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

Abstract

We report the discovery of 47 low surface brightness objects in deep images of a 3 • × 3 • field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness µ(g, 0) ranging from 24 -26 mag arcsec −2 and effective radii r eff = 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 eff = 1.5 kpc -4.6 kpc their sizes are similar to those of L * galaxies even though their median stellar mass is only ∼ 6 × 10 7 M ⊙ . 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 µ 475 = 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 these "ultra-diffuse galaxies" (UDGs) may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.

A Universe of ultradiffuse galaxies: theoretical predictions from ΛCDM simulations

“…in Coma (van Dokkum et al 2015a), A168 (Román & Trujillo 2017a), A2744 (Janssens et al 2017), and other eight low-redshift clusters (van der Burg et al 2016, hereafter vdB16). Here, we focus on the comparison of the UDGs in clusters between the model predictions and observations.…”

“…044-0.063 (van der Burg, Muzzin & Hoekstra 2016) and PiscesPerseus supercluster (Martinez-Delgado et al 2016), as well as several galaxy groups (Makarov et al 2015;Toloba et al 2016;Román & Trujillo 2017b;Trujillo et al 2017). While their stellar masses are similar to typical dwarf galaxies, their effective radii are similar to the L * galaxies (van Dokkum et al 2015a;). These galaxies are generally referred to as ultradiffuse galaxies (UDGs).…”

The Survey of Centaurus A's Baryonic Structures (SCABS) – II. The extended globular cluster system of NGC 5128 and its nearby environment

“…In this case, these clumps could represent the last coherent structures resulting from these violent events. On the other hand, the recent discovery of "ultra-diffuse" galaxies (UDGs) in large galaxy clusters (van Dokkum et al 2015;Koda et al 2015;Mihos et al 2015;Muñoz et al 2015;Martínez-Delgado et al 2016) have been proposed to represent failed L * galaxies with deep potentials relative to their stellar masses. Little is known about their respective GC systems, but recent work has indicated that they may have high SN 30 with as many as dozens of GCs (Beasley & Trujillo 2016;Peng & Lim 2016;van Dokkum et al 2016).…”

The IAC Stripe 82 Legacy Project: a wide-area survey for faint surface brightness astronomy

“…Recently, a population of extremely diffuse galaxies with central surface brightness 24 µ(g, 0) 26 mag arcsec −2 has been identified in the Coma cluster (Koda et al 2015;van Dokkum et al 2015). One third of these roughly 1000 UDGs are Milky Way sized with large effective radii of R E > 1.5 kpc, however small stellar masses of only few 10 7 − 10 8 M ⊙ .…”