1606.06291
A high stellar velocity dispersion and ~100 globular clusters for the ultra diffuse galaxy dragonfly 44
van Dokkum, et al
Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these UDGs (ultra-diffuse galaxies) in a rich cluster suggests that they have very high masses. Present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Kick II telescope. Find a velocity dispersion of 47 km/s, which implies a dynamical mass of M_dyn=0.7e10Msun within its reprojected half-light radius of r_1/2=4.6 kpc. The M/L ratio is 48 Msun/Lsun, and the DM fraction is 98 % within the HLR. The high mass of Dragonfly 44 is accompanies by a large globular cluster population. From deep Gemini imaging taken in 0.4" seeing, infer that Dragonfly 44 has 94 globular clusters, similar to the counts for other galaxies in this mass range. Results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, DM content, and globular cluster systems of much more luminous objects. Estimate the total dark halo mass of Dragonfly 44 by comparing the amount of DM within r=4.6 kpc to enclosed mass profiles of NFW haloes. The enclosed mass suggests a total mass of 1e12 Msun, similar to the mass of the MW. The existence of nearly-dark objects with this mass was unexpected, as galaxy formation was thought to be maximally-efficient in this regime.
No comments:
Post a Comment