Dynamical Histories of the Crater II and Hercules Dwarf Galaxies

We investigate the possibility that the dwarf galaxies Crater II and Hercules have previously been tidally stripped by the Milky Way. We present Magellan/IMACS spectra of candidate member stars in both objects. We identify 37 members of Crater II, 25 of which have velocity measurements in the literature, and we classify three stars within that subset as possible binaries. We find that including or removing these binary candidates does not change the derived velocity dispersion of Crater II. Excluding the binary candidates, we measure a velocity dispersion of sigma V-los = 2.7(-0.4)(+0.5) km s(-1), corresponding to M/L = 47(-13)(+17) M-circle dot/L-circle dot. We measure a mean metallicity of [Fe/H] = -1.95(-0.05)(+0.06),with a dispersion of sigma([Fe/H]) = 0.18(-0.08)(+0.06). Our velocity dispersion and metallicity measurements agree with previous measurements for Crater II, and confirm that the galaxy resides in a kinematically cold dark-matter halo. We also search for spectroscopic members stripped from Hercules in the possible extratidal stellar overdensities surrounding the dwarf. For both galaxies, we calculate proper motions using Gaia DR2 astrometry, and use their full 6D phase space information to evaluate the probability that their orbits approach sufficiently close to the Milky Way to experience tidal stripping. Given the available kinematic data, we find a probability of similar to 40% that Hercules has suffered tidal stripping. The proper motion of Crater II makes it almost certain to be stripped.