A Closer Look at an Unusual Ultradiffuse Galaxy^*

Abstract We present a spectroscopic study of the “Disco Ball” (SMDG0038365-064207), a rotationally supported, green-valley, ultradiffuse galaxy (UDG) with a nuclear star cluster, multiple stellar clusters, and active star-forming regions using data obtained with the Keck Cosmic Web Imager on the Keck II Telescope. We calculate that the galaxy hosts 31 ± 11 “globular” clusters (GCs). Kinematic measurements confirm rotation with a peak rotational velocity of at least 39 ± 9 km s −1 and a dynamical mass within the effective radius of at least 10 9.1 ± 0.2 M ⊙ . Our dynamical estimates of the halo mass are consistent with that obtained using the number of GCs and together suggest M h = 10 10.9 ± 0.2 M ⊙ . Our findings challenge two common assumptions: (1) clusters in some UDGs may be younger than generally assumed, and thus more luminous than standard 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.