1,861 research outputs found
The MicroJansky Radio Galaxy Population
We use highly spectroscopically complete observations of the radio sources
from the VLA 1.4 GHz survey of the HDF-N region to study the faint radio galaxy
population and its evolution. We spectrally classify the sources into four
spectral types: absorbers, star formers, Seyfert galaxies, and broad-line AGNs,
and we analyze their properties by type. We supplement the spectroscopic
redshifts with photometric redshifts measured from the rest-frame UV to MIR
spectral energy distributions. Using deep X-ray observations of the field, we
do not confirm the existence of an X-ray-radio correlation for star-forming
galaxies. We also do not observe any correlations between 1.4 GHz flux and R
magnitude or redshift. We find that the radio powers of the host galaxies rise
dramatically with increasing redshift, while the optical properties of the host
galaxies show at most small changes. Assuming that the locally determined
FIR-radio correlation holds at high redshifts, we estimate total FIR
luminosities for the radio sources. We note that the FIR luminosity estimates
for any radio-loud AGNs will be overestimates. Considering only the radio
sources with quasar-like bolometric luminosities, we find a maximum ratio of
candidate highly-obscured AGNs to X-ray-luminous (>10^42 ergs/s) sources of
about 1.9. We use source-stacking analyses to measure the X-ray surface
brightnesses of various X-ray and radio populations. We find the contributions
to the 4-8 keV light from our candidate highly-obscured AGNs to be very small,
and hence these sources are unable to account for the light that has been
suggested may be missing at these energies.Comment: 20 pages, Accepted by The Astrophysical Journal (scheduled for 1 Jan
2007), color figures 2 and 3 can be found at
http://www.astro.wisc.edu/~barger/radiopaper.htm
Faint M-dwarfs and the structure of the Galactic disk
We use broadband photometry and low-resolution spectra of a complete sample
of late-K and M dwarfs brighter than I=22 in three fields at high galactic
latitude to study issues relating to galactic structure and large scale
abundance gradients in the Galaxy. The observed starcounts in each field are a
good match to the predictions of models based on deep starcount data in other
intermediate-latitude fields, and these models identify the late-type stars as
members of the Galactic disk. Abundances for these late type stars are
estimated via narrowband indices that measure the strength of the TiO and CaH
bands in their spectra. Our results show that the average abundance in the
Galactic disk remains close to solar even at heights of more than 2 kpc above
the Plane.Comment: to appear in PASP; 17 pages, including 7 embedded, postscript figures
and 1 embedded table; uses AAS LaTeX style files (not included); also
available at http://astro.caltech.edu/~map/map.bibliography.htm
A systematic review of associations between environmental exposures and development of asthma in children aged up to 9 years
Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.Peer reviewedPublisher PD
Resolving the Submillimeter Background: the 850-micron Galaxy Counts
Recent deep blank field submillimeter surveys have revealed a population of
luminous high redshift galaxies that emit most of their energy in the
submillimeter. The results suggest that much of the star formation at high
redshift may be hidden to optical observations. In this paper we present
wide-area 850-micron SCUBA data on the Hawaii Survey Fields SSA13, SSA17, and
SSA22. Combining these new data with our previous deep field data, we establish
the 850-micron galaxy counts from 2 mJy to 10 mJy with a >3-sigma detection
limit. The area coverage is 104 square arcmin to 8 mJy and 7.7 square arcmin to
2.3 mJy. The differential 850-micron counts are well described by the function
n(S)=N_0/(a+S^3.2), where S is the flux in mJy, N_0=3.0 x 10^4 per square
degree per mJy, and a=0.4-1.0 is chosen to match the 850-micron extragalactic
background light. Between 20 to 30 per cent of the 850-micron background
resides in sources brighter than 2 mJy. Using an empirical fit to our >2 mJy
data constrained by the EBL at lower fluxes, we argue that the bulk of the
850-micron extragalactic background light resides in sources with fluxes near 1
mJy. The submillimeter sources are plausible progenitors of the present-day
spheroidal population.Comment: 5 pages, accepted by The Astrophysical Journal Letter
The Submillimeter Properties of the 1 Ms Chandra Deep Field North X-ray Sample
We present submillimeter observations for 136 of the 370 X-ray sources
detected in the 1 Ms exposure of the Chandra Deep Field North. Ten of the X-ray
sources are significantly detected in the submillimeter. The average X-ray
source in the sample has a significant 850 micron flux of 1.69+/-0.27 mJy. This
value shows little dependence on the 2-8 keV flux from 5e-16 erg/cm^2/s to
1e-14 erg/cm^2/s. The ensemble of X-ray sources contribute about 10% of the
extragalactic background light at 850 microns. The submillimeter excess is
found to be strongest in the optically faint X-ray sources that are also seen
at 20 cm, which is consistent with these X-ray sources being obscured and at
high redshift (z>1).Comment: 5 pages, submitted to The Astrophysical Journal Letter
The evolution of mass loaded supernova remnants: II. Temperature dependent mass injection rates
We investigate the evolution of spherically symmetric supernova remnants in which mass loading takes place due to conductively driven evaporation of embedded clouds. Numerical simulations reveal significant differences between the evolution of conductively mass loaded and the ablatively mass loaded remnants studied in Paper I. A main difference is the way in which conductive mass loading is extinguished at fairly early times, once the interior temperature of the remnant falls below ~ 107 K. Thus, at late times remnants that ablatively mass load are dominated by loaded mass and thermal energy, while those that conductively mass load are dominated by swept-up mass and kinetic energy. Simple approximations to the remnant evolution, complementary to those in Paper I, are given
The Calar Alto Deep Imaging Survey: K-band Galaxy Number Counts
We present K-band number counts for the faint galaxies in the Calar Alto Deep
Imaging Survey (CADIS). We covered 4 CADIS fields, a total area of 0.2deg^2, in
the broad band filters B, R and K. We detect about 4000 galaxies in the K-band
images, with a completeness limit of K=19.75mag, and derive the K-band galaxy
number counts in the range of 14.25 < K < 19.75mag. This is the largest medium
deep K-band survey to date in this magnitude range. The B- and R-band number
counts are also derived, down to completeness limits of B=24.75mag and
R=23.25mag. The K-selected galaxies in this magnitude range are of particular
interest, since some medium deep near-infrared surveys have identified breaks
of both the slope of the K-band number counts and the mean B-K color at
K=17\sim18mag. There is, however, a significant disagreement in the K-band
number counts among the existing surveys. Our large near-infrared selected
galaxy sample allows us to establish the presence of a clear break in the slope
at K=17.0mag from dlogN/dm = 0.64 at brighter magnitudes to dlogN/dm = 0.36 at
the fainter end. We construct no-evolution and passive evolution models, and
find that the passive evolution model can simultaneously fit the B-, R- and
K-band number counts well. The B-K colors show a clear trend to bluer colors
for K > 18mag. We also find that most of the K=18-20mag galaxies have a B-K
color bluer than the prediction of a no-evolution model for an L_* Sbc galaxy,
implying either significant evolution, even for massive galaxies, or the
existence of an extra population of small galaxies.Comment: Accepted for A&A, 10 pages, 7 figure
Mass-loaded spherical accretion flows
We have calculated the evolution of spherical accretion flows undergoing mass-loading from embedded clouds through either conduction or hydrodynamical ablation. We have observed the effect of varying the ratios of the mass-loading timescale and the cooling timescale to the ballistic crossing timescale through the mass-loading region.
We have also varied the ratio of the potential energy of a particle injected into the flow near the outer region of mass-loading to the temperature at which a minimum occurs in the cooling curve. The two types of mass-loading produce qualitatively different types of behaviour in the accretion flow, since mass-loading through conduction requires the ambient gas to be hot, whereas mass ablation from clumps occurs throughout the flow. Higher ratios of injected to accreted mass typically occur with hydrodynamical ablation, in agreement with previous work on wind-blown bubbles and supernova remnants. We find that mass-loading damps the radiative overstability of such flows, in agreement with our earlier work. If the mass-loading is high enough it can stabilize the accretion shock at a constant radius, yielding an almost isothermal subsonic post-shock flow. Such solutions may be relevant to cooling flows onto massive galaxies. Mass-loading can also lead to the formation of isolated shells of high temperature material, separated by gas at cooler temperatures
An Extremely Deep Wide-Field Near-Infrared Survey: Bright Galaxy Counts and Local Large Scale Structure
We present a deep, wide-field near-infrared (NIR) survey over five widely
separated fields at high Galactic latitude covering a total of ~ 3 deg^2 in J,
H, and Ks. The deepest areas of the data (~ 0.25 deg^2) extend to a 5 sigma
limiting magnitude of JHKs > 24 in the AB magnitude system. Although depth and
area vary from field to field, the overall depth and large area of this dataset
make it one of the deepest wide-field NIR imaging surveys to date. This paper
discusses the observations, data reduction, and bright galaxy counts in these
fields. We compare the slope of the bright galaxy counts with the Two Micron
All Sky Survey (2MASS) and other counts from the literature and explore the
relationship between slope and supergalactic latitude. The slope near the
supergalactic equator is sub- Euclidean on average pointing to the possibility
of a decreasing average space density of galaxies by ~ 10-15% over scales of ~
250-350 Mpc. On the contrary, the slope at high supergalactic latitudes is
strongly super-Euclidean on average suggesting an increase in the space density
of galaxies as one moves from the voids just above and below the supergalactic
plane out to distances of ~ 250-350 Mpc. These results suggest that local large
scale structure could be responsible for large discrepancies in the measured
slope between different studies in the past. In addition, the local universe
away from the supergalactic plane appears to be underdense by ~ 25-100%
relative to the space densities of a few hundred megaparsecs distant. Subject
headings: cosmology: observations and large scale structure of
universe-galaxies: fundamental parameters (counts)-infrared: galaxiesComment: Accepted to ApJS, 18 Pages, 14 Figures, 8 Table
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