25,507 research outputs found
NGC 3312: A victim of ram pressure sweeping
Researchers are undertaking a volume limited survey of the Hydra I cluster in neutral hydrogen using the National Radio Astronomy Observatory's Very Large Array (VLA). The main purpose is to study the effects of a dense environment on the gaseous component of the galaxies. Observational evidence has been accumulating recently that ram pressure sweeping does occur in the centers of clusters, but it is possible that tidal interactions play a role as well. Results of high resolution HI imaging of NGC 3312, the large peculiar spiral near the cluster center are presented. Hydra I (= A1060) is the nearest rich cluster beyond Virgo and, as such, presents a unique opportunity to do a complete survey of a cluster. It is similar to the Virgo cluster in many of its general physical characteristics, such as size, x ray luminosity, velocity dispersion, and galaxy content (high spiral fraction). However, Hydra I appears to be more regular and relaxed. This is evident in the x ray distribution in its central region, which is radially symmetric and centered on the dominant galaxy, NGC 3311, a cD-like elliptical. The observed x ray luminosity implies a central gas density of 4.5 x 10 to the 3rd power cm(-3). Gallagher (1978) argued from optical images of NGC 3312 that this galaxy might be an ideal candidate to directly study effects of the ram pressure process; it might currently be undergoing stripping of its interstellar medium. The researchers' data are consistent with this suggestion, but other origins of the peculiar appearance cannot yet be ruled out
Counts and Sizes of Galaxies in the Hubble Deep Field - South: Implications for the Next Generation Space Telescope
Science objectives for the Next Generation Space Telescope (NGST) include a
large component of galaxy surveys, both imaging and spectroscopy. The Hubble
Deep Field datasets include the deepest observations ever made in the
ultraviolet, optical and near infrared, reaching depths comparable to that
expected for NGST spectroscopy. We present the source counts, galaxy sizes and
isophotal filling factors of the HDF-South images. The observed integrated
galaxy counts reach >500 galaxies per square arcminute at AB<30. We extend
these counts to faint levels in the infrared using models. The trend previously
seen that fainter galaxies are smaller, continues to AB=29 in the high
resolution HDF-S STIS image, where galaxies have a typical half-light radius of
0.1 arcseconds. Extensive Monte Carlo simulations show that the small measured
sizes are not due to selection effects until >29mag. Using the HDF-S NICMOS
image, we show that galaxies are smaller in the near infrared than they are in
the optical. We analyze the isophotal filling factor of the HDF-S STIS image,
and show that this image is mostly empty sky even at the limits of galaxy
detection, a conclusion we expect to hold true for NGST spectroscopy. At the
surface brightness limits expected for NGST imaging, however, about a quarter
of the sky is occupied by the outer isophotes of AB<30 galaxies. We discuss the
implications of these data on several design concepts of the NGST near-infrared
spectrograph. We compare the effects of resolution and the confusion limit of
various designs, as well as the multiplexing advantages of either multi-object
or full-field spectroscopy. We argue that the optimal choice for NGST
spectroscopy of high redshift galaxies is a multi-object spectrograph (MOS)
with target selection by a micro electro mechanical system (MEMS) device.Comment: 27 pages including 10 figures, accepted for publication in the
Astronomical Journal, June 2000, abridged abstrac
Electrically detected magnetic resonance using radio-frequency reflectometry
The authors demonstrate readout of electrically detected magnetic resonance
at radio frequencies by means of an LCR tank circuit. Applied to a silicon
field-effect transistor at milli-kelvin temperatures, this method shows a
25-fold increased signal-to-noise ratio of the conduction band electron spin
resonance and a higher operational bandwidth of > 300 kHz compared to the kHz
bandwidth of conventional readout techniques. This increase in temporal
resolution provides a method for future direct observations of spin dynamics in
the electrical device characteristics.Comment: 9 pages, 3 figure
The relationship between weather and caribou productivity for the La-Poile Caribou Herd, Newfoundland
To describe the relationship between weather and caribou (Rangifer tarandus) productivity, we compared weather variables (snow on ground, winter temperature and measures of growing season) with measures of productivity (calves seen by hunters, calves and yearlings in the harvest and percent calves and yearlings and pregnancy rate for caribou classified during fall and spring surveys) for the LaPoile Caribou Herd in southwestern Newfoundland. Hunter statistics reliably estimated changes in population demography. Percent calves seen by hunters was correlated with calves/100 females classified in fall. Weather may have influenced productivity for the LaPoile Caribou Herd in Newfoundland. Colder winter temperatures were associated with fewer calves the next fall and pregnancy rates and yearlings/100 females in the spring were negatively correlated with snow on ground the previous winter. These relationships appear to be density related
The Dust Content of Galaxy Clusters
We report on the detection of reddening toward z ~ 0.2 galaxy clusters. This
is measured by correlating the Sloan Digital Sky Survey cluster and quasar
catalogs and by comparing the photometric and spectroscopic properties of
quasars behind the clusters to those in the field. We find mean E(B-V) values
of a few times 10^-3 mag for sight lines passing ~Mpc from the clusters'
center. The reddening curve is typical of dust but cannot be used to
distinguish between different dust types. The radial dependence of the
extinction is shallow near the cluster center suggesting that most of the
detected dust lies at the outskirts of the clusters. Gravitational
magnification of background z ~ 1.7 sources seen on Mpc (projected) scales
around the clusters is found to be of order a few per cent, in qualitative
agreement with theoretical predictions. Contamination by different spectral
properties of the lensed quasar population is unlikely but cannot be excluded.Comment: 4 pages, 3 figure
The AGN Outflow in the HDFS Target QSO J2233-606 from a High-Resolution VLT/UVES Spectrum
We present a detailed analysis of the intrinsic UV absorption in the central
HDFS target QSO J2233-606, based on a high-resolution, high S/N (~25 -- 50)
spectrum obtained with VLT/UVES. This spectrum samples the cluster of intrinsic
absorption systems outflowing from the AGN at radial velocities v ~ -5000 --
-3800 km/s in the key far-UV diagnostic lines - the lithium-like CNO doublets
and H I Lyman series. We fit the absorption troughs using a global model of all
detected lines to solve for the independent velocity-dependent covering factors
of the continuum and emission-line sources and ionic column densities. This
reveals increasing covering factors in components with greater outflow
velocity. Narrow substructure is revealed in the optical depth profiles,
suggesting the relatively broad absorption is comprised of a series of multiple
components. We perform velocity-dependent photoionization modeling, which
allows a full solution to the C, N, and O abundances, as well as the velocity
resolved ionization parameter and total column density. The absorbers are found
to have supersolar abundances, with [C/H] and [O/H] ~0.5 -- 0.9, and [N/H] ~
1.1 -- 1.3, consistent with enhanced nitrogen production expected from
secondary nucleosynthesis processes. Independent fits to each kinematic
component give consistent results for the abundances. The lowest-ionization
material in each of the strong absorbers is modeled with similar ionization
parameters. Components of higher-ionization (indicated by stronger O VI
relative to C IV and N V) are present at velocities just redward of each
low-ionization absorber. We explore the implications of these results for the
kinematic-geometric-ionization structure of the outflow.Comment: 12 pages, 10 figures, emulateapj, accepted for publication in Ap
Delta-like and gtl2 are reciprocally expressed, differentially methylated linked imprinted genes on mouse chromosome 12
AbstractThe distal portion of mouse chromosome 12 is imprinted. To date, however, Gtl2 is the only imprinted gene identified on chromosome 12. Gtl2 encodes multiple alternatively spliced transcripts with no apparent open reading frame. Using conceptuses with maternal or paternal uniparental disomy for chromosome 12 (UPD12), we found that Gtl2 is expressed from the maternal allele and methylated at the 5′ end of the silent paternal allele. A reciprocally imprinted gene, Delta-like (Dlk), with homology to genes involved in the Notch signalling pathway was identified 80kb upstream of Gtl2. Dlk was expressed exclusively from the paternal allele in both the embryo and placenta, but the CpG-island promoter of Dlk was completely unmethylated on both parental alleles. Rather, a paternally methylated region was identified in the last exon of the active Dlk allele. The proximity, reciprocal imprinting and methylation in this domain are reminiscent of the co-ordinately regulated Igf2–H19 imprinted domain on mouse chromosome 7. Like H19 and Igf2, Gtl2 and Dlk were found to be co-expressed in the same tissues throughout development, though not after birth. These results have implications for the regulation, function and evolution of imprinted domains
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