Evolução química e história de formação estelar no universo local

Abstract

Analisamos as galáxias do Sloan Digital Sky Survey com o nosso código de síntese espectral Starlight. O Starlight acha a combinação de populações estelares simples de diferentes idades e metalicidades que melhor modela o espectro de uma galáxia. Ele permite derivar vários parâmetros associados a uma galáxia, como a massa em estrelas, a história de formação estelar e a evolução química. A partir do espectro residual puramente nebular (subtraindo do espectro observado o modelado), medimos as linhas de emissão, das quais derivamos propriedades do gás dentro das galáxias. Estudamos a evolução das galáxias com formação estelar. Constatamos que as galáxias de maior massa formaram suas estrelas e seus metais mais rapidamente. A evolução da metalicidade das estrelas é estudada diretamente. Calibramos também a taxa de formação estelar atual medida pela síntese com a medida pela luminosidade de Halfa. Derivamos a relação massa estelar-metalicidade estelar (M*-Z*) em diferentes redshifts. Esta é a primeira vez que a relação M*-Z* é calculada para o mesmo conjunto de galáxias. Observamos que a metalicidade estelar observada tem uma evolução compatível com um modelo simples de evolução química de caixa fechada. Para as galáxias classificadas como LINERs, encontramos que a luminosidade observada em Halfa é compatível com o número de fótons ionizantes emitidos pelas populações estelares velhas dessas galáxias. Este resultado implica em uma profunda revisão da taxa de atividade nuclear nas galáxias do Universo local.

Not long ago the adjective astronomical solely described the many stars and galaxies in the sky still to be observed. This has changed in the last decade, and now astronomers take pride in the astronomical amount of publicly available observations. This revolution happened thanks to huge surveys which scanned large parts of the sky for months and months. One such mega-survey is the Sloan Digital Sky Survey (sdss), whose over one million galaxy, quasar and star spectra have driven astronomy to a new era. Our semi-empirical stellar population spectral synthesis code starlight is in line with this new era in astronomy. starlight finds the combination of simple stellar populations (SSPs) of different ages and metallicities that best fits a galaxy spectrum. This allows us to derive several galaxy properties, such as the mass in stars, the star formation history and the chemical evolution. All this is done in a highly automated fashion, suitable to analyse huge samples like those coming from the sdss. Furthermore, from the pure nebular residual spectrum (obtained by subtracting the modelled from the observed spectrum), we measure emission lines. We then obtain several gas properties, such as chemical abundances and nebular extinction. Emission lines also provide important information on the ionization sources in a galaxy through line-ratio diagnostic diagrams. For instance, galaxies form two “seagull wings” on the [N ii]/Hα vs. [O iii]/Hβ diagram: Galaxies whose gas is ionized by a harder source, such as active galactic nuclei (AGNs), live on the right wing; those whose gas is ionized by young stars are found on the left wing. This thesis encompasses some studies on sdss galaxies which put constrains on the evolution of galaxies in the local Universe. Chapter 1 is a short introduction to the evolution of galaxies and the scientific context of this thesis. Chapter 2 reviews the sdss data, our stellar population synthesis algorithm starlight, and our emission-line detection code. In Chapter 3 we obtain the star formation history and chemical evolution for a sample of over 80000 star-forming (SF) sdss galaxies. We study these galaxies according to their position on the [N ii]/Hα vs. [O iii]/Hβ diagram by grouping them into nebular metallicity (Zneb) bins. We also study them in stellar mass bins. We find that the more metal-rich (and more massive) galaxies have formed stars faster and enriched their gas more quickly. On the other hand, the less metal-rich (and less massive) ones are slower in forming stars and in polluting their gas with metals. This was the first time the evolution of stellar metallicity was directly studied. Chapter 4 explores the properties of SF galaxies. We show how Zneb correlates with some physical and observational properties, such as stellar mass, mean stellar age, mass-weighted stellar metallicity, brightness and dust content. We calibrate the current star-formation rate derived from the synthesis with the one from Hα luminosity. This calibration can be applied in future studies of AGN hosts, for which Hα cannot be used as a star formation indicator. We also study more deeply their star formation histories, distributions of histories and possible bias and selection effects. Chapter 5 deals with the evolution of the stellar mass–stellar metallicity relation (M?–Z?) with look-back time. From the stellar mass and metallicity histories of sdss galaxies recovered with starlight, we derive M?–Z? for different redshifts. This was the first time the evolution of M?–Z? relation was derived for the same set of galaxies. We find that M?–Z? steepens and spans to lower mass and metallicities with look-back time. We find that the metallicity evolution is compatible with that expected from a simple closed box model. This means that, as a first approximation, the M?–Z? relation for galaxies with stellar mass greater than ∼ 1010 M may be explained by variations in their star formation histories, and not by inflows or outflows of gas. Chapter 6 focuses on LINERs, galaxies classified as AGN hosts, but which could in fact be ionized by hot old stars. For a fraction of the galaxies classified as LINERs in the sdss, we show that their observed Hα luminosity is compatible with the number of ionizing photons emitted by SSPs older than 107.5 yr in those galaxies. In addition, photoionization models using the ionization field obtained for old stellar populations almost completely cover diagnostic diagrams, such as the [N ii]/Hα vs. [O iii]/Hβ, which means that emission-line ratios are also compatible with ionization by old stellar populations. The “seagull” shape of the [N ii]/Hα vs. [O iii]/Hβ diagram is due to selection effects. This result implies a deep review of the rate of nuclear activity in galaxies in the local Universe. Chapter 7 summarises our main results and proposes some future studies. This thesis presents new methods to study the evolution of galaxies by using properties recovered from stellar population spectral synthesis. These methods open the path, among others, to more detailed studies about the connection between AGN and star formation.