145 research outputs found
The Large-scale Distribution of Cool Gas around Luminous Red Galaxies
We present a measurement of the correlation function between luminous red
galaxies and cool gas traced by Mg II \lambda \lambda 2796, 2803 absorption, on
scales ranging from about 30 kpc to 20 Mpc. The measurement is based on
cross-correlating the positions of about one million red galaxies at z~0.5 and
the flux decrements induced in the spectra of about 10^5 background quasars
from the Sloan Digital Sky Survey. We find that: (i) This galaxy-gas
correlation reveals a change of slope on scales of about 1 Mpc, consistent with
the expected transition from a dark matter halo dominated environment to a
regime where clustering is dominated by halo-halo correlations. Assuming that,
on average, the distribution of Mg II gas follows that of dark matter up to a
gas-to-mass ratio, we find the standard halo model to provide an accurate
description of the gas distribution over three orders of magnitude in scale.
Within this framework we estimate the average host halo mass of luminous red
galaxies to be about 10^{13.5} M_solar, in agreement with other methods. We
also find the Mg II gas-to-mass ratio around LRGs to be consistent with the
cosmic value estimated on Mpc scales. Combining our galaxy-gas correlation and
the galaxy-mass correlation function from galaxy-galaxy lensing analyses we can
directly measure the Mg II gas-to-mass ratio as a function of scale and reach
the same conclusion. (ii) From line-width estimates, we show that the velocity
dispersion of the gas clouds also shows the expected 1- and 2-halo behaviors.
On large scales the gas distribution follows the Hubble flow, whereas on small
scales we observe the velocity dispersion of the Mg II gas clouds to be lower
than that of collisionless dark matter particles within their host halo. This
is in line with the fact that cool clouds are subject to the pressure of the
virialized hot gas.Comment: 18 pages, 11 figures, 1 table, submitted to MNRA
Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey
We report the discovery of five very cool white dwarfs in the Sloan Digital
Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced
absorption (CIA) from molecular hydrogen and are similar in color to the three
previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402.
The fifth, an ultracool white dwarf candidate, shows milder CIA flux
suppression and has a color and spectral shape similar to WD 0346+246. All five
new white dwarfs are faint (g > 18.9) and have significant proper motions. One
of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system
with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.Comment: 15 pages, 3 figures, submitted to ApJL. Higher resolution versions of
finding charts are available at
http://astro.uchicago.edu/~gates/findingchart
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
A New Giant Stellar Structure in the Outer Halo of M31
The Sloan Digital Sky Survey has revealed an overdensity of luminous red
giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we
have called Andromeda NE. The line-of-sight distance to Andromeda NE is within
approximately 50 kpc of M31; Andromeda NE is not a physically unrelated
projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central
surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of
magnitude more diffuse than any known Local Group dwarf galaxy at that
luminosity. Based on its distance and morphology, Andromeda NE is likely
undergoing tidal disruption. Andromeda NE's red giant branch color is unlike
that of M31's present-day outer disk or the stellar stream reported by Ibata et
al. (2001), arguing against a direct link between Andromeda NE and these
structures. However, Andromeda NE has a red giant branch color similar to that
of the G1 clump; it is possible that these structures are both material torn
off of M31's disk in the distant past, or that these are both part of one
ancient stellar stream.Comment: 11 pages, 3 figures; ApJ Letters accepted versio
A Strategy for Finding Near Earth Objects with the SDSS Telescope
We present a detailed observational strategy for finding Near Earth Objects
(NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate
strategies in normal, unbinned mode as well as binning the CCDs 2x2 or 3x3,
which affects the sky coverage rate and the limiting apparent magnitude. We
present results from 1 month, 3 year and 10 year simulations of such surveys.
For each cadence and binning mode, we evaluate the possibility of achieving the
Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H <= 18 for
an albedo of 0.1). We find that an unbinned survey is most effective at
detecting H <= 20 NEOs in our sample. However, a 3x3 binned survey reaches the
Spaceguard Goal after only seven years of operation. As the proposed large
survey telescopes (PanStarss; LSST) are at least 5-10 years from operation, an
SDSS NEO survey could make a significant contribution to the detection and
photometric characterization of the NEO population.Comment: Accepted by AJ -- 12 pages, 11 figure
High-resolution, H band Spectroscopy of Be Stars with SDSS-III/APOGEE: I. New Be Stars, Line Identifications, and Line Profiles
APOGEE has amassed the largest ever collection of multi-epoch,
high-resolution (R~22,500), H-band spectra for B-type emission line (Be) stars.
The 128/238 APOGEE Be stars for which emission had never previously been
reported serve to increase the total number of known Be stars by ~6%. We focus
on identification of the H-band lines and analysis of the emission peak
velocity separations (v_p) and emission peak intensity ratios (V/R) of the
usually double-peaked H I and non-hydrogen emission lines. H I Br11 emission is
found to preferentially form in the circumstellar disks at an average distance
of ~2.2 stellar radii. Increasing v_p toward the weaker Br12--Br20 lines
suggests these lines are formed interior to Br11. By contrast, the observed IR
Fe II emission lines present evidence of having significantly larger formation
radii; distinctive phase lags between IR Fe II and H I Brackett emission lines
further supports that these species arise from different radii in Be disks.
Several emission lines have been identified for the first time including
~16895, a prominent feature in the spectra for almost a fifth of the sample
and, as inferred from relatively large v_p compared to the Br11-Br20, a tracer
of the inner regions of Be disks. Unlike the typical metallic lines observed
for Be stars in the optical, the H-band metallic lines, such as Fe II 16878,
never exhibit any evidence of shell absorption, even when the H I lines are
clearly shell-dominated. The first known example of a quasi-triple-peaked Br11
line profile is reported for HD 253659, one of several stars exhibiting intra-
and/or extra-species V/R and radial velocity variation within individual
spectra. Br11 profiles are presented for all discussed stars, as are full
APOGEE spectra for a portion of the sample.Comment: accepted in A
Update on the Nature of Virgo Overdensity
We use the Eighth Data Release of Sloan Digital Sky Survey (SDSS DR8) catalog
with its additional sky coverage of the southern Galactic hemisphere, to
measure the extent and study the nature of the Virgo Overdensity (VOD; Juric et
al. 2008). The data show that the VOD extends over no less than 2000 deg^2,
with its true extent likely closer to 3000 deg^2. We test whether the VOD can
be attributed to a tilt in the stellar halo ellipsoid with respect to the plane
of the Galactic disk and find that the observed symmetry of the north-south
Galactic hemisphere star counts excludes this possibility. We argue that the
Virgo Overdensity, in spite of its wide area and cloud-like appearance, is
still best explained by a minor merger. Its appearance and position is
qualitatively similar to a near perigalacticon merger event and, assuming that
the VOD and the Virgo Stellar Stream share the same progenitor, consistent with
the VSS orbit determined by Casetti-Dinescu et al. (2009).Comment: 9 pages,6 figures; accepted for publication in A
Tracing Sagittarius Structure with SDSS and SEGUE Imaging and Spectroscopy
We show that the Sagittarius dwarf tidal stream can be traced with very red
K/M-giant stars selected from SDSS photometry. A subset of these stars are
spectroscopically confirmed with SEGUE and SDSS spectra, and the distance scale
of 2MASS and SDSS M giants is calibrated to the RR Lyrae distance scale. The
absolute g band magnitude of the K/M-giant stars at the tip of the giant branch
is M_g=-1.0. The line-of-sight velocities of the M giant and BHB stars that are
spatially coincident with the Sgr dwarf tidal stream are consistent with those
of previous authors, reinforcing the need for new models that can explain all
of the Sgr tidal debris stream observations. We estimate stellar densities
along the tidal tails that can be used to help constrain future models. The
K/M-giant, BHB, and F-turnoff stars in the lower surface brightness tidal
stream that is adjacent to the main leading Sgr dwarf tidal tail have
velocities and metallicities that are similar to those of the stars in the
leading tidal tail. The ratio of K/M giants to BHBs and BHBs to F-turnoff stars
are also similar for both branches of the leading tidal tail. We show that
there is an additional low-metallicity tidal stream near the Sgr trailing tidal
tail.Comment: 19 figures, accepted for publication in ApJ, references update
The kinematics of late type stars in the solar cylinder studied with SDSS data
We study the velocity distribution of Milky Way disk stars in a
kiloparsec-sized region around the Sun, based on ~ 2 million M-type stars from
DR7 of SDSS, which have newly re-calibrated absolute proper motions from
combining SDSS positions with the USNO-B catalogue. We estimate photometric
distances to all stars, accurate to ~ 20 %, and combine them with the proper
motions to derive tangential velocities for this kinematically unbiased sample
of stars. Based on a statistical de-projection method we then derive the
vertical profiles (to heights of Z = 800 pc above the disk plane) for the first
and second moments of the three dimensional stellar velocity distribution. We
find that = -7 +/- 1 km/s and = -9 +/- 1 km/s, independent of height
above the mid-plane, reflecting the Sun's motion with respect to the local
standard of rest. In contrast, changes distinctly from -20 +/- 2 km/s in
the mid-plane to = -32 km/s at Z = 800 pc, reflecting an asymmetric drift
of the stellar mean velocity that increases with height. All three components
of the M-star velocity dispersion show a strong linear rise away from the
mid-plane, most notably \sigma_{ZZ}, which grows from 18 km/s (Z = 0) to 40
km/s (at Z = 800 pc). We determine the orientation of the velocity ellipsoid,
and find a significant vertex deviation of 20 to 25 degrees, which decreases
only slightly to heights of Z = 800 pc. Away from the mid-plane, our sample
exhibits a remarkably large tilt of the velocity ellipsoid towards the Galactic
plane, which reaches 20 deg. at Z = 800 pc and which is not easily explained.
Finally, we determine the ratio \sigma^2_{\phi\phi}/\sigma^2_{RR} near the
mid-plane, which in the epicyclic approximation implies an almost perfectly
flat rotation curve at the Solar radius.Comment: 18 pages, 9 figures, accepted to Astron.
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