THE CHEMICAL AND DYNAMICAL EVOLUTION OF THE GALAXY

Abstract

ABSTRACT Theoretical models of the chemical and dynamical evolution of galaxies are developed and a Galactic structure survey is presented -- both are invaluable tools for uncovering the history of the formation of our Galaxy. Important advances in my theoretical models are non-instantaneous recycling of newly synthesized elements such as iron from Type Ia supernovae (mergers of white dwarf binaries). For the first time, Type Ia supernovae are incorporated in models of stellar remnants as baryonic dark matter. If dark halos consist mainly of white dwarfs, as has been proposed, then an excessive Type Ia supernovae rate is predicted unless the binary fraction of precursor stars is virtually zero. The observed constancy of chemical abundance ratios in halo stars e.g., [O/Fe] ~/= +0.5, signifies enrichment from only Type II supernovae. I explore the timescale for the onset of Type Ia supernovae as an upper limit to the timescale of halo formation. The apparent inconsistency between age differences of 2-3 Gyr in halo stars and the timescale inferred from the onset of Type Ia supernovae (~<0.1 Gyr for a range of models) suggests galaxies formed hierarchically from substructures in which star formation was disrupted before the onset of Type Ia supernovae. Chemical and dynamical evolution models of viscous disks with non-instantaneous recycling are also explored. The patterns of chemical element ratios and scatter in the age-metallicity relationship are calculated. Characterized by radial flow of gas, these models are ideal for exploring new ideas on the formation of galactic bulges from inflow of disk gas. The observed high metallicity of bulge stars may not require an ad hoc variation of the stellar initial mass function, if the bulge is an extension of the disk metallicity gradient. In addition, I present data from an unbiased spectroscopic survey of F-G stars in Selected Areas 51, 57 and 68. Radial velocities accurate to ~<10 km/s are derived. Comparison of the velocity distributions with predictions of Galactic structure models suggests Vrot ~/= 170, sigmaV ~/= 50, sigmaU ~/= 80 km/s for the thick disk. Preliminary distances are derived by photometric parallax, and 3-dimensional velocities calculated using published proper motions. Stellar metallicities can be derived by analysis of spectral line indices composed of weak iron features. Calibration against metallicity standard stars is presented. Evaluation of metallicities of program stars are forthcoming. These data are useful for constraining the existence of a metallicity gradient in the Galactic halo, and determining the correlation of kinematics and chemistry in the disk/halo transition.