The Metal-Enriched Outer Disk of NGC 2915

TL;DR

This study investigates the metallicity distribution in the outer disk of NGC 2915, revealing an unexpectedly high metal content likely influenced by mechanisms beyond local star formation, impacting dwarf galaxy evolution models.

Contribution

It provides the first detailed metallicity measurements of the outer disk of NGC 2915 and explores potential processes responsible for metal enrichment beyond star formation.

Findings

Outer disk metallicity reaches 0.4 Z_solar

Metallicity gradient is flat or increasing outwards

Star formation alone cannot explain observed metal levels

Abstract

We present optical emission-line spectra for outlying HII regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be enriched to a metallicity of 0.4 Z_solar. An analysis of the metal yields shows that the outer disk of NGC 2915 is overabundant for its gas fraction, while the central star-foming core is similarly under-abundant for its gas fraction. Star formation rates derived from very deep ~14 ks GALEX FUV exposures indicate that the low-level of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider 3 plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC 2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.