2009MNRAS.400.1472L

We present the results of a spectroscopic survey of the recently discovered M31 satellites AndXV and AndXVI, lying at projected distances from the centre of M31 of 93 and 130 kpc, respectively. These satellites lie to the south of M31, in regions of the stellar halo which wide-field imaging has revealed as relative voids (compared to the ∼degree-scale coherent stream-like structures). Using the Deep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ∼6km/s down to i ∼ 21.5. While the distance to AndXVI remains the same as previously reported (525±50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch in AndXV are in fact Galactic, and AndXV is actually likely to be much more distant at 770 ±70 kpc (compared to the previous 630 kpc), increasing the luminosity from M_V~ -9.4 to -9.8. The AndXV velocity dispersion is resolved with v_r= -339⁺⁷_–6km/s and σ_v= 11⁺⁷_–5km/s. The AndXVI dispersion is not quite resolved at 1σ with v_r_= -385⁺⁵_–6km/s and σ = 0⁺¹⁰_–indefkm/s. Using the photometry of the confirmed member stars, we find metallicities of AndXV (median [Fe/H] = -1.58, interquartile range ±0.08) and AndXVI (median [Fe/H] = -2.23, interquartile range ±0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H] = -1.8 (-2.1) for AndXV (AndXVI), with an uncertainty of ∼0.2 dex in both cases. Our measurements of AndXV reasonably resolve its mass (∼10⁸M_☉) and suggest a polar orbit, while the velocity of AndXVI suggests it is approaching the M31 escape velocity given its large M31 centric distance.