Detailed stellar population analysis of Early-Typegalaxies with redshift to constrain their evolution

Abstract

Una de las cuestiones básicas de la Astronomía moderna es cómo las galaxias de las diferentes familias han evolucionado desde su formación. Una de estas familias, las galaxias de tipos tempranos (ETGs, del inglés Early-Type Galaxy), formadas por galaxias elípticas y lenticulares, que contienen la mayor parte de la materia luminosa del Universo, ha sido profundamente estudiada durante décadas. En esta tesis se propone esclarecer este tema, analizando las poblaciones estelares de las ETGs masivas a trav´es del tiempo cósmico desde diferentes puntos de vista. Para tal fin, explotamos las nuevas herramientas y métodos de última generación, como el ajuste completo del espectro (full-spectral-fitting) combinado con el análisis de los índices espectrales mediante los diagramas índice-índice. La novedad de este trabajo est´a en el estudio detallado de las poblaciones estelares, llevado a cabo galaxia por galaxia incluso a alto desplazamiento al rojo. Este conjunto de observaciones y métodos utilizados, permite acotar los diferentes escenarios propuestos para la formación y evolución de las ETGs.

One of the long-standing questions in modern Astronomy is how galaxies of different families have evolved since they formed. One of these families, the Early-Type Galaxies (ETGs), which is formed by elliptical and lenticulars galaxies, containing most of the luminous matter in the Universe, have been deeply studied for decades. Several scenarios for the formation and evolution of these galaxies have been proposed but, so far, non of them seem to be conclusive. In the Local Universe, ETGs follow very tight relations on their properties, indicative of a slow evolution since their formation. But the early epochs of the Universe are still to be unraveled as they add new unknowns into the equation. For example, massive ETGs at high redshift. It has been recently discovered that they were much more compact in the early Universe (z∼2) than the massive ones we see today. However, the mechanisms regulating this size evolution are not yet clearly understood and the ones proposed do not completely match the predictions from the competing scenarios to explain this evolution. In this thesis, we aim to shed some light into this puzzling topic by analyzing the stellar populations of massive ETGs over cosmic time from different points of view. For this purpose, we exploit new state-of-the-art analysis tools and methods, such as the full-spectrum-fitting approach combined with a more classical index-index analysis from the spectroscopic absorption features. The novelty of this work is the detailed analysis of the stellar populations, performed on an individual galaxy basis. Altogether, these methods can pose strong constrains on the competing scenarios for the formation and evolution of ETGs. Most of the mechanisms proposed to explain the size evolution of massive ETGs predict the presence (although scarce) of relic compact massive galaxies in the local Universe. The first part of the thesis presents a full characterization of a unique sample of local compact massive galaxies. We address the question whether they are the descendants of those that we observe in the early Universe. However, our results from a detailed analysis of their kinematics, morphologies and stellar population properties suggest that local compact massive galaxies are nearly the exact copies of those at high redshift instead of their descendants. The vicinity of these objects place them as the perfect laboratory for the exploration of the mechanisms responsible for the formation of their counterparts at high redshift. Further constrains on the evolution of massive galaxies are possible from a detailed study of the stellar populations of ETGs in clusters at varying redshift. Such detailed analysis is possible out to z∼ 0.8, i.e. approximately half the age of the Universe, where obtaining spectra with high enough quality is still possible pushing current observing facilities to their limit. We have derived mean ages, metallicities, abundance ratios and SFHs on an individual galaxy basis at such high redshift. The obtained results have been compared to those for their local counterparts in clusters with similar properties to discriminate between different evolutionary paths. Our results point out that a relation between the position within the cluster, the velocity dispersion and the SFH may allow us to understand better the galaxy cluster evolution. We find that the most massive galaxies evolve passively while the lower-mass ones, generally located in the surroundings, experience a more extended star formation history. However, all these studies are performed under the assumption of a universal Initial Mass Function (IMF). The debate on the possible variations of the IMF for explaining several aspects of ETGs evolution, such as e.g. their chemical enrichment and abundance pattern, uses to peak almost every decade. The most recent contributions point towards a bottom-heavier IMF for more massive galaxies. We present an analysis about the biased results that can be obtained when adopting a standard, universal IMF. We have quantified the impact of varying the IMF slope and shape into various galaxy properties. For this purpose we use two samples: a sample of ETGs with varying masses and a sample of local compact massive galaxies. We find that galaxies are rejuvenated and get more massive as we steepen the slope of a Salpeter-like power-law IMF. However, this trend is milder if we consider a Kroupa-like multi-segmented IMF, where the weight of the very low-mass stars is decreased. In addition, we find that by tuning each IMF slope according to the central velocity dispersion of the galaxy, as recently claimed, their derived SFHs tend to be more similar, although still different. This pattern involves a varying amount of recent residual star formation, for the most massive ones.