Keck Spectroscopy of the Coma Cluster Ultra-Diffuse Galaxy Y358: Dynamical Mass in a Wider Context

TL;DR

This study presents Keck spectroscopy of the Coma cluster UDG Y358, revealing its dark matter core, and compares its dynamical mass with other UDGs and simulations to understand their evolutionary pathways.

Contribution

It provides the first dynamical mass measurement of Y358, compares UDG types with simulations, and explores their evolutionary connections in mass-radius-luminosity space.

Findings

Y358 is confirmed as a Coma cluster UDG with a dark matter core.

GC-poor UDGs resemble isolated, gas-rich UDGs in dynamical mass.

GC-rich UDGs are more massive and do not match gas-rich UDGs or simulations.

Abstract

We examine ultra-diffuse galaxies (UDGs) and their relation to non-UDGs in mass-radius-luminosity space. We begin by publishing Keck/KCWI spectroscopy for the Coma cluster UDG Y358, for which we measure both a recessional velocity and velocity dispersion. Our recessional velocity confirms association with the Coma cluster and Y358's status as a UDG. From our velocity dispersion (19 $\pm$ 3 km s$^{-1}$) we calculate a dynamical mass within the half-light radius which provides evidence for a core in Y358's dark matter halo. We compare this dynamical mass, along with those for globular cluster (GC)-rich/-poor UDGs in the literature, to mass profiles for isolated, gas-rich UDGs and UDGs in the NIHAO/FIRE simulations. We find GC-poor UDGs have dynamical masses similar to isolated, gas-rich UDGs, suggesting an evolutionary pathway may exist between the two. Conversely, GC-rich UDGs have dynamical masses too massive to be easily explained as the evolution of the isolated, gas-rich UDGs. The simulated UDGs match the dynamical masses of the GC-rich UDGs. However, once compared in stellar mass -- halo mass space, the FIRE/NIHAO simulated UDGs do not match the halo masses of either the isolated, gas-rich UDGs or the GC-rich UDGs at the same stellar mass. Finally, we supplement our data for Y358 with other UDGs that have measured velocity dispersions in the literature. We compare this sample to a wide range of non-UDGs in mass-radius-luminosity space, finding UDGs have a similar locus to non-UDGs of similar luminosity with the primary difference being their larger half-light radii.