162 research outputs found
Spatially Correlated Cluster Populations in the Outer Disk of NGC 3184
We use deep (~27.5 mag V-band point-source limiting magnitude) V- and U-band
LBT imaging to study the outer disk (beyond the optical radius R_25) of the
non-interacting, face-on spiral galaxy NGC 3184 (D = 11.1 Mpc; R_25 = 11.1 kpc)
and find that this outer disk contains >1000 objects (or marginally-resolved
'knots') resembling star clusters with masses ~10^2 - 10^4 M_sun and ages up to
~1 Gyr. We find statistically significant numbers of these cluster-like knots
extending to ~1.4 R_25, with the redder knots outnumbering bluer at the largest
radii. We measure clustering among knots and find significant correlation to
galactocentric radii of 1.5 R_25 for knot separations <1 kpc. The effective
integrated surface brightness of this outer disk cluster population ranges from
30 - 32 mag arcsec^-2 in V. We compare the HI extent to that of the correlated
knots and find that the clusters extend at least to the damped Lyman-alpha
threshold of HI column density (2e20 cm^-2; 1.62 R_25). The blue knots are
correlated with HI spiral structure to 1.5 R_25, while the red knots may be
correlated with the outer fringes of the HI disk to 1.7 R_25. These results
suggest that outer disks are well-populated, common, and long-lasting features
of many nearby disk galaxies.Comment: Accepted for publication in The Astrophysical Journal. 12 pages, 10
figure
Stellar Population Variations in the Milky Way's Stellar Halo
If the stellar halos of disk galaxies are built up from the disruption of
dwarf galaxies, models predict highly structured variations in the stellar
populations within these halos. We test this prediction by studying the ratio
of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor
populations) to main-sequence turn-off stars (MSTO stars; a feature of all
populations) in the stellar halo of the Milky Way using data from the Sloan
Digital Sky Survey. We develop and apply an improved technique to select BHB
stars using ugr color information alone, yielding a sample of ~9000 g<18
candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across
~1/4 of the sky at the distance resolution permitted by the absolute magnitude
distribution of MSTO stars. We find large variations of BHB/MSTO star ratio in
the stellar halo. Previously identified, stream-like halo structures have
distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo
features, e.g., the low-latitude structure, appear to be almost completely
devoid of BHB stars, whereas other structures appear to be rich in BHB stars.
The Sagittarius tidal stream shows an apparent variation in BHB/MSTO ratio
along its extent, which we interpret in terms of population gradients within
the progenitor dwarf galaxy. Our detection of coherent stellar population
variations between different stellar halo substructures provides yet more
support to cosmologically motivated models for stellar halo growth.Comment: Astronomical Journal, in press. 10 pages, 5 color figures. Much
better printed in colo
An Improved Photometric Calibration of the Sloan Digital Sky Survey Imaging Data
We present an algorithm to photometrically calibrate wide field optical
imaging surveys, that simultaneously solves for the calibration parameters and
relative stellar fluxes using overlapping observations. The algorithm decouples
the problem of "relative" calibrations, from that of "absolute" calibrations;
the absolute calibration is reduced to determining a few numbers for the entire
survey. We pay special attention to the spatial structure of the calibration
errors, allowing one to isolate particular error modes in downstream analyses.
Applying this to the Sloan Digital Sky Survey imaging data, we achieve ~1%
relative calibration errors across 8500 sq.deg. in griz; the errors are ~2% for
the u band. These errors are dominated by unmodelled atmospheric variations at
Apache Point Observatory. These calibrations, dubbed "ubercalibration", are now
public with SDSS Data Release 6, and will be a part of subsequent SDSS data
releases.Comment: 16 pages, 17 figures, matches version accepted in ApJ. These
calibrations are available at http://www.sdss.org/dr
A New Milky Way Dwarf Galaxy in Ursa Major
In this Letter, we report the discovery of a new dwarf satellite to the Milky
Way, located at () (158.72,51.92) in the
constellation of Ursa Major. This object was detected as an overdensity of red,
resolved stars in Sloan Digital Sky Survey data. The color-magnitude diagram of
the Ursa Major dwarf looks remarkably similar to that of Sextans, the lowest
surface brightness Milky Way companion known, but with approximately an order
of magnitude fewer stars. Deeper follow-up imaging confirms this object has an
old and metal-poor stellar population and is 100 kpc away. We roughly
estimate M -6.75 and 250 pc for this dwarf. Its luminosity is
several times fainter than the faintest known Milky Way dwarf. However, its
physical size is typical for dSphs. Even though its absolute magnitude and size
are presently quite uncertain, Ursa Major is likely the lowest luminosity and
lowest surface brightness galaxy yet known.Comment: Replaced with ApJL accepted version. Includes some additional
details, corrected references, and minor changes to Figure
The Dependence of Star Formation History and Internal Structure on Stellar Mass for 10^5 Low-Redshift Galaxies
We study the relations between stellar mass, star formation history, size and
internal structure for a complete sample of 122,808 galaxies drawn from the
Sloan Digital Sky Survey. We show that low-redshift galaxies divide into two
distinct families at a stellar mass of 3 \times 10^10 M_sol. Lower mass
galaxies have young stellar populations, low surface mass densities and the low
concentrations typical of disks. A significant fraction of the lowest mass
galaxies in our sample have experienced recent starbursts. At given stellar
mass, the sizes of low mass galaxies are log- normally distributed with
dispersion sigma(ln R_50) \sim 0.5, in excellent agreement with the idea that
they form with little angular momentum loss through cooling and condensation in
a gravitationally dominant dark matter halo. Their median stellar surface mass
density scales with stellar mass as mu* propto M_*^0.54, suggesting that the
stellar mass of a disk galaxy is proprtional to the three halves power of its
halo mass. This suggests that the efficiency of the conversion of baryons into
stars in low mass galaxies increases in propor- tion to halo mass, perhaps as a
result of supernova feedback processes. At stellar masses above 3 \times 10^10
M_sol, there is a rapidly increasing frac- tion of galaxies with old stellar
populations, high surface mass densities and high concentrations typical of
bulges. In this regime, the size distribution is log-normal, but its dispersion
decreases rapidly with increasing stellar mass and the median mass surface
density is approximately constant. This suggests that the star formation
efficiency decreases in the highest mass halos, and that little star formation
occurs in massive galaxies once they have assembled.Comment: accepted by MNRAS, some changes to results as a result of
improvements in stellar mass estimates as decribed in Paper
The Atacama Cosmology Telescope: Physical Properties and Purity of a Galaxy Cluster Sample Selected via the Sunyaev-Zel'dovich Effect
We present optical and X-ray properties for the first confirmed galaxy
cluster sample selected by the Sunyaev-Zel'dovich Effect from 148 GHz maps over
455 square degrees of sky made with the Atacama Cosmology Telescope. These
maps, coupled with multi-band imaging on 4-meter-class optical telescopes, have
yielded a sample of 23 galaxy clusters with redshifts between 0.118 and 1.066.
Of these 23 clusters, 10 are newly discovered. The selection of this sample is
approximately mass limited and essentially independent of redshift. We provide
optical positions, images, redshifts and X-ray fluxes and luminosities for the
full sample, and X-ray temperatures of an important subset. The mass limit of
the full sample is around 8e14 Msun, with a number distribution that peaks
around a redshift of 0.4. For the 10 highest significance SZE-selected cluster
candidates, all of which are optically confirmed, the mass threshold is 1e15
Msun and the redshift range is 0.167 to 1.066. Archival observations from
Chandra, XMM-Newton, and ROSAT provide X-ray luminosities and temperatures that
are broadly consistent with this mass threshold. Our optical follow-up
procedure also allowed us to assess the purity of the ACT cluster sample.
Eighty (one hundred) percent of the 148 GHz candidates with signal-to-noise
ratios greater than 5.1 (5.7) are confirmed as massive clusters. The reported
sample represents one of the largest SZE-selected sample of massive clusters
over all redshifts within a cosmologically-significant survey volume, which
will enable cosmological studies as well as future studies on the evolution,
morphology, and stellar populations in the most massive clusters in the
Universe.Comment: 20 pages, 15 figures, 6 tables. Accepted for publication in ApJ.
Higher resolution figures available at:
http://peumo.rutgers.edu/~felipe/e-prints
Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data Release
The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained
five-band optical-wavelength imaging near the Galactic plane outside of the
nominal survey boundaries. These additional data were obtained during
commissioning and subsequent testing of the SDSS observing system, and they
provide unique wide-area imaging data in regions of high obscuration and star
formation, including numerous young stellar objects, Herbig-Haro objects and
young star clusters. Because these data are outside the Survey regions in the
Galactic caps, they are not part of the standard SDSS data releases. This paper
presents imaging data for 832 square degrees of sky (including repeats), in the
star-forming regions of Orion, Taurus, and Cygnus. About 470 square degrees are
now released to the public, with the remainder to follow at the time of SDSS
Data Release 4. The public data in Orion include the star-forming region NGC
2068/NGC 2071/HH24 and a large part of Barnard's loop.Comment: 31 pages, 9 figures (3 missing to save space), accepted by AJ, in
press, see http://photo.astro.princeton.edu/oriondatarelease for data and
paper with all figure
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