Early type galaxies: Mapping out the two-dimensional space of galaxy star formation histories

Early type galaxies form a multi-parameter family, as evidenced by the two-dimensional (2-D) Fundamental Plane relationship. However, their star formation histories are often treated as a one-dimensional mass sequence. This dissertation presents a systematic study of the relationship between the multi-parameter structural properties of early type galaxies and their star formation histoires. We demonstrate that the stellar populations of early type galaxies span a 2-D space, which means that their star formation histories form a two-parameter family. This 2-D family is then mapped onto several familiar early type galaxy scaling relations, including the color-magnitude relation, the Fundamental Plane, and a cross-section through the Fundamental Plane.;We find that the stellar population properties, and therefore the star formation histories of early type galaxies depend most strongly on galaxy velocity dispersion (sigma), rather than on luminosity (L), stellar mass (M*), or dynamical mass ( Mdyn). Interestingly, stellar populations are independent of the radius (Re) of the galaxies. At fixed sigma, they show correlated residuals through the thickness of the Fundamental Plane (FP) in the surface-brightness (Ie) dimension, such that low-surface-brightness galaxies are older, less metal-enriched, and more enhanced in Mg relative to Fe than their counterparts at the same sigma and Re on the FP midplane. Similarly, high-surface-brightness galaxies are younger, more metal-rich, and less Mg-enhanced than their counterparts on the FP midplane. These differences suggest that the duration of star formation varies through the thickness of the FP.;If the dynamical mass-to-light ratios of early type galaxies ( Mdyn/L) were constant for all such galaxies, the FP would be equivalent to the plane predicted by the virial relation. However, the observed FP does not exactly match the virial plane. The FP is tilted from the virial plane, indicating that Mdyn/L varies systematically across it. Furthermore the FP relation, although relatively tight, shows more scatter in surface brightness (at fixed sigma and Re) than is predicted by observational errors. This finite thickness indicates that Mdyn/L also varies at a fixed point on the FP.;We observe that the stellar populations of early type galaxies vary through the thickness of the FP. These differences translate into variations in the stellar mass-to-light ratio (M*/L) that contribute to both the tilt and the thickness of the FP. However, the mass-to-light variations due to stellar population differences are too small to explain either the tilt of the FP or its thickness. This implies that the tilt and thickness of the FP are driven by systematic variations in either the central dark matter fraction in galaxies or in the IMF with which they form stars. Furthermore, because star formation histories can be mapped onto locations in FP-space, the variations in central dark matter fraction or IMF differences must be correlated with differences in the galaxies' star formation histories.