96 research outputs found
The Milky Way's stellar halo - lumpy or triaxial?
We present minimum chi-squared fits of power law and Hernquist density
profiles to F-turnoff stars in eight 2.5 deg wide stripes of SDSS data: five in
the North Galactic Cap and three in the South Galactic cap. Portions of the
stellar Galactic halo that are known to contain large streams of tidal debris
or other lumpy structure, or that may include significant contamination from
the thick disk, are avoided. The data strongly favor a model that is not
symmetric about the Galaxy's axis of rotation. If included as a free parameter,
the best fit to the center of the spheroid is surprisingly approx 3 kpc from
the Galactic center in the direction of the Sun's motion. The model fits favor
a low value of the density of halo stars at the solar position. The alternative
to a non-axisymmetric stellar distribution is that our fits are contaminated by
previously unidentified lumpy substructure.Comment: 10 pages, 10 figs, to appear in proceedings of conference "Physics at
the end of the Galactic Cosmic Ray Spectrum", Journal of Physics: Conf.
series, eds. G. Thomson and P. Sokolsk
Kinematic Discovery of a Stellar Stream Located in Pisces
We report the kinematic discovery of the Pisces Stellar Stream (PSS), at Galactic longitude l â 135° and â39° < b < â36°. We originally identified this halo substructure from velocities of red giant branch stars in the Sloan Digital Sky Survey (SDSS) Data Release 8, and confirmed its presence in turnoff stars from SDSS photometric data. The PSS is a narrow, kinematically cold tidal stream, with Ï_(v, 0) â 8 km s^(â1). Its metallicity is [Fe/H] â â2.2, with ~0.3 dex dispersion. The color-magnitude signature of the stream turnoff, combined with our measured metallicity, places the PSS at a distance of 35 ± 3 kpc. The PSS is the same as the previously announced "Triangulum stream" and part of the proposed "stream a." We rule out an association of the PSS with other previously known Milky Way substructures in the same region of the sky
The First Hypervelocity Star from the LAMOST Survey
We report the first hypervelocity star (HVS) discovered from the LAMOST
spectroscopic survey. It is a B-type star with a heliocentric radial velocity
about 620 km/s, which projects to a Galactocentric radial velocity component of
~477 km/s. With a heliocentric distance of ~13 kpc and an apparent magnitude of
~13 mag, it is the nearest bright HVS currently known. With a mass of ~9Msun,
it is one of the three most massive HVSs discovered so far. The star is
clustered on the sky with many other known HVSs, with the position suggesting a
possible connection to Galactic center structures. With the current
poorly-determined proper motion, a Galactic center origin of this HVS remains
consistent with the data at the 1sigma level, while a disk run-away origin
cannot be excluded. We discuss the potential of the LAMOST survey to discover a
large statistical sample of HVSs of different types.Comment: 5 pages, 5 figures, accepted for publication in ApJL, updated contour
plot for the ejection positions after correcting a mistake in the calculatio
Spectroscopy of Quasar Candidates from SDSS Commissioning Data
The Sloan Digital Sky Survey has obtained images in five broad-band colors
for several hundred square degrees. We present color-color diagrams for stellar
objects, and demonstrate that quasars are easily distinguished from stars by
their distinctive colors. Follow-up spectroscopy in less than ten nights of
telescope time has yielded 22 new quasars, 9 of them at , and one with
, the second highest-redshift quasar yet known. Roughly 80% of the
high-redshift quasar candidates selected by color indeed turn out to be
high-redshift quasars.Comment: 4 pages, 3 figures, to appear in the proceedings of "After the Dark
Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October
Astrophysics Conference in Marylan
An Algorithm for Preferential Selection of Spectroscopic Targets in LEGUE
We describe a general target selection algorithm that is applicable to any
survey in which the number of available candidates is much larger than the
number of objects to be observed. This routine aims to achieve a balance
between a smoothly-varying, well-understood selection function and the desire
to preferentially select certain types of targets. Some target-selection
examples are shown that illustrate different possibilities of emphasis
functions. Although it is generally applicable, the algorithm was developed
specifically for the LAMOST Experiment for Galactic Understanding and
Exploration (LEGUE) survey that will be carried out using the Chinese Guo Shou
Jing Telescope. In particular, this algorithm was designed for the portion of
LEGUE targeting the Galactic halo, in which we attempt to balance a variety of
science goals that require stars at fainter magnitudes than can be completely
sampled by LAMOST. This algorithm has been implemented for the halo portion of
the LAMOST pilot survey, which began in October 2011.Comment: 17 pages, 7 figures, accepted for publication in RA
A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey
(Abridged) We perform an analysis of spectra and photometry for 22,770 stars
included in the third data release (DR3) of the SDSS. We measure radial
velocities and, based on a model-atmosphere analysis, derive estimates ofthe
atmospheric parameters (effective temperature, surface gravity, and [Fe/H]) for
each star. Stellar evolution models are then used to estimate distances. The
SDSS sample covers a range in stellar brightness of 14 < V < 22, and comprises
large numbers of F- and G-type stars from the thick-disk and halo populations
(up to 100 kpc from the galactic plane), therefore including some of the oldest
stars in the Milky Way. In agreement with previous results from the literature,
we find that halo stars exhibit a broad range of iron abundances, with a peak
at [Fe/H] ~ -1.4. This population exhibits essentially no galactic rotation.
Thick-disk G-dwarf stars at distances from the galactic plane in the range
1<|z|<3 kpc show a much more compact metallicity distribution, with a maximum
at [Fe/H] ~ -0.7, and a median galactic rotation velocity at that metallicity
of 157 +/- 4 km/s (a lag relative to the thin disk of 63 km/s). A comparison of
color indices and metal abundances with isochrones indicates that no
significant star formation has taken place in the halo in the last ~ 11 Gyr,
but there are thick-disk stars which are at least 2 Gyr younger. We find the
metallicities of thick-disk stars to be nearly independent of galactocentric
distance between 5 and 14 kpc, in contrast with the marked gradients found in
the literature for the thin disk. No vertical metallicity gradient is apparent
for the thick disk, but we detect a gradient inits rotational velocity of -16
+/- 4 km/s/kpc between 1 and 3 kpc from the plane.Comment: 18 pages, 16 figures; accepted for publication in the ApJ; also
available from http://hebe.as.utexas.edu
SDSS Catalog of Stars in the Draco Dwarf Spheroidal Galaxy
The Sloan Digital Sky Survey (SDSS) has scanned the entire region containing
the Draco dwarf spheroidal galaxy to 23rd magnitude in g*. We present a catalog
of stars found in a 453 square arcminute, elliptical region centered on the
Draco dwarf spheroidal galaxy. Objects in the catalog are matched with five
previously published catalogs. The catalog contains SDSS photometry for 5634
individual objects, and also the photometry from matches to any of the other
catalogs. A comparison of the photometry between catalogs allows us to identify
142 candidate variable objects. One hundred and twelve of the suspected
variables have colors consistent with RR Lyrae variables.Comment: To appear in ApJS 14 pages, 6 figure
Fixing the Reference Frame for PPMXL Proper Motions Using Extragalactic Sources
We quantify and correct systematic errors in PPMXL proper motions using
extragalactic sources from the first two LAMOST data releases and the
Veron-Cetty & Veron Catalog of Quasars. Although the majority of the sources
are from the Veron catalog, LAMOST makes important contributions in regions
that are not well-sampled by previous catalogs, particularly at low Galactic
latitudes and in the south Galactic cap. We show that quasars in PPMXL have
measureable and significant proper motions, which reflect the systematic
zero-point offsets present in the catalog. We confirm the global proper motion
shifts seen by Wu, Ma, & Zhou (2011), and additionally find smaller-scale
fluctuations of the QSO-derived corrections to an absolute frame. We average
the proper motions of 158,106 extragalactic objects in bins of 3x3 degrees and
present a table of proper motion corrections.Comment: Accepted for publication in RAA; 12 pages, 6 figures (Fig. 1 at
reduced resolution); full table of corrections available in online journal,
with arxiv ancillary files (as ASCII table), or by reques
The Origin of the Virgo Stellar Substructure
We present three-dimensional space velocities of stars selected to be
consistent with membership in the Virgo stellar substructure. Candidates were
selected from SA 103, a single 40x40 arcmin field from our proper motion (PM)
survey in Kapteyn's Selected Areas (SAs), based on the PMs, SDSS photometry,
and follow-up spectroscopy of 215 stars. The signature of the Virgo
substructure is clear in the SDSS color-magnitude diagram (CMD) centered on SA
103, and 16 stars are identified that have high Galactocentric-frame radial
velocities (V_GSR > 50 km/s) and lie near the CMD locus of Virgo. The implied
distance to the Virgo substructure from the candidates is 14+/-3 kpc. We derive
mean kinematics from these 16 stars, finding a radial velocity V_GSR = 153+/-22
km/s and proper motions (mu_alpha*cos(delta), mu_delta) = (-5.24,
-0.91)+/-(0.43, 0.46) mas/yr. From the mean kinematics of these members, we
determine that the Virgo progenitor was on an eccentric (e ~ 0.8) orbit that
recently passed near the Galactic center (pericentric distance R_p ~ 6 kpc).
This destructive orbit is consistent with the idea that the substructure(s) in
Virgo originated in the tidal disruption of a Milky Way satellite. N-body
simulations suggest that the entire cloud-like Virgo substructure (encompassing
the "Virgo Overdensity" and the "Virgo Stellar Stream") is likely the tidal
debris remnant from a recently-disrupted massive (~10^9 M_sun) dwarf galaxy.
The model also suggests that some other known stellar overdensities in the
Milky Way halo (e.g., the Pisces Overdensity and debris near NGC 2419 and SEGUE
1) are explained by the disruption of the Virgo progenitor.Comment: Accepted to ApJ; preprint format, 41 pages, 17 figures (some with
degraded resolution). Full-resolution version (in emulateapj format)
available at
http://homepages.rpi.edu/~carlij/virgo_paper/carlin_etal2012_virgo.pd
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