Title: Stealth Galaxies and Mass Bias the  Milky Way Dwarf Spheroidal Population

Abstract:
Ultrafaint dwarf spheroidal galaxies of the Milky Way appear  to be extremely dark matter dominated and 
are consistent with inhabiting dark matter subhalos that are just as massive as those of dwarfs 10,000 
times more luminous.  I show that this apparent common mass for MW dwarfs may be an artifact of 
selection bias.  Halos with circular velocities smaller than about 15 km/s and masses within 300 pc less 
than ~10^7 Msun are likely to host stealth galaxies, with surface brightnesses too low to have been 
detected in current surveys.  This bias may be an important limitation in the hunt for low-mass H-2 cooling 
fossils from the epoch of reionization in the halo.  Finally,  I will briefly discuss the degree to which the 
presence of binary stars and external tidal forces could be affecting mass estimates for ultrafaint dwarfs.  
For Segue 1, a simultaneous Bayesian analysis of both membership probabilities and the contribution of 
binary orbital motion to a the observed velocity dispersion shows that Segue -1 is a dark-matter dominated 
dwarf to high significance.  Comprehensive multi-epoch data sets are required to confidently constrain the 
binary contribution in other faint dwarfs.