The globular clusters and star formation history of the isolated, quiescent ultra-diffuse galaxy DGSAT I

We investigate the isolated, quiescent ultra-diffuse galaxy (UDG) DGSAT I and its globular cluster (GC) system using two orbits of Hubble Space Telescope Advanced Camera for Surveys imaging in the F606W and F814W filters. This is the first study of GCs around a UDG in a low-density environment. DGSAT I was previously found to host an irregular blue low surface brightness clump, which we confirm as very likely belonging to the galaxy rather than being a chance projection, and represents a recent episode of star formation (∼500 Myr) that challenges some UDG formation scenarios. We select GC candidates based on colours and magnitudes, and construct a self-consistent model of the GC radial surface density profile along with the background. We find a half-number radius of RGC = 2.7 ± 0.1 kpc (more compact than the diffuse starlight) and a total of 12 ± 2 GCs. The total mass fraction in GCs is relatively high, supporting an overmassive dark matter halo as also implied by the high velocity dispersion previously measured. The GCs extend to higher luminosities than expected, and have colours that are unusually similar to their host galaxy colour, with a very narrow spread, all of which suggest an early, intense burst of cluster formation. The nature and origin of this galaxy remain puzzling, but the most likely scenario is a ‘failed galaxy’ that formed relatively few stars for its halo mass, and could be related to cluster UDGs whose size and quiescence pre-date their infall. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
SRJ acknowledges funding support from the Australian Research Council through Discovery Project grant DP200102574 during the course of this work. AJR was supported as a Research Corporation for Science Advancement Cottrell Scholar. DMD acknowledges financial support from the Talentia Senior Program (through the incentive ASE-136) from Secretaría General de Universidades, Investigación y Tecnología, de la Junta de Andalucía. DMD acknowledges funding from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and project (PDI2020-114581GB-C21/ AEI / 10.13039/501100011033). Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Support for Program number HST-GO-14846 was provided through a grant from the STScI under NASA contract NAS5-26555. This research has used the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research used ASTROPY,11 a community-developed core Python package for Astronomy (Astropy Collaboration 2013, 2018). This research used PHOTUTILS, an Astropy package for detection and photometry of astronomical sources (Bradley et al. 2021). This research has used the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has used NASA’s Astrophysics Data System Bibliographic Services.