1995A&A...294..432R

Stellar and nebular abundance indicators reveal that there exists significant abundance fluctuations in the interstellar medium (ISM) of gas-rich galaxies. It is shown that at the present observed solar level of O/H∼6x10^–4, abundance differences of a factor of two, such as existing between the Sun and the nearby Orion Nebula, are many times larger than expected. We examine a variety of hydrodynamical processes operating at scales ranging from 1pc to greater than 10kpc, and show that the ISM should appear better homogenized chemically than it actually is: (i) on large galactic scales (1≥l≥10kpc), turbulent diffusion of interstellar clouds in the shear flow of galactic differential rotation is able to wipe out azimuthal O/H fluctuations in less than 10⁹yr; (ii) at the intermediate scale (100≥l≥1000pc), cloud collisions and expanding supershells driven by evolving associations of massive stars, differential rotation and triggered star formation will re-distribute and mix gas efficiently in about 10⁸yr; (iii) at small scales (1≥l≥100pc), turbulent diffusion may be the dominant mechanism in cold clouds, while Rayleigh-Taylor and Kelvin-Helmhotz instabilities quickly develop in regions of gas ionized by massive stars, leading to full mixing in ≤2x10⁶yr. It is suggested that the relatively large O/H fluctuations observed in large disk galaxies may be due to retention, in sites favored by triggered star formation, of freshly enriched ejecta from SNR and supershells expanding in a differentially rotating disk, plus, possibly, infall of low metallicity material from individual clouds like high velocity clouds which splash on the disk on timescales shorter than the local mixing time. In low-mass galaxies, stimulated star formation is much less efficient, and the most effective mixing mechanisms are absent; the escape of newly enriched material due to galactic winds powered by the starburst events, the lack of large-scale stirring, and the long dormant phase between successive star forming episodes make possible the survival of large abundance discontinuities.