Some deviations
from pure power-law shape are observed, hinting at the presence of a high-mass
truncation in the cluster mass function. However, upon inspection of the
cluster mass function, we find it is consistent with a pure power-law of index
despite a slight bend at M. No
statistically significant truncation is observed. From the cluster age
distributions, we find a low rate of disruption () in both the
disk and circumnuclear ring. Finally, we estimate the cluster formation
efficiency () over the last 100 Myr in each region, finding %
for the disk, % for the circumnuclear ring, and % for the
entire UBVI footprint. This study highlights the need for wide-field UBVI
coverage of galaxies to study cluster populations in detail, though a small
sample of clusters can provide significant insight into the characteristics of
the population.Comment: 31 pages, 9 figures, accepted to the A
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Marker-assisted selection for resistance to potato virus Y
Potato Virus Y (PVY) imposes serious limitations on potato yield, quality and tuber seed
production. Traditional PVY screening requires artificial inoculation under controlled
conditions followed by ELISA to test for PVY. This method is very tedious and time
consuming thus prohibiting the screening of large segregating populations. The genes
RYadg from S. tuberosum ssp. andigena and Rysto from S. stolonferum provide extreme
resistance to PVY. The objective of this work is to assess the usefulness of molecular
markers for determining the allelic configuration of parental material containing R.yadg
and Rysto genes and as an early selection tool for predicting PVY resistance. To achieve
this, two segregating populations for RYadg and Rysto were screened with molecular
markers, inoculated artificially and tested for PVY by ELISA. Ninety-six percent of the
segregating lines for the RYadg gene showed coincidence between results for molecular
markers and ELISA at 40 days after inoculations. Both ELISA and molecular marker
results fit a 1:1 (resistant:susceptible) segregation ratio indicating the presence of Ryadg as
a simplex. In the population segregating for Rysto only 84% of the segregating lines showed coincidence between results for molecular markers and ELISA at 40 days after
inoculations. The molecular markers results fit a 1:1 segregation ratio whereas the
ELISA results indicated that a second gene/allele was likely providing resistance to PVY.
Markers associated with RYadg and Rysto were successfully used for Marker-Assisted
Selection in the Pacific NW Potato Breeding program. In addition, advanced potato
clones were evaluated for presence Of Ryadg and Rysto PVY resistance sources. The lines
LBR2, B071 8-3 and EGA97061-4 showed patterns of resistance for PVY associated
with RYadg based on molecular marker evaluations. Additional sources of PVY resistance,
not detected with the markers used in this study, are likely present in other advanced
potato germplasm evaluated
Properties of the cluster population of NGC 1566 and their implications
We present results of a photometric study into the cluster population of NGC 1566, a nearby grand design spiral galaxy, sampled out to a Galactocentric radius of kpc. The shape of the mass-limited age distribution shows negligible variation with radial distance from the centre of the galaxy, and demonstrates three separate sections, with a steep beginning, flat middle and steep end. The luminosity function can be approximated by a power law at lower luminosities with evidence of a truncation at higher luminosity. The power law section of the luminosity function of the galaxy is best fitted by an index , in agreement with other studies, and is found to agree with a model luminosity function, which uses an underlying Schechter mass function. The recovered power law slope of the mass distribution shows a slight steepening as a function of galactocentric distance, but this is within error estimates. It also displays a possible truncation at the high mass end. Additionally, the cluster formation efficiency () and the specific U-band luminosity of clusters () are calculated for NGC 1566 and are consistent with values for similar galaxies. A difference in NGC 1566, however, is that the fairly high star formation rate is in contrast with a low and , indicating that can only be said to depend strongly on , not the star formation rate
Sizes and Shapes of Young Star Cluster Light Profiles in M83
We measure the radii and two-dimensional light profiles of a large sample of
young, massive star clusters in M83 using archival HST/WFC3 imaging of seven
adjacent fields. We use GALFIT to fit the two-dimensional light profiles of the
clusters, from which we find effective (half-light) radii, core radii, and
slopes of the power-law (EFF) profile (). We find lognormal distributions
of effective radius and core radius, with medians of 2.5 pc and
1.3 pc, respectively. Our results provide strong evidence for a
characteristic size of young, massive clusters. The average effective radius
and core radius increase somewhat with cluster age. Little to no change in
effective radius is observed with increasing galactocentric distance, except
perhaps for clusters younger than 100 Myr. We find a shallow correlation
between effective radius and mass for the full cluster sample, but a stronger
correlation is present for clusters 200-300 Myr in age. Finally, the majority
of the clusters are best fit by an EFF model with index . There is
no strong evidence for change in with cluster age, mass, or
galactocentric distance. Our results suggest that clusters emerge from early
evolution with similar radii and are not strongly affected by the tidal field
of M83. Mass loss due to stellar evolution and/or GMC interactions appear to
dominate cluster expansion in the age range we study.Comment: 34 pages, 11 figures, 3 tables, accepted by MNRAS. Machine-readable
table attached (full version of Table 3). To obtain, download the source file
from the "Other formats" link abov
Filamentary Star Formation in NGC 1275
We examine the star formation in the outer halo of NGC~1275, the central
galaxy in the Perseus cluster (Abell 426), using far ultraviolet and optical
images obtained with the Hubble Space Telescope. We have identified a
population of very young, compact star clusters with typical ages of a few Myr.
The star clusters are organised on multiple-kiloparsec scales. Many of these
star clusters are associated with "streaks" of young stars, the combination of
which has a cometary appearance. We perform photometry on the star clusters and
diffuse stellar streaks, and fit their spectral energy distributions to obtain
ages and masses. These young stellar populations appear to be normal in terms
of their masses, luminosities and cluster formation efficiency; <10% of the
young stellar mass is located in star clusters. Our data suggest star formation
is associated with the evolution of some of the giant gas filaments in NGC~1275
that become gravitationally unstable on reaching and possibly stalling in the
outer galaxy. The stellar streaks then could represent stars moving on
ballistic orbits in the potential well of the galaxy cluster. We propose a
model where star-forming filaments, switched on ~50~Myr ago and are currently
feeding the growth of the NGC~1275 stellar halo at a rate of ~2-3 solar masses
per year. This type of process may also build stellar halos and form isolated
star clusters in the outskirts of youthful galaxies.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Suppression of human immunodeficiency virus replication during acute measles.
To determine the effect of measles virus coinfection on plasma human immunodeficiency virus (HIV) RNA levels, a prospective study of hospitalized children with measles was conducted between January 1998 and October 2000 in Lusaka, Zambia. Plasma HIV RNA levels were measured during acute measles and 1 month after hospital discharge. The median plasma HIV RNA level in 33 children with measles who were followed longitudinally was 5339 copies/mL at study entry, 60,121 copies/mL at hospital discharge, and 387,148 copies/mL at 1-month follow-up. The median plasma HIV RNA level in children without acute illness was 228,454 copies/mL. Plasma levels of immune activation markers were elevated during the period of reduced plasma HIV RNA. Plasma levels of several potential HIV suppressive factors also were elevated during acute measles. HIV replication is transiently suppressed during acute measles at a time of intense immune activation
The properties, origin and evolution of stellar clusters in galaxy simulations and observations
We investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one ‘cluster’, for the isolated galaxies we are able to model features we term ‘clusters’ with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myrs) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas onto the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback
Hubble Tarantula Treasury Project. III. Photometric Catalog and Resulting Constraints on the Progression of Star Formation in the 30 Doradus Region
We present and describe the astro-photometric catalog of more than 800,000
sources found in the Hubble Tarantula Treasury Project (HTTP). HTTP is a Hubble
Space Telescope (HST) Treasury program designed to image the entire 30 Doradus
region down to the sub-solar (~0.5 solar masses) mass regime using the Wide
Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS). We observed 30
Doradus in the near ultraviolet (F275W, F336W), optical (F555W, F658N, F775W),
and near infrared (F110W, F160W) wavelengths. The stellar photometry was
measured using point-spread function (PSF) fitting across all the bands
simultaneously. The relative astrometric accuracy of the catalog is 0.4 mas.
The astro-photometric catalog, results from artificial star experiments and the
mosaics for all the filters are available for download. Color-magnitude
diagrams are presented showing the spatial distributions and ages of stars
within 30 Dor as well as in the surrounding fields. HTTP provides the first
rich and statistically significant sample of intermediate and low mass pre-main
sequence candidates and allows us to trace how star formation has been
developing through the region. The depth and high spatial resolution of our
analysis highlight the dual role of stellar feedback in quenching and
triggering star formation on the giant HII region scale. Our results are
consistent with stellar sub-clustering in a partially filled gaseous nebula
that is offset towards our side of the Large Magellanic Cloud.Comment: 20 pages, 22 Figures, 3 Tables, Photometric Catalogs and Mosaiced
images will be available for download upon publication, accepted for
publication on ApJ
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