469 research outputs found
Gamma-Ray Emission from PWNe Interacting with Molecular Clouds
We consider a situation in which a pulsar (and its nebula) is formed inside
or close to a high density regions of a molecular cloud. We apply a recent
model for the gamma radiation of pulsar wind nebulae (PWN), which includes not
only radiation processes due to injected leptons but also processes due to
injection of relativistic hadrons, in order to calculate the expected gamma-ray
emission from such interacting PWNe. The example calculations have been
performed for two objects of this type from which directions TeV gamma-ray
sources have recently been observed (IC443 and W41). We show that the gamma-ray
emission below a few TeV can be produced by leptons accelerated in the past in
the vicinity of the pulsars. gamma-rays with energies above ~10 TeV can be
produced by hadrons interacting with the matter inside the supernova remnant
and surrounding dense clouds. In contrary to the low energy TeV emission, this
high energy TeV emission should be correlated with the location of dense clouds
able to capture hadrons due to their strong magnetic fields.Comment: Proceedings of the 30th ICRC, Merida, Mexico, 2007, in pres
GRAVITY: getting to the event horizon of Sgr A*
We present the second-generation VLTI instrument GRAVITY, which currently is
in the preliminary design phase. GRAVITY is specifically designed to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the
massive black hole at center of the Milky Way. We have identified the key
design features needed to achieve this goal and present the resulting
instrument concept. It includes an integrated optics, 4-telescope, dual feed
beam combiner operated in a cryogenic vessel; near infrared wavefront sensing
adaptive optics; fringe tracking on secondary sources within the field of view
of the VLTI and a novel metrology concept. Simulations show that the planned
design matches the scientific needs; in particular that 10 microarcsecond
astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given
the availability of suitable phase reference sources.Comment: 13 pages, 11 figures, to appear in the conference proceedings of SPIE
Astronomical Instrumentation, 23-28 June 2008, Marseille, Franc
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
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
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
The Transformation of Cluster Galaxies at Intermeidate Redshift
We combine imaging data from the Advanced Camera for Surveys (ACS) with
VLT/FORS optical spectroscopy to study the properties of star-forming galaxies
in the z=0.837 cluster CL0152-1357. We have morphological information for 24
star-forming cluster galaxies, which range in morphology from late-type and
irregular to compact early-type galaxies. We find that while most star-forming
galaxies have colors bluer than 1.0, eight are in the red
cluster sequence. Among the star-forming cluster population we find five
compact early-type galaxies which have properties consistent with their
identification as progenitors of dwarf elliptical galaxies. The spatial
distribution of the star-forming cluster members is nonuniform. We find none
within Mpc of the cluster center, which is highly suggestive of an
intracluster medium interaction. We derive star formation rates from [OII]
line fluxes, and use these to compare the global star
formation rate of CL0152-1357 to other clusters at low and intermediate
redshifts. We find a tentative correlation between integrated star formation
rates and , in the sense that hotter clusters have lower integrated star
formation rates. Additional data from clusters with low X-ray temperatures is
needed to confirm this trend. We do not find a significant correlation with
redshift, suggesting that evolution is either weak or absent between z=0.2-0.8.Comment: ApJ accepte
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
Evaluation of the health-related quality of life of children in Schistosoma haematobium-endemic communities in Kenya: a cross-sectional study.
BACKGROUND: Schistosomiasis remains a global public health challenge, with 93% of the ~237 million infections occurring in sub-Saharan Africa. Though rarely fatal, its recurring nature makes it a lifetime disorder with significant chronic health burdens. Much of its negative health impact is due to non-specific conditions such as anemia, undernutrition, pain, exercise intolerance, poor school performance, and decreased work capacity. This makes it difficult to estimate the disease burden specific to schistosomiasis using the standard DALY metric.
METHODOLOGY/PRINCIPAL FINDINGS: In our study, we used Pediatric Quality of Life Inventory (PedsQL), a modular instrument available for ages 2-18 years, to assess health-related quality of life (HrQoL) among children living in a Schistosoma haematobium-endemic area in coastal Kenya. The PedsQL questionnaires were administered by interview to children aged 5-18 years (and their parents) in five villages spread across three districts. HrQoL (total score) was significantly lower in villages with high prevalence of S. haematobium (-4.0%, p<0.001) and among the lower socioeconomic quartiles (-2.0%, p<0.05). A greater effect was seen in the psychosocial scales as compared to the physical function scale. In moderate prevalence villages, detection of any parasite eggs in the urine was associated with a significant 2.1% (p<0.05) reduction in total score. The PedsQL reliabilities were generally high (Cronbach alphas ≥0.70), floor effects were acceptable, and identification of children from low socioeconomic standing was valid.
CONCLUSIONS/SIGNIFICANCE: We conclude that exposure to urogenital schistosomiasis is associated with a 2-4% reduction in HrQoL. Further research is warranted to determine the reproducibility and responsiveness properties of QoL testing in relation to schistosomiasis. We anticipate that a case definition based on more sensitive parasitological diagnosis among younger children will better define the immediate and long-term HrQoL impact of Schistosoma infection
Strong Lensing Analysis of A1689 from Deep Advanced Camera Images
We analyse deep multi-colour Advanced Camera images of the largest known
gravitational lens, A1689. Radial and tangential arcs delineate the critical
curves in unprecedented detail and many small counter-images are found near the
center of mass. We construct a flexible light deflection field to predict the
appearance and positions of counter-images. The model is refined as new
counter-images are identified and incorporated to improve the model, yielding a
total of 106 images of 30 multiply lensed background galaxies, spanning a wide
redshift range, 1.0z5.5. The resulting mass map is more circular in
projection than the clumpy distribution of cluster galaxies and the light is
more concentrated than the mass within . The projected mass profile
flattens steadily towards the center with a shallow mean slope of
, over the observed range,
r, matching well an NFW profile, but with a relatively high
concentration, . A softened isothermal profile
(\arcs) is not conclusively excluded, illustrating that
lensing constrains only projected quantities. Regarding cosmology, we clearly
detect the purely geometric increase of bend-angles with redshift. The
dependence on the cosmological parameters is weak due to the proximity of
A1689, , constraining the locus, .
This consistency with standard cosmology provides independent support for our
model, because the redshift information is not required to derive an accurate
mass map. Similarly, the relative fluxes of the multiple images are reproduced
well by our best fitting lens model.Comment: Accepted by ApJ. For high quality figures see
http://wise-obs.tau.ac.il/~kerens/A168
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