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Gallazzi, Anna (2006): Modeling and interpretation of galaxy spectra: the stellar populations of nearby galaxies. Dissertation, LMU München: Fakultät für Physik



Our current understanding of structure formation in the Universe seems to be well described by a hierarchical scenario, in which small units assemble first to produce more massive systems. In recent years, much observational evidence has been accumulated, indicating that star formation proceeded instead in an antihierarchical fashion. Constraining the age and chemical composition of the stellar populations in galaxies should help shed light on this apparent dichotomy between mass assembly and star formation activity. The integrated spectra of galaxies contain valuable clues about the ages and metallicities of the stars producing the light. However, at first order, they are affected in a similar way by age and metallicity. Studies of more refined spectral diagnostics, such as individual stellar absorption features, are thus needed to provide more stringent constraints on these parameters. This method has been limited so far to small samples of elliptical galaxies, using population synthesis models with limited spectral resolution and restricted coverage in stellar effective temperatures. The objective of this thesis is the interpretation of the optical spectra of large samples of nearby galaxies in terms of the light-weighted metallicity, age and mass of their stellar populations. I have developed a new method to simultaneously derive median-likelihood estimates of each physical parameter and the associated confidence intervals. The method, based on a recent highresolution population synthesis code with full temperature coverage, consists in comparing each observed spectrum with a comprehensive library of star formation histories. The constraints are set by the simultaneous fit of an optimally selected set of spectral absorption features. I have applied this method to a sample of 200,000 galaxies from the Sloan Digital Sky Survey, including galaxies with any star formation history, from quiescent early-type to actively star forming galaxies. Thanks to the unprecedented statistics, I could give an accurate description of the galaxy distribution in the full physical parameters space. The relation between stellar metallicity, age and stellar mass shows a rapid transition from low-mass, young, metal-poor to high-mass, old, metal-rich galaxies at a stellar mass of 3×10^10 solar masses, the same characteristic scale of several observed bi-modalities in galaxy properties. The stellar metallicity-mass relation is interpreted as a manifestation of galactic winds, which are more efficient in removing metals from the shallow potential well of low-mass galaxies. I then explored the implications of the above relations to re-assess the physical origin of observed scaling relations of elliptical galaxies, linking their luminous and dynamical mass to the properties of their stellar populations. The relations are driven by an increase in metallicity, age and element abundance ratios with galaxy mass. The scatter is contributed by a similar amount by both age and metallicity. The increasing spread towards younger ages at low stellar masses indicates that low-mass ellipticals either formed their stars later or have a more extended star formation history. This hints at a shift in stellar growth towards less massive galaxies in recent epochs. The large ranges in observational and physical properties covered by SDSS galaxies make it a representative sample of the local Universe. I could thus derive the total mass density of metals and baryons locked up in stars today. I have also studied how metals and stellar mass are distributed as a function of various galaxy properties. The galaxies containing the bulk of the total stellar mass (massive, bulge-dominated galaxies with old stellar populations) are also those that contribute the largest fraction of metals, as expected from the mass-metallicity relation. These quantities set the fundamental constraints at the present epoch of the cosmic star formation and chemical enrichment histories. The more detailed knowledge of the relations between galaxy physical parameters allows a more direct comparison with predictions from semi-analytic models of galaxy formation and evolution. Moreover, the more robust constraints represent an important calibration at redshift zero for similar studies at higher redshifts.