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Gas and stellar metallicities in HII galaxies

We examine the gas and stellar metallicities in a sample of HII galaxies from the Sloan Digital Sky Survey, which possibly contains the largest homogeneous sample of HII galaxy spectra to date. We eliminated all spectra with an insufficient signal-to-noise ratio, without strong emission lines, and without the [OII] lambda3727 Å line, which is necessary for the determination of the gas metallicity. This excludes galaxies with redshift <~ 0.033. Our final sample contains ~700 spectra of HII galaxies. Through emission line strength calibrations and a detailed stellar population analysis employing evolutionary stellar synthesis methods, which we already used in previous works, we determined the metallicities of both the gas and the stellar content of these galaxies. We find that in HII galaxies up to stellar masses of 5\cdot10^9 M_sol, enrichment mechanisms do not vary with galactic mass, being the same for low- and high-mass galaxies on average. They do seem to present a greater variety at the high-mass end, though, indicating a more complex assembly history for high-mass galaxies. In around 23 per cent of our HII galaxies we find a metallicity decrease over the last few Gyr. Our results favour galaxy evolution models featuring constantly infalling low-metallicity clouds that retain part of the galactic winds. Above 5\cdot10^9 M_sol stellar mass, the retention of high metallicity gas by the galaxies' gravitational potential dominates.