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Session 47 - Old Stellar Populations Beyond The Milky Way - II.
Oral session, Wednesday, June 11
North Main Hall A,
For old stellar populations that are so distant or so densely packed that HST cannot resolve individual stars we turn to integrated starlight for clues about age and metal abundance. Using Balmer lines versus sensitive metallic spectral features one can derive light-weighted-mean ages in a differential sense. Es and S0s tend to have metallicities near solar and show real scatter in mean age. Smaller galaxies show larger age scatter than larger galaxies, and there may be a cluster/field environment effect in which cluster galaxies form the more homogeneous population. Elemental abundance ratios in galaxy nuclei vary from those of solar neighborhood stars. In addition to real galaxy-to-galaxy scatter, larger galaxies tend to have 0.3-dex enhancements of lighter elements (Mg, Na, N) compared to iron-peak elements (Ca, Fa) relative to a scaled solar mixture, in a pattern that compares better to the Galactic bulge than the halo, with the screaming exception of Nitrogen, which is low in the bulge, solar in the disk and halo, high in large galaxy nuclei, and higher still in metal-rich M31 globular clusters. Abundance ratio effects cause major uncertainty in mean age zero point. Abundance ratio measurements provide insight into galaxy formation to an extent limited by nucleosynthesis theory.
