641 research outputs found
Detection of a population gradient in the Sagittarius Stream
We present a quantitative comparison between the Horizontal Branch morphology
in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide
field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc
away from the center of the parent galaxy. We find that the Blue Horizontal
Branch (BHB) stars in that part of the Stream are five times more abundant than
in the Sgr core, relative to Red Clump stars. The difference in the ratio of
BHB to RC stars between the two fields is significant at the 4.8 sigma level.
This indicates that the old and metal-poor population of Sgr was preferentially
stripped from the galaxy in past peri-Galactic passages with respect to the
intermediate-age metal rich population that presently dominates the bound core
of Sgr, probably due to a strong radial gradient that was settled within the
galaxy before its disruption. The technique adopted in the present study allows
to trace population gradients along the whole extension of the Stream.Comment: 4 pages, 3 .ps figures (fig. 1 at low resolution); Accepted for
publication by A&A Letter
Rings and Radial Waves in the Disk of the Milky Way
We show that in the anticenter region, between Galactic longitudes of
, there is an oscillating asymmetry in the main sequence
star counts on either side of the Galactic plane using data from the Sloan
Digital Sky Survey. This asymmetry oscillates from more stars in the north at
distances of about 2 kpc from the Sun to more stars in the south at 4-6 kpc
from the Sun to more stars in the north at distances of 8-10 kpc from the Sun.
We also see evidence that there are more stars in the south at distances of
12-16 kpc from the Sun. The three more distant asymmetries form roughly
concentric rings around the Galactic center, opening in the direction of the
Milky Way's spiral arms. The northern ring, 9 kpc from the Sun, is easily
identified with the previously discovered Monoceros Ring. Parts of the southern
ring at 14 kpc from the Sun (which we call the TriAnd Ring) have previously
been identified as related to the Monoceros Ring and others have been called
the Triangulum Andromeda Overdensity. The two nearer oscillations are
approximated by a toy model in which the disk plane is offset by of the order
100 pc up and then down at different radii. We also show that the disk is not
azimuthally symmetric around the Galactic anticenter and that there could be a
correspondence between our observed oscillations and the spiral structure of
the Galaxy. Our observations suggest that the TriAnd and Monoceros Rings (which
extend to at least 25 kpc from the Galactic center) are primarily the result of
disk oscillations.Comment: 19figures, 2tables, ApJ accepte
Proper Motions in Kapteyn Selected Area 103: A Preliminary Orbit for the Virgo Stellar Stream
We present absolute proper motions in Kapteyn Selected Area (SA) 103. This
field is located 7 degrees west of the center of the Virgo Stellar Stream (VSS,
Duffau et al. 2006), and has a well-defined main sequence representing the
stream. In SA 103 we identify one RR Lyrae star as a member of the VSS
according to its metallicity, radial velocity and distance. VSS candidate
turnoff stars and subgiant stars have proper motions consistent with that of
the RR Lyrae star. The 3D velocity data imply an orbit with a pericenter of 11
kpc and an apocenter of ~90 kpc. Thus, the VSS comprises tidal debris found
near the pericenter of a highly destructive orbit. Examining the six globular
clusters at distances larger than 50 kpc from the Galactic center, and the
proposed orbit of the VSS, we find one tentative association, NGC 2419. We
speculate that NGC 2419 is possibly the nucleus of a disrupted system of which
the VSS is a part.Comment: ApJL accepte
Kinematic Discovery of a Stellar Stream Located in Pisces
We report the kinematic discovery of the Pisces Stellar Stream (PSS), at Galactic longitude l ≈ 135° and –39° < b < –36°. 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 σ_(v, 0) ≈ 8 km s^(–1). Its metallicity is [Fe/H] ≈ –2.2, 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
Stellar over-densities in the halo: the extent of the Virgo over-density
We map the three dimensional extent of the Virgo Over-density by combining
distance information from RR Lyrae variables and projected spatial information
from SEKBO (Keller et al. 2008) and Sloan Digital Sky Survey (SDSS) DR6
photometry. The Virgo Over-density is seen to comprise two filaments 14.5 x 3
degrees and 10 x 3 degrees and a circular structure 3 degrees in diameter.
Together the three features span 38 degrees of right ascension and declinations
of +2 to -15 degrees. RR Lyrae variables place the two filamentary features at
heliocentric distances of 20 and 17 kpc respectively, with projected dimensions
of 5 x 1 kpc and 3 x 1 kpc.Comment: 6 pages, 5 figures, MNRAS accepte
The LEGUE Input Catalogue for Dark Night Observing in the LAMOST Pilot Survey
We outline the design of the dark nights portion of the LAMOST Pilot Survey,
which began observations in October 2011. In particular, we focus on Milky Way
stellar candidates that are targeted for the LEGUE (LAMOST Experiment for
Galactic Understanding and Exploration) survey. We discuss the regions of sky
in which spectroscopic candidates were selected, and the motivations for
selecting each of these sky areas. Some limitations due to the unique design of
the telescope are discussed, including the requirement that a bright (V < 8)
star be placed at the center of each plate for wavefront sensing and active
optics corrections. The target selection categories and scientific goals
motivating them are briefly discussed, followed by a detailed overview of how
these selection functions were realized. We illustrate the difference between
the overall input catalog - Sloan Digital Sky Survey (SDSS) photometry - and
the final targets selected for LAMOST observation.Comment: 11 pages, 7 figures, accepted for publication in RA
Mapping the Milky Way with LAMOST II: the stellar halo
The radial number density and flattening of the Milky Way's stellar halo is
measured with metal-poor ([Fe/H]) K giants from LAMOST
DR3, using a nonparametric method which is model independent and largely avoids
the influence of halo substucture. The number density profile is well described
by a single power law with index , and flattening that
varies with radius. The stellar halo traced by LAMOST K giants is more
flattened at smaller radii, and becomes nearly spherical at larger radii. The
flattening, , is about 0.64, 0.8, 0.96 at , 20 and 30 kpc (where
), respectively. Moreover, the
leading arm of the Sagittarius dwarf galaxy tidal stream in the north, and the
trailing arm in the south, are significant in the residual map of density
distribution. In addition, an unknown overdensity is identified in the residual
map at (R,Z)=(30,15) kpc.Comment: 16 pages, 24 figures, accepted by MNRA
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