Crater 2: An Extremely Cold Dark Matter Halo

We present results from MMT/Hectochelle spectroscopy of 390 red giant candidate stars along the line of sight to the recently discovered Galactic satellite Crater 2. Modeling the joint distribution of stellar positions, velocities, and metallicities as a mixture of Crater 2 and Galactic foreground populations, we identify ∼62 members of Crater 2, for which we resolve a line-of-sight velocity dispersion of = km s^-1 and a mean velocity of = km s^-1 (solar rest frame). We also resolve a metallicity dispersion of = dex and a mean of = dex that is 0.28 ±0.14 dex poorer than estimated from photometry. Despite Crater 2's relatively large size (projected halflight radius R _h ∼ 1 kpc) and intermediate luminosity (M _V ∼ -8), its velocity dispersion is the coldest that has been resolved for any dwarf galaxy. These properties make Crater 2 the most extreme low-density outlier in dynamical as well as structural scaling relations among the Milky Way's dwarf spheroidals. Even so, under assumptions of dynamical equilibrium and negligible contamination by unresolved binary stars, the observed velocity distribution implies a gravitationally dominant dark matter halo, with a dynamical mass of M _o and a mass-to-light ratio of enclosed within a radius of ∼1 kpc, where the equivalent circular velocity is km s^-1.