1,321 research outputs found
Star Clusters in M31: II. Old Cluster Metallicities and Ages from Hectospec Data
We present new high signal-to-noise spectroscopic data on the M31 globular
cluster system, obtained with the Hectospec multifiber spectrograph on the 6.5m
MMT. More than 300 clusters have been observed at a resolution of 5A. The
primary focus of this paper is the determination of mean cluster metallicities,
ages and reddenings. Metallicities were estimated using a calibration of Lick
indices with [Fe/H] provided by Galactic GCs. The metallicity distribution is
not generally bimodal, in strong distinction with the bimodal Galactic globular
distribution. Rather, the M31 distribution shows a broad peak, centered at
[Fe/H]=-1, suggesting that the cluster systems of M31 and the Milky Way had
different formation histories. Ages for clusters with [Fe/H] > -1 were
determined using the automatic stellar population analysis program EZ_Ages. We
find no evidence for massive clusters in M31 with intermediate ages. Moreover,
we find that the mean ages of the old GCs are remarkably constant over about a
decade in metallicity (-0.95 < [Fe/H] < 0.0). (abridged)Comment: 25 pages, 24 figures, 3 table
Mapping the Galactic Halo. V. Sgr dSph Tidal Debris 60 degrees from the Main Body
As part of the Spaghetti Project Survey (SPS) we have detected a
concentration of giant stars well above expectations for a smooth halo model.
The position (l~350, b~50) and distance (~50 kpc) of this concentration match
those of the Northern over-density detected by SDSS (Yanny et al. 2000, Ivezic
et al. 2000). We find additional evidence for structure at ~80 kpc in the same
direction. We present radial velocities for many of these stars, including the
first published results from the 6.5m Magellan telescope. The radial velocities
for stars in these structures are in excellent agreement with models of the
dynamical evolution of the Sgr dwarf tidal debris, whose center is 60 degrees
away. The metallicity of stars in these streams is lower than that of the main
body of the Sgr dwarf, which may indicate a radial metallicity gradient prior
to disruption.Comment: 10 pages, 3 figures accepted in Astrophysical Journal Letter
Diffuse Light in the Virgo Cluster
We present deep optical imaging of the inner 1.5 x 1.5 degrees of the Virgo
cluster to search for diffuse intracluster light (ICL). Our image reaches a 1
sigma depth of mu_v=28.5 mag/arcsec^2 -- 1.5 mag/arcsec^2 deeper than previous
surveys -- and reveals an intricate web of diffuse intracluster light. We see
several long (>100 kpc) tidal streamers, as well as a myriad of smaller-scale
tidal tails and bridges between galaxies. The diffuse halo of M87 is traced out
to nearly 200 kpc, appearing very irregular on these scales, while significant
diffuse light is also detected around the M84/M86 pair. Several galaxies in the
core are embedded in common envelopes, suggesting they are true physical
subgroups. The complex substructure of Virgo's diffuse ICL reflects the
hierarchical nature of cluster assembly, rather than being the product of
smooth accretion around a central galaxy.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood to look
for halo substructure in velocity and angular momentum space. Our sample
includes red giants, RR Lyrae, and red horizontal branch stars within 2.5 kpc
of the Sun with [Fe/H] less than -1.0. It was chosen to include stars with
accurate distances, space velocities, and metallicities as well as
well-quantified errors. We confirm the existence of the streams found by Helmi
and coworkers, which we refer to as the H99 streams. These streams have a
double-peaked velocity distribution in the z direction. We use the results of
modeling of the H99 streams by Helmi and collaborators to test how one might
use v_z velocity information and radial velocity information to detect
kinematic substructure in the halo. We find that detecting the H99 streams with
radial velocities alone would require a large sample. We use the velocity
distribution of the H99 streams to estimate their age. From our model of the
progenitor of the H99 streams, we determine that it was accreted between 6 and
9 Gyr ago. The H99 streams have [alpha/Fe] abundances similar to other halo
stars in the solar neighborhood, suggesting that the gas that formed these
stars were enriched mostly by Type II SNe. We have also discovered in angular
momentum space two other possible substructures, which we refer to as the
retrograde and prograde outliers. The retrograde outliers are likely to be halo
substructure, but the prograde outliers are most likely part of the smooth
halo. The retrograde outliers have significant structure in the v_phi direction
and show a range of [alpha/Fe]. The methods presented in this paper can be used
to exploit the kinematic information present in future large databases like
RAVE, SDSSII/SEGUE, and Gaia.Comment: 46 pages, 13 figures, and 9 tables. Minor changes to text to match
proofed version of the paper. Low resolution figures. High resolution version
at http://www.astro.wisc.edu/~kepley/solar_streams.p
Andromeda VIII - a New Tidally Distorted Satellite of M31
We report the detection of a new satellite of M31, projected close to M32.
And VIII is tidally distorted, with length ~10 kpc and width a few kpc. It
contains 5-12 planetary nebulae (PNe) and 1-3 globular clusters, and has a
velocity of -204 km/s with respect to M31, some 350 km/s away from M32's
velocity. There are also about 4 x 10^5 solar masses of HI, well-separated from
the disk, at the same position and velocity. The satellite has luminosity of
1.2-2.4 x 10^8 solar luminosities, and a central surface brightness of order
\mu_V=24. Both these values are typical of Local Group dwarf galaxies. Its
surface brightness is some 6 magnitudes brighter than any of the stellar
streams found in the Milky Way or M31. The three associated globular clusters
have reddening consistent with foreground reddening from the Milky Way only,
making it likely that the satellite is in front of M31, unlike the giant tidal
stream of Ibata et al.(2001), which is behind M31 in the SE quadrant. However,
the major axis of And VIII is aligned with the western edge of this giant
stream, and we suggest that its unusual fan shape is caused by superposition of
two streams, the westernmost of which was tidally stripped from And VIII.Comment: 11 pages, 2 figures. ApJ Letters, in pres
Planetary Nebulae Kinematics in M31
We present kinematics of 135 planetary nebulae in M31 from a survey covering
3.9 square degrees and extending out to 15 kpc from the southwest major axis
and more than 20 kpc along the minor axis. The majority of our sample, even
well outside the disk, shows significant rotational support (mean line-of-sight
velocity 116 km/s). We argue that these PN belong to the outer part of M31's
large de Vaucouleurs bulge. Only five PN have velocities clearly inconsistent
with this fast rotating bulge. All five may belong to tidal streams in M31's
outer halo. One is projected on the Northern Spur, and is counter-rotating with
respect to the disk there. Two are projected along the major axis at X=-10 kpc
and have M32-like velocities; they could be debris from that galaxy. The
remaining two halo PN are located near the center of the galaxy and their
velocities follow the gradient found by Ibata et al. (2004), implying that
these PN could belong to the Southern Stream. If M31 has a non-rotating,
pressure-supported halo, we have yet to find it, and it must be a very minor
component of the galaxy.Comment: accepted to ApJ; main body of paper is 36 pages, including 14 figure
Star Clusters in M 31. IV. A Comparative Analysis of Absorption Line Indices in Old M 31 and Milky Way Clusters
We present absorption line indices measured in the integrated spectra of
globular clusters both from the Galaxy and from M 31. Our samples include 41
Galactic globular clusters, and more than 300 clusters in M 31. The conversion
of instrumental equivalent widths into the Lick system is described, and
zero-point uncertainties are provided. Comparison of line indices of old M 31
clusters and Galactic globular clusters suggests an absence of important
differences in chemical composition between the two cluster systems. In
particular, CN indices in the spectra of M 31 and Galactic clusters are
essentially consistent with each other, in disagreement with several previous
works. We reanalyze some of the previous data, and conclude that reported CN
differences between M 31 and Galactic clusters were mostly due to data
calibration uncertainties. Our data support the conclusion that the chemical
compositions of Milky Way and M 31 globular clusters are not substantially
different, and that there is no need to resort to enhanced nitrogen abundances
to account for the optical spectra of M 31 globular clusters.Comment: 72 pages, including 15 figures and 14 tables. Published by the
Astronomical Journa
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