Recently a population of large , very low surface brightness , spheroidal galaxies was identified in the Coma cluster .
The apparent survival of these Ultra Diffuse Galaxies ( UDGs ) in a rich cluster suggests that they have very high masses .
Here we present the stellar kinematics of Dragonfly 44 , one of the largest Coma UDGs , using a 33.5 hr integration with DEIMOS on the Keck II telescope .
We find a velocity dispersion of \sigma = 47 ^ { +8 } _ { -6 } km s ^ { -1 } , which implies a dynamical mass of M _ { dyn } ( < r _ { 1 / 2 } ) = 0.7 ^ { +0.3 } _ { -0.2 } \times 10 ^ { 10 } M _ { \odot } within its deprojected half-light radius of r _ { 1 / 2 } = 4.6 \pm 0.2 kpc .
The mass-to-light ratio is M / L _ { I } ( < r _ { 1 / 2 } ) = 48 ^ { +21 } _ { -14 } M _ { \odot } /L _ { \odot } , and the dark matter fraction is 98 \% within r _ { 1 / 2 } .
The high mass of Dragonfly 44 is accompanied by a large globular cluster population .
From deep Gemini imaging taken in 0 \farcs 4 seeing we infer that Dragonfly 44 has 94 ^ { +25 } _ { -20 } globular clusters , similar to the counts for other galaxies in this mass range .
Our results add to other recent evidence that many UDGs are ‘ ‘ failed ’ ’ galaxies , with the sizes , dark matter content , and globular cluster systems of much more luminous objects .
We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r = 4.6 kpc to enclosed mass profiles of NFW halos .
The enclosed mass suggests a total mass of \sim 10 ^ { 12 } M _ { \odot } , similar to the mass of the Milky Way .
The existence of nearly-dark objects with this mass is unexpected , as galaxy formation is thought to be maximally-efficient in this regime .