We present the first spectroscopic analysis of the faint M31 satellite galaxies , And XI and And XIII , as well as a reanalysis of existing spectroscopic data for two further faint companions , And IX ( correcting for an error in earlier geometric modelling ) 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 \sim 22 ) .
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 ] \sim - 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 S : N , but we set a one sigma upper limit of \sigma _ { v } < 4.5 kms ^ { -1 } .
For And IX , And XII and And XIII we resolve velocity dispersions of \sigma _ { v } =4.5 ^ { +3.6 } _ { -3.4 } , 2.6 ^ { +5.1 } _ { -2.6 } and 9.7 ^ { +8.9 } _ { -4.5 } kms ^ { -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 } \times 10 ^ { 6 } { M _ { \odot } } , 2.4 ^ { +6.5 } _ { -2.4 } \times 10 ^ { 6 } { M _ { \odot } } and 1.1 ^ { +1.4 } _ { -0.7 } \times 10 ^ { 7 } { M _ { \odot } } respectively .
We discuss each satellite in the context of the Mateo relations for dwarf spheroidal galaxies , and in reference to the Universal halo profiles established for Milky Way dwarfs ( ) .
Both And IX and XII fall below the universal halo profiles of , indicating that they are less massive than would be expected for objects of their half-light radius .
When combined with the findings of , 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 halos that with regard to their central densities – are significantly different from those of the Milky Way .