Abundance Profiles in Low-Mass Galaxies

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The nitrogen and oxygen abundances in the warm ionized gas of low-mass, metal-poor galaxies appear surprisingly homogeneous considering the prevalence of large HII regions, which contain hundreds of massive stars. Of the six galaxies with extensive optical spectroscopy, only the largest and most metal-rich, the LMC, shows evidence for a chemical gradient akin to those commonly seen in spirals. Furthermore, no significant localized chemical fluctuations are found in the vicinity of young star clusters, despite large expected chemical yields of massive stars. An ad-hoc fine-tuning of the release, dispersal and mixing rates could give rise to the observed homogeneity, but a more probable explanation is that fresh ejecta reside in a hard-to-observe hot or cold phase. In any case, the observations indicate that heavy elements which have already mixed with the warm interstellar medium are homogeneously dispersed. Mixing of fresh ejecta with the surrounding warm ISM apparently requires longer than the lifetimes of typical HII regions (>10⁷ yrs). The lack of observed localized chemical enrichments is consistent with a scenario whereby freshly-synthesized metals are expelled into the halos of galaxies in a hot, 10⁶ K phase by supernova-driven winds before they cool and ``rain'' back down upon the galaxy, creating gradual enrichments on spatial scales >1 kpc.