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 .
Modelling the joint distribution of stellar positions , velocities and metallicities as a mixture of Crater 2 and Galactic foreground populations , we identify \sim 62 members of Crater 2 , for which we resolve line-of-sight velocity dispersion \sigma _ { v _ { los } } = 2.7 _ { -0.3 } ^ { +0.3 } km s ^ { -1 } about mean velocity of \langle v _ { \mathrm { los } } \rangle = 87.5 _ { -0.4 } ^ { +0.4 } km s ^ { -1 } ( solar rest frame ) .
We also resolve a metallicity dispersion \sigma _ { \mathrm { [ Fe / H ] } } = 0.22 _ { -0.03 } ^ { +0.04 } dex about a mean of \langle [ Fe/H ] \rangle = -1.98 _ { -0.1 } ^ { +0.1 } dex that is 0.28 \pm 0.14 dex poorer than is estimated from photometry .
Despite Crater 2 ’ s relatively large size ( projected halflight radius R _ { h } \sim 1 kpc ) and intermediate luminosity ( M _ { V } \sim - 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 dynamical mass 4.4 _ { -0.9 } ^ { +1.2 } \times 10 ^ { 6 } M _ { \odot } and mass-to-light ratio 53 _ { -11 } ^ { +15 } M _ { \odot } / L _ { V, \odot } enclosed within a radius of \sim 1 kpc , where the equivalent circular velocity is 4.3 _ { -0.5 } ^ { +0.5 } km s ^ { -1 } .