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PhD Thesis: Semi-Empirical Analysis of Galaxy Spectra

Abstract

In this Thesis we investigate the stellar population properties of galaxies by means of a semi-empirical analysis of their spectral data. The galaxy samples used in this work are drawn from the Sloan Digital Sky Survey (SDSS), which is the state-of-the-art project on surveying astronomical objects in the local universe (z ~ 0.1). Physical properties of galaxies are obtained with the application of a spectral synthesis method to galaxy spectra, which are modelled as a combination of simple stellar populations of distinct ages and metallicities derived from recent evolutionary synthesis models. This approach results in a set of physical parameters related to the stellar content of each galaxy, like mean ages and metallicities, mass and extinction. The analysis of emission lines is also favoured from the inspection of starlight subtracted spectra. In addition to the excelent database available by the SDSS, the spectral synthesis provides a detailed study of the stellar populations and the general properties of galaxies.

We discuss the spectral classification of emission line galaxies based on diagnostic diagrams formed by line ratios of the strongest lines present in their spectra, mainly the [OIII]/Hbeta versus [NII]/Halpha diagram. In particular, we show that diagrams involving the ratio [OII]/[OIII] can be useful to distinguish normal star-forming galaxies and hosts of active galactic nuclei. Based on this spectral classification, we investigate the bimodality observed in the galaxy population through the analysis of the stellar population properties of galaxies. We find that the light-weighted mean stellar age is the parameter which best describes the two galaxy populations of the local universe, characterized by star-forming galaxies at one side, and passive ones at another. The stellar mass has an additional role in the sense that the star formation occurs essentially in low-mass systems. Our results give support to the existence of a downsizing in galaxy formation, in which the star formation is shifted from massive galaxies at early times to low-mass galaxies as the universe evolves.

These results are complemented with an analysis on the environmental dependence of properties related to the stellar populations of nearby galaxies. The environment is defined by the projected local galaxy density estimated from a nearest neighbour approach. We recover the star formation--density relation in terms of the mean light-weighted stellar age, which is strongly correlated with star formation parameters derived from Halpha. We find that the age--density relation is distinct when we divide galaxies according to luminosity or stellar mass. The relation is remarkable for galaxies in all bins of luminosity. On the other hand, only for an intermediate stellar mass interval, associated to a transition in galaxy properties, the relation shows a change in galaxy properties with environment. Such distinct behaviours are associated to the large stellar masses of galaxies with the same luminosity in high-density environments. The age--density relation results from the prevalence of massive systems in high-density environments, independently of galaxy luminosity, with the additional observed downsizing in galaxy formation. Finally, our results support that a natural path for galaxy evolution proceeds via a nurture way that took place mainly at high-redshifts.

MSc Dissertation: Environmental Dependence of Star Formation in Galaxies

Abstract

We investigate the environmental dependence of galaxies with star formation from a volume-limited sample of 1188 nearby galaxy spectra extracted from the 2dF Galaxy Redshift Survey. The environment is characterized by the local number density of galaxies. We discriminate the star-forming galaxies in distinct spectral classes by the use of a set of equivalent widths. The frequency of galaxies of different classes are then evaluated as a function of the environment. We show that the fraction of star-forming galaxies decreases in dense regions, where the population of passive galaxies is high. On the other hand, when we analyse the fraction of short starburst galaxies we note that it is not environment dependent, and in denser regions some of these objects show morphological distortions. We also show that the star formation rate of galaxies, based on the Halpha equivalent width, decreases with increasing density. We propose that tidal interactions are responsible for decreasing the star formation in high density environments, by means of the suppression of galaxies' gaseous reservoirs. However, these interactions can also produce short starbursts and morphological peculiarities in galaxies localized in high density environments. Thus, we argue that the evolution of star formation properties of galaxies, although more intense and important in clusters, occurs in all environments.