paper

Keck Spectroscopy of Red Giant Stars in the Vicinity of M31's Massive Globular Cluster G1

arXiv:astro-ph/0309295 · doi:10.1086/382517

Abstract

We present results from a Keck spectroscopic survey of red giants in a field near M31's major axis, ~34kpc in projection from the nucleus, near the globular cluster G1. We use multislit LRIS spectroscopy to measure the CaII triplet in 41 stars with apparent mags 20<I<22. Of these, 23 stars are found to have radial velocities v<-200km/s indicating they are giants in M31; the rest are probably foreground Galactic stars. About 2/3 of the M31 members concentrate at v=-451km/s, with a velocity spread of sigma=27km/s, which suggests that they belong to the outer disk or perhaps a cold debris trail in the halo. The group's mean velocity is consistent with that of nearby HI gas and models of M31's disk velocity field but not with G1 or M31's systemic velocity. We use V,I photometry to estimate a mean metallicity of <[Fe/H]>=-0.8 for these potential M31 outer disk stars. Six of the 23 M31 members v and [Fe/H] values consistent with those of G1 (after accounting forits intrinsic spread in both): one of these stars lies within G1's projected tidal radius and is a likely member; the remaining 5 stars are not physically close to G1 but may represent its tidal debris. More data are needed to confirm the nature of these 5 stars as it is likely that they represent M31's smooth halo population. We might have expected to detect tidal debris if G1 were the remnant core of a dwarf galaxy being accreted by M31; instead, the majority of M31 giants in this field are metal-rich and belong to what is evidently the outer disk of M31, and only a small fraction (<~20%) could possibly have originated in G1.

11 pages, 2 figures, accepted for publication in the AJ

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