4.2. Spatial Abundance Profiles

The spatial distribution of abundances in galaxies depends coarsely on the Hubble type. Spectroscopic study of H II regions in unbarred or weakly barred spiral galaxies typically reveals a strong radial gradient in metallicity (Figure 8), as determined from O/H (Zaritsky, Kennicutt, & Huchra (1994; ZKH); Vila-Costas & Edmunds 1992; VCE). The derived O/H can drop by a factor of ten to thirty or fifty from the nucleus of a galaxy to the outer disk as demonstrated in galaxies with well-sampled data. In those spirals with spectroscopy of more than ten H II regions covering the full radial extent of the disk, there is little evidence that O/H gradients deviate from exponential profiles. Irregular galaxies, by contrast, show little spatial variation in abundances, to high levels of precision (Kobulnicky & Skillman 1996), indicating a well-mixed ISM. The data for strongly barred spiral galaxies shows evidence that their O/H gradients are more shallow than in unbarred spirals. I will discuss these galaxies in more detailed in a later section.

Figure 8. The gradient in O/H across the disk of the spiral galaxy M101 vs. galactocentric radius (Kennicutt & Garnett 1996).