We present the results of a spectroscopic survey of the recently discovered M31 satel- lites And XVÂ and And XVI , lying at projected distances from the centre of M31 of 93 and 130Â 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 \sim 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 \sim 6 ~ { } { km s ^ { -1 } } Â down to i \sim 21.5 .
While the distance to And XVIÂ remains the same as previously reported ( 525 \pm 50 kpc ) , we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch ( TRGB ) in And XVÂ are in fact Galactic , and And XVÂ is actually likely to be much more distant at 770 \pm 70 kpc ( compared to the previous 630Â kpc ) , increasing the luminosity from M _ { V } \approx - 9.4 to M _ { V } \approx - 9.8 .
The And XVÂ velocity dispersion is resolved with v _ { r } =-339 ^ { +7 } _ { -6 } { km s ^ { -1 } } and \sigma _ { v } = 11 ^ { +7 } _ { -5 } { km s ^ { -1 } } .
The And XVIÂ dispersion is not quite resolved at 1 \sigma with v _ { r } =-385 ^ { +5 } _ { -6 } { km s ^ { -1 } } and \sigma = 0 ^ { +10 } _ { - indef } { km s ^ { -1 } } .
Using the photometry of the confirmed member stars , we find metallicities of And XV ( median [ Fe/H ] = -1.58 , interquartile range \pm 0.08 ) , and And XVI ( median [ Fe/H ] = -2.23 , interquartile range \pm 0.12 ) .
Stacking the spectra of the member stars , we find spectroscopic [ Fe/H ] = -1.8 ( -2.1 ) for And XVÂ ( And XVI ) , with a uncertainty of \sim 0.2 Â dex in both cases .
Our measurements of And XVÂ reasonably resolve its mass ( \sim 10 ^ { 8 } M _ { \odot } ) and suggest a polar orbit , while the velocity of And XVIÂ suggests it is approaching the M31 escape velocity given its large M31-centric distance .