Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster

TL;DR

This paper reports the discovery of 47 ultra-diffuse galaxies in the Coma cluster, revealing their large sizes, low stellar masses, and potential dark matter dominance, challenging previous galaxy formation models.

Contribution

The study presents the first large sample of ultra-diffuse galaxies in the Coma cluster, with detailed measurements and implications for galaxy evolution and dark matter content.

Findings

Most objects are confirmed as Coma cluster galaxies.

Galaxies have sizes comparable to L* galaxies despite low stellar mass.

One galaxy observed with Hubble shows a spheroidal shape with low surface brightness.

Abstract

We report the discovery of 47 low surface brightness objects in deep images of a 3 x 3 degree field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness mu(g,0) ranging from 24 - 26 mag/arcsec^2 and effective radii r_e = 3"-10", as measured from archival Canada France Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r_e = 1.5 - 4.6 kpc their sizes are similar to those of L* galaxies even though their median stellar mass is only ~6 x 10^7 Solar masses. The galaxies are relatively red and round, with <g-i> = 0.8 and <b/a> = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope ACS observation. The ACS imaging shows a large spheroidal object with a central surface brightness mu(g,0) = 25.8 mag/arcsec^2, a Sersic index n=0.6, and an effective radius of 7", corresponding to 3.4 kpc at the distance of Coma. The galaxy is not resolved into stars, consistent with expectations for a Coma cluster object. We speculate that UDGs may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.