The Distribution of Heavy Elements in Spiral and Elliptical Galaxies

Abstract

In large disk and spheroidal galaxies, spatially resolved abundance information can be extracted by analysis of either emission lines, absorption lines, or both, depending on the situation. This review recaps significant results as they apply to nondwarf galaxies, including the Milky Way, spiral disks and bulges, and elliptical and lenticular galaxies. Methods for determining abundances are explained in appendices. Conclusions that span the galaxy types treated here are as follows. All galaxies, on average, have heavy‐element abundances (metallicities) that systematically decrease outward from their galactic centers while their global metallicities increase with galaxy mass. Abundance gradients are steepest in normal spirals and are seen to be progressively flatter going in order from barred spirals to lenticulars to ellipticals. The distribution of abundances N(Z) versus Z is strongly peaked compared with simple closed‐box model predictions of chemical enrichment in all galaxy types. That is, a "G dwarf problem," commonly known in the solar cylinder, exists for all large galaxies. For spiral galaxies, local metallicity appears to be correlated with total (disk+bulge) surface density. Examination of N/O versus O/H in spiral disks indicates that production of N is dominated by primary processes at low metallicity and secondary processes at high metallicity. Carbon production increases with increasing metallicity. Abundance ratios Ne/O, S/O, and Ar/O appear to be universally constant and independent of metallicity, which argues either that the initial mass function (IMF) is universally constant or that these ratios are not sensitive to IMF variations. In the Milky Way, there is a rough age‐metallicity trend with much scatter, in the sense that older stars are more metal poor. In elliptical galaxies, nuclear abundances are in the range [Z/H] = 0.0–0.4, but the element mixture is not scaled‐solar. In large elliptical galaxies [Mg/Fe] is in the range 0.3–0.5, decreasing to ≈0 in smaller elliptical galaxies. Other light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity dispersion appears to be a key parameter in the modulation of [Mg/Fe], but the cause of the connection is unclear.