Metallicities and stellar populations in spiral bulges

Abstract

A major aim of current astrophysical research is the development of models of galaxy formation that are consistent with observational data. Until recently, the main tools of such research were broadband colours. However, the Lick system of indices has recently provided researchers with a more accurate tool for probing galaxy populations. The work in this thesis compares Lick indices in bright elliptical galaxies with those in the bulges of spiral galaxies, in an effort to compare, contrast and constrain their star formation histories.
Long-slit spectroscopic observations of line-strengths and kinematics were carried out along the minor axes of 32 galaxies (ellipticals, SOs and spiral bulges). This study shows bulges to have central line-strengths comparable with those of single stellar populations of approximately solar metallicity or above. In bulges, central line-strengths are shown to lie close to, or within, the loci defining solar abundance ratio SSPs. The large number (24) of indices measured in this study, in conjunction with models of the effects of varying abundance ratios, permit the breaking of age/metallicity degeneracy and allow estimation of enhancements in specific light elements. Estimates of luminosity-weighted age, metallicity and abundance ratio show that, while early-type galaxies exhibit a large range of ages (2 to 13 Gyr), bulges are generally younger, and less enhanced in light elements. Trends involving age and metallicity also differ between early and late types. An anti-correlation
is found between age and metallicity in early-type galaxies in the sample, while, in bulges, metallicity is correlated with velocity dispersion. Metallicity gradients are detected in all galaxies in the sample. Age gradients are also detected. No gradients are detected in abundance ratios.
A "single zone with in-fall" model of galactic chemical evolution is used to constrain star formation histories of the galaxies observed. It is shown that monolithic collapse models can not reproduce the line-strengths observed in either early- or late-type galaxies.
However, models including more extended star formation (e.g. mergers or secular evolution) can. The results of this work are, therefore, at odds with the received wisdom that bulges and elliptical galaxies are old and coeval, with indices in bulges more consistent with secular evolution models of formation, and those in early-type galaxies more consistent with merger models.