3,370 research outputs found
Cool White Dwarfs Identified in the Second Data Release of the UKIRT Infrared Deep Sky Survey
We have paired the Second Data Release of the Large Area Survey of the UKIRT
Infrared Deep Sky Survey with the Fifth Data Release of the Sloan Digital Sky
Survey to identify ten cool white dwarf candidates, from their photometry and
astrometry. Of these ten, one was previously known to be a very cool white
dwarf. We have obtained optical spectroscopy for seven of the candidates using
the GMOS-N spectrograph on Gemini North, and have confirmed all seven as white
dwarfs. Our photometry and astrometry indicates that the remaining two objects
are also white dwarfs. Model analysis of the photometry and available
spectroscopy shows that the seven confirmed new white dwarfs, and the two new
likely white dwarfs, have effective temperatures in the range Teff = 5400-6600
K. Our analysis of the previously known white dwarf confirms that it is cool,
with Teff = 3800 K. The cooling age for this dwarf is 8.7 Gyr, while that of
the nine ~6000 K white dwarfs is 1.8-3.6 Gyr. We are unable to determine the
masses of the white dwarfs from the existing data, and therefore we cannot
constrain the total ages of the white dwarfs. The large cooling age for the
coolest white dwarf in the sample, combined with its low estimated tangential
velocity, suggests that it is an old member of the thin disk, or a member of
the thick disk of the Galaxy, with an age 10-11 Gyr. The warmer white dwarfs
appear to have velocities typical of the thick disk or even halo; these may be
very old remnants of low-mass stars, or they may be relatively young thin disk
objects with unusually high space motion.Comment: 37 pages (referee format), 4 tables, 7 figures, accepted to Ap
On the Connection Between Metal Absorbers and Quasar Nebulae
We establish a simple model for the distribution of cold gas around L*
galaxies using a large set of observational constraints on the properties of
strong MgII absorber systems. Our analysis suggests that the halos of L*
galaxies are filled with cool gaseous clouds having sizes of order 1kpc and
densities of ~10^{-2} cm^{-3}. We then investigate the physical effects of
cloud irradiation by a quasar and study the resulting spectral signatures. We
show that quasar activity gives rise to (i) extended narrow-line emission on
~100kpc scales and (ii) an anisotropy in the properties of the absorbing gas
arising from the geometry of the quasar radiation field. Provided that quasars
reside in halos several times more massive than those of L* galaxies, our model
predictions appear to be in agreement with observations of narrow emission-line
nebulae around quasars and the recent detections of ~100kpc cold gaseous
envelopes around those objects, suggesting a common origin for these phenomena.
We discuss the implications of our results for understanding absorption
systems, probing quasar environments at high redshifts, and testing the quasar
unification scheme.Comment: 15 pages, 13 figures (ApJ submitted
Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey
We present the results of a search for cool white dwarfs in the United
Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area
Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky
Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their
neutral optical colors and blue near-infrared colors, as well as faint Reduced
Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini
Observatory, and showed the majority of the candidates to be newly identified
cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants.
Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with
effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky
has produced thirteen new white dwarfs. Model fits to the photometry show that
seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and
cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s
and are likely to be thick disk 10-11 Gyr-old objects. The other half of the
sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr,
and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with
unusually high velocities, or lower-mass remnants of thick disk or halo late-F
or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures
and 5 Table
Finding Nested Common Intervals Efficiently
International audienceIn this paper, we study the problem of effi ciently fi nding gene clusters formalized by nested common intervals between two genomes represented either as permutations or as sequences. Considering permutations, we give several algorithms whose running time depends on the size of the actual output rather than the output in the worst case. Indeed, we first provide a straightforward O(n^3) time algorithm for finding all nested common intervals. We reduce this complexity by providing an O(n^2) time algorithm computing an irredundant output. Finally, we show, by providing a third algorithm, that fi nding only the maximal nested common intervals can be done in linear time. Considering sequences, we provide solutions (modi cations of previously de ned algorithms and a new algorithm) for di fferent variants of the problem, depending on the treatment one wants to apply to duplicated genes
Lyman-alpha absorption around nearby galaxies
We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption lines
in the outer regions of eight nearby galaxies using background QSOs and AGN as
probes. Lya lines are detected within a few hundred km/s of the systemic
velocity of the galaxy in all cases. We conclude that a background
line-of-sight which passes within 26-200 h-1 kpc of a foreground galaxy is
likely to intercept low column density neutral hydrogen with log N(HI) >~ 13.0.
The ubiquity of detections implies a covering factor of ~ 100% for low N(HI)
gas around galaxies within 200 h-1 kpc. We discuss the difficulty in trying to
associate individual absorption components with the selected galaxies and their
neighbors, but show that by degrading our STIS data to lower resolutions, we
are able to reproduce the anti-correlation of Lya equivalent width and impact
parameter found at higher redshift. We also show that the equivalent width and
column density of Lya complexes (when individual components are summed over ~
1000 km/s) correlate well with a simple estimate of the volume density of
galaxies brighter than M(B) = -17.5 at the same redshift as a Lya complex. We
do not reject the hypothesis that the selected galaxies are directly
responsible for the observed Lya lines, but our analysis indicates that
absorption by clumpy intragroup gas is an equally likely explanation. (Abriged)Comment: Accepted for publication in Nov 20, 2002 issue of ApJ. Paper with all
figures can be found at http://www.astro.princeton.edu/~dvb/lyapaper.ps
(preferable). Minor typos fixe
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