Elemental Abundance Variations and Chemical Enrichment from Massive Stars in Starbursts. II. NGC 1569

We present a long-slit optical spectrophotometric survey covering 0.05 kpc² in the nearby irregular "post-starburst" galaxy NGC 1569 to search for chemical gradients and inhomogeneities in the interstellar medium. Despite the presence of two massive evolved star clusters and numerous H II regions, we find no evidence for chemical gradients or inhomogeneities that may be attributed to enrichment from the recent star formation activity. The chemical properties at all locations are consistent with the results from the highest signal-to-noise ratio spectra: 12 + log (O/H) = 8.19 ± 0.02, log (N/O) = -1.39 ± 0.05, He/H = 0.080 ± 0.003. No localized chemical self-enrichment ("pollution") from massive star evolution is found, even though the data are sensitive to the chemical yields from as few as two or three massive stars.

Flat chemical abundance profiles appear to be the rule rather than the exception in low-mass galaxies, even though the expected yield of heavy elements produced by massive stars in young starbursts is substantial. Based on a typical IMF, a dynamical mass of ~3 × 10⁵ M_☉, and an age of 20 Myr, roughly 450 stars in excess of 20 M_☉ should have already exploded as supernovae within the star cluster, A, in NGC 1569, releasing ~1000 M_☉ of oxygen and ~8 M_☉ of nitrogen. Strong chemical signatures in the surrounding interstellar material should be detected unless one or more of the following are true: (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 <10⁷ 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 10⁶ K phase or a cold, dusty, molecular phase. We advance the third scenario as the most plausible, and we suggest ways to locate the chemical products of massive star formation in starburst galaxies. Any successful model for chemical enrichment in these systems must be able to reproduce the appearance of chemical homogeneity on spatial scales of ~20-1000 pc and on temporal scales that are longer than the lifetimes of prominent H II regions (~10⁷ yr). Such long timescales imply that the instantaneous recycling approximation sometimes used in galactic chemical evolution modeling is not generally applicable.