A Headless Tadpole Galaxy: The High Gas-phase Metallicity of the Ultra-diffuse Galaxy UGC 2162

Abstract

The cosmological numerical simulations tell us that accretion of external metal-poor gas drives star formation (SF) in galaxy disks. One the best pieces of observational evidence supporting this prediction is the existence of low-metallicity star-forming regions in relatively high-metallicity host galaxies. The SF is thought to be fed by metal-poor gas recently accreted. Since the gas accretion is stochastic, there should be galaxies with all the properties of a host but without the low-metallicity starburst. These galaxies have not been identified yet. The exception may be UGC 2162, a nearby ultra-diffuse galaxy (UDG) that combines low surface brightness and relatively high metallicity. We confirm the high metallicity of UGC 2162 () using spectra taken with the 10 m GTC telescope. UGC 2162 has the stellar mass, metallicity, and star formation rate surface density expected for a host galaxy in between outbursts. This fact suggests a physical connection between some UDGs and metal-poor galaxies, which may be the same type of object in a different phase of the SF cycle. UGC 2162 is a high-metallicity outlier of the mass-metallicity relation, a property shared by the few UDGs with known gas-phase metallicity.