A Closer Look at an Unusual Ultra-Diffuse Galaxy

Abstract

We present a spectroscopic study of the ``Disco Ball'' (SMDG0038365-064207), a rotationally-supported, red-sequence, ultra-diffuse galaxy (UDG) with a nuclear star cluster (NSC), multiple stellar clusters, and active star-forming regions using data obtained with KCWI on the Keck II Telescope. We calculate that the galaxy hosts $34\pm11$ ``globular" clusters. Kinematic measurements confirm rotation with a peak rotational velocity of at least 53km s$^{-1}$ and a dynamical mass within $r_{\rm e}$ of at least $10^{9.3 \pm 0.2}M_{\odot}$. Our dynamical estimates of the halo mass are consistent with that obtained using the number of globular clusters and together suggest $M_{\rm h}=10^{11.1\pm0.2}M_{\odot}$. The NSC may exhibit signatures of weak AGN activity. Our findings challenge two common assumptions: (1) clusters in some UDGs may be younger than generally assumed, and thus more luminous than standard globular clusters (GCs), affecting GC counts and the derived GC luminosity function in these UDGs, and (2) quiescent UDGs can be rotationally supported, making kinematic measurements viewing-angle dependent in such cases. The Disco Ball, while unremarkable in mass, size, projected structural properties, or color, reveals surprising complexity, highlighting the need for detailed studies of more UDGs.