paper

A headless tadpole galaxy: the high gas-phase metallicity of the ultra-diffuse galaxy UGC 2162

arXiv:1810.08453 · doi:10.3847/1538-4357/aae9f3

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) 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.

arXiv commentAccepted for publication in ApJ. 10 pages. 5 figures

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