A headless tadpole galaxy: the high gas-phase metallicity of the
  ultra-diffuse galaxy UGC 2162
  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) which combines low surface brightness and relatively high metallicity. We
confirm the high metallicity of UGC 2162 (12 + log(O/H) = 8.52+0.27-0.24 )
using spectra taken with the 10-m GTC telescope. GC2162 has the stellar mass,
metallicity, and star-formation rate (SFR) 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.
