A Keck/DEIMOS spectroscopic survey of the faint M31 satellites AndIX, AndXI, AndXII and AndXIII†

M. L. M. Collins, S. C. Chapman, M. J. Irwin, N. F. Martin, R. A. Ibata, D. B. Zucker, A. Blain, A. M. N. Ferguson, G. F. Lewis, A. W. McConnachie, J. Peñarrubia, Annette Ferguson

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We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, as well as a re-analysis of existing spectroscopic data for two further faint companions, AndIX (correcting for an error in earlier geometric modelling that caused a misclassification of member stars in previous work) and AndXII. By combining data obtained using the Deep Imaging Multi-Object Spectrograph (DEIMOS) mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have identified the most probable members for each of the satellites based on their radial velocities (precise to several down to i ~ 22), distance from the centre of the dwarf spheroidal galaxies (dSphs) and their photometric [Fe/H]. Using both the photometric and spectroscopic data, we have also calculated global properties for the dwarfs, such as systemic velocities, metallicities and half-light radii. We find each dwarf to be very metal poor ([Fe/H] ~ -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve dispersion for AndXI due to small sample size and low signal-to-noise ratio, but we set a 1σ upper limit of σv <4.5kms-1. For AndIX, AndXII and AndXIII we resolve velocity dispersions of σv = 4.5+3.6-3.4, 2.6+5.1-2.6 and 9.7+8.9-4.5kms-1, though we note that the dispersion for AndXIII is based on just three stars. We derive masses within the half-light radii for these galaxies of 6.2+5.3-5.1 × 106, 2.4+6.5-2.4 × 106 and 1.1+1.4-0.7 × 107Msolar, respectively. We discuss each satellite in the context of the Mateo relations for dSphs, and in reference to the universal halo profiles established for Milky Way dwarfs. Both AndIX and AndXII fall below the universal halo profiles of Walker et al., indicating that they are less massive than would be expected for objects of their half-light radius. When combined with the findings of McConnachie & Irwin, which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit haloes that with regard to their central densities are significantly different from those of the Milky Way. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. ‡ E-mail: mlmc2@ast.cam.ac.uk
Original languageEnglish
Pages (from-to)2411-2433
JournalMonthly Notices of the Royal Astronomical Society
Publication statusPublished - 1 Oct 2010


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