7 research outputs found
Spectroscopy of QUEST RR Lyrae Variables: the new Virgo Stellar Stream
Eighteen RR Lyrae variables (RRLs) that lie in the "12\fh 4 clump"
identified by the QUEST survey have been observed spectroscopically to measure
their radial velocities and metal abundances. Ten blue horizontal branch (BHB)
stars identified by the Sloan Digital Sky Survey (SDSS) were added to this
sample. Six of the 9 stars in the densest region of the clump have a mean
radial velocity in the galactic rest frame () of 99.8 and
= 17.3 , which is slightly smaller than the average error of
the measurements. The whole sample contains 8 RRLs and 5 BHB stars that have
values of suggesting membership in this stream. For 7 of these
RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex,
yield and = 0.40. These values suggest that
the stream is a tidally disrupted dwarf spheroidal galaxy of low luminosity.
Photometry from the database of the SDSS indicates that this stream covers at
least 106 deg of the sky in the constellation Virgo. The name Virgo Stellar
Stream is suggested.Comment: Replaced with revised version accepted for publication in ApJ Letters
13 pages 4 figure
Spectroscopy of Bright QUEST RR Lyrae Stars: Velocity Substructures toward Virgo
Using a sample of 43 bright (V<16.1, distance <13 kpc) RR Lyrae stars (RRLS)
from the QUEST survey with spectroscopic radial velocities and metallicities,
we find that several separate halo substructures contribute to the Virgo
overdensity (VOD). While there is little evidence for halo substructure in the
spatial distribution of these stars, their distribution in radial velocity
reveals two moving groups. These results are reinforced when the sample is
combined with a sample of blue horizontal branch stars that were identified in
the SDSS, and the combined sample provides evidence for one additional moving
group. These groups correspond to peaks in the radial velocity distribution of
a sample of F type main-sequence stars that was recently observed in the same
directon by SEGUE, although in one case the RRLS and F star groups may not lie
at the same distance. One of the new substructures has a very narrow range in
metallicity, which is more consistent with it being the debris from a destroyed
globular cluster than from a dwarf galaxy. A small concentration of stars have
radial velocities that are similar to the Virgo Stellar Stream (VSS) that was
identified previously in a fainter sample of RRLS. Our results suggest that
this feature extends to distances as short as ~12 kpc from its previous
detection at ~19 kpc. None of the new groups and only one star in the sample
have velocities that are consistent with membership in the leading tidal stream
from the Sagittarius Dwarf Spheroidal Galaxy, which some authors have suggested
is the origin of the VOD.Comment: Accepted for publication in the A
The Chemical Enrichment History of the Fornax Dwarf Spheroidal Galaxy from the Infrared Calcium Triplet
Near infrared spectra were obtained for 117 red giants in the Fornax dwarf
spheroidal galaxy with the FORS1 spectrograph on the VLT, in order to study the
metallicity distribution of the stars and to lift the age-metallicity
degeneracy of the red giant branch (RGB) in the color-magnitude diagram (CMD).
Metallicities are derived from the equivalent widths of the infrared Calcium
triplet lines at 8498, 8542, and 8662 A, calibrated with data from globular
clusters, the open cluster M67 and the LMC. For a substantial portion of the
sample, the strength of the Calcium triplet is unexpectedly high, clearly
indicating that the main stellar population of Fornax is significantly more
metal-rich than could be inferred from the position of its RGB in the CMD. We
show that the relative narrowness of the RGB in Fornax is caused by the
superposition of stars of very different ages and metallicities. The
metallicity distribution in Fornax is centered at [Fe/H]= -0.9, with a
metal-poor tail extending to [Fe/H] ~= -2. While the distribution to higher
metallicities is less well determined by our observations, the comparison with
LMC data indicates that it extends to [Fe/H] ~ -0.4. By comparing the
metallicities of the stars with their positions in the CMD, we have derived the
complex age-metallicity relation of Fornax. In the first few Gyr, the metal
abundance rose to [Fe/H] ~ -1.0 dex. The enrichment accelerated in the past ~
1-4 Gyr to reach [Fe/H] ~ -0.4 dex. More than half the sample is constituted of
star younger than ~ 4 Gyr, thus indicating sustained recent star formation in
Fornax. These results indicate that the capacity of dwarf spheroidal galaxies
to retain the heavy elements that they produce is larger than expected.
(Abridged)Comment: 36 pages, 19 figures, to appear in Astronomical Journal, January 200