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Session 76 - Star Formation in Galaxies.
Display session, Friday, January 09
The chemical properties of galaxies are like fossils which record the past star formation activity and the evolution of the interstellar medium. Massive stars evolving on 10^6 year timescales contribute the bulk of the ionizing photons, mechanical energy, and heavy elements that enrich the interstellar, and perhaps the hot, intra-cluster medium. They should be the ``biggest polluters'', yet ground-based and HST ultraviolet spectroscopy of HII regions in a sample of young starburst galaxies show no signs of localized chemical enrichment (i.e. ``self-enrichment'') in the vicinity of 3-12 Myr old starbursts. Furthermore, the C and N abundances in HII regions appear to be correlated, as expected if the chemical enrichment is caused, not by local, temporary enrichments from massive stars, but by global, secular enrichment resulting from the particular star formation history of low and intermediate mass stars in each object. The lack of strong chemical signatures in the ISM surrounding giant HII regions suggests that chemical enrichment proceeds via one of the following scenarios: 1) Different star forming regions throughout the studied galaxies ``conspire'' to keep star formation rates and global abundances uniform at all times, 2) ejecta from stellar winds and supernovae are transported to all corners of the galaxy on timescales of <10^7 yr, and are mixed instantaneously and uniformly, or 3) freshly synthesized elements remain unmixed with the surrounding interstellar medium and reside in a hard-to-observe hot 10^6 K phase detectable only in the X-ray regime. I argue that chemical enrichment from starbursts probably proceeds via this third scenario which can reproduce the appearance of chemical homogeneity on spatial scales of \sim20 pc to 1000 pc and on time scales that are longer than the lifetimes of prominent HII regions (\sim10^7 yrs).
Program listing for Friday