The Stellar Populations of Two Quiescent Low Surface Brightness Dwarf Galaxies in Low Density Environments

Abstract: Dwarf galaxies located in low density environments that have quiescent stellar populations are extremely rare and the mechanism for quenching their stars is highly uncertain. Here we present two low surface brightness dwarf galaxies with large (>1.2 kpc) effective radii that were previously identified in the Dragonfly Ultrawide Survey for which we have obtained spectra using the KCWI instrument on the Keck II telescope. We derive their stellar populations and star formation histories in several radial bins out to 1.25 effective radii. Globally both galaxies are old (~9 Gyr) and metal-poor ([M/H] ~ -1) and reveal quenching on a relatively rapid timescale (~1 Gyr). We find flat to rising age and metallicity gradients in both galaxies. This is similar to those seen in ultra diffuse galaxies but is in contrast with typical classical dwarf galaxies and predictions from simulations. One galaxy, DFUWS 68, hosts several globular clusters which we find to share similar ages, metallicities and quenching timescales to that of the host galaxy stars.We briefly discuss possible quenching mechanisms and suggest that neither cosmic web stripping nor internal feedback processes alone can explain our results.