A Rich Globular Cluster System in Dragonfly 17: Are Ultra-Diffuse Galaxies Pure Stellar Halos?*

Abstract

Observations of nearby galaxy clusters at low surface brightness have identified galaxies with low luminosities, but sizes as large as L å galaxies, leading them to be dubbed "ultra-diffuse galaxies" (UDGs). The survival of UDGs in dense environments like the Coma cluster suggests that UDGs could reside in much more massive dark halos. We report the detection of a substantial population of globular clusters (GCs) around a Coma UDG, Dragonfly 17 (DF17). We find that DF17 has a high GC specific frequency of S N = 26 ± 13. The GC system is extended, with an effective radius of 12″ ± 2″, or 5.6 ± 0.9 kpc at Coma distance, 70% larger than the galaxy itself. We also estimate the mean of the GC luminosity function to infer a distance of -+ 97 14 17 Mpc, providing redshift-independent confirmation that one of these UDGs is in the Coma cluster. The presence of a rich GC system in DF17 indicates that, despite its low stellar density, star formation was intense enough to form many massive star clusters. If DF17ʼs ratio of total GC mass to total halo mass is similar to those in other galaxies, then DF17 has an inferred total mass of ∼1011 M e , only ∼10% the mass of the Milky Way, but extremely dominated by dark matter, with M/L V ≈ 1000. We suggest that UDGs like DF17 may be "pure stellar halos," i.e., galaxies that formed their stellar halo components, but then suffered an early cessation in star formation that prevented the formation of any substantial central disk or bulge.

How large are the globular cluster systems of early-type galaxies and do they scale with galaxy halo properties?

“…HR17 used weak lensing results to connect their measured stellar masses to halo masses (M 200 ) and virial radii (R 200 (Peng et al 2016), and assign approximate virial radii of 130 and 175 kpc respectively. The four most massive galaxies in our sample lack halo masses and virial radii.…”

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

“…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). These works find that UDGs are likely to host primarily blue GC populations more similar to dwarfs than giants, although with only one UDG GC system studied so far, few general conclusions can be drawn.…”

A Universe of ultradiffuse galaxies: theoretical predictions from ΛCDM simulations

“…Using the abundance and kinematics of globular clusters around two UDGs, VCC 1287 and DF17, and Beasley & Trujillo (2016) estimated the corresponding dynamical masses to be around m vir ∼ (8 ± 4) × 10 10 M and m vir ∼ (9 ± 2) × 10 10 M , respectively, similar to that of the typical dwarf galaxies. Using the relation between the mass of the globular cluster system and the halo mass (Harris, Harris & Alessi 2013;Harris, Harris & Hudson 2015), Peng & Sungsoon (2016) also inferred the total mass of DF17 to be (9.3 ± 4.7) × 10 10 M ; Amorisco, Monachesi & White (2016) estimated the dynamical masses of 54 Coma UDGs to be lower than 1.3 × 10 11 M . Román & Trujillo (2017a) also found that the distribution of UDGs around A168 is similar to the normal dwarfs, but significantly different from the distribution of massive galaxies with masses similar to the Milky Way.…”