A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII and And XIII

Abstract

We present the first spectroscopic analysis of the faint M31 satellite galaxies, And XI and And XIII, as well as a re-analysis of existing spectroscopic data for two further faint companions, And IX (correcting for an error in earlier geometric modelling that caused a misclassification of member stars in previous work) and And XII. 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 km s^(-1) 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 And XI due to small sample size and low signal-to-noise ratio, but we set a 1σ upper limit of σ_v < 4.5 km s^(−1). For And IX, And XII and And XIII we resolve velocity dispersions of σ_v= 4.5^(+3.6)_(−3.4), 2.6^(+5.1)_(−2.6) and 9.7^(+8.9)_(−4.5) km s^(−1), though we note that the dispersion for And XIII is based on just three stars. We derive masses within the half-light radii for these galaxies of 6.2^(+5.3)_(−5.1)× 10^6, 2.4^(+6.5)_(−2.4)× 10^6 and 1.1^(+1.4)_(−0.7)× 10_7 M_⊙, 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 And IX and And XII 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.

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