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