Deep HST/UVIS Imaging of the Candidate Dark Galaxy CDG-1

CDG-1 is a tight grouping of four likely globular clusters in the Perseus cluster, and a candidate dark galaxy with little or no diffuse light. Here we provide new constraints on the luminosity of any underlying stellar emission, using Hubble Space Telescope/UVIS F200LP imaging. No diffuse emission is detected, with a 2σ upper limit of F200LP > 28.1 mag arcsec^‑2 on the 5″ scale of CDG-1. This surface brightness limit corresponds to a 2σ lower limit of >0.5 for the fraction of the total luminosity that is in the form of globular clusters. The most likely alternative, although improbable, is that CDG-1 is a chance grouping of four globular clusters in the halo of the Perseus galaxy IC 312.