A new perspective in the evolution of galaxiesFrom global to local scales

Resumen

This thesis dissertation addresses the topic of galaxy formation and evolution with the main purpose of shedding some light on the main mechanism(s) responsible for the self- regulation of the star formation activity in the nearby Universe. As a first step, we revisit the classic “nature or nurture” debate in this context by using a sample of ∼82000 objects extracted from the Sloan Digital Sky Survey. Our analysis cross- correlates two proxies of the specific star formation rate, the equivalent width (EW) of the Hα line and the (u-r) colour, with other physical properties (mass, metallicity, environment, morphology, and the presence of close companions) leading to the discovery of a relatively tight “ageing sequence” in the colour–EW plane. This trend favours a scenario where the secular conversion of gas into stars (i.e. nature) is the main physical driver of star formation and the gradual transition from a “chemically primitive” (metal-poor and intensely star- forming) state to a “chemically evolved” (metal-rich and passively-evolving) system. To address the dependence on local and global properties, we investigate in further detail the resolved colour-EW diagram of ∼40 matching objects from the CALIFA survey. The IFS data reveal that the smooth “ageing” process always takes place, inside-out (i.e. the central parts are more evolved than the outskirts), across the entire extent of all galaxies. The current state of the different regions seems to be mostly driven by local processes, albeit global properties (in particular, galaxy morphology) may play an important role. The analysis of low signal-to-noise (S/N) measurements was critical in order to reach these conclusions. To make an optimal use of this valuable information, we developed the Bayesian Technique for Multi-image Analysis (BaTMAn) algorithm for the segmentation of multidimensional data, with emphasis on the statistically-meaningful binning of IFS obser- vations. When applied to our dataset, the method significantly reduces the scatter in the colour-EW diagram, convincingly demonstrating that there may be different ageing paths depending on the details of the local star formation history. In our view, the new generation of IFS surveys calls for new analysis tools and method- ologies, and a rigorous assessment of their accuracy (and that of the associated errors!) requires a battery of realistic test cases where the correct solution is known. The final part of this thesis describes the production of synthetic IFS observations, with similar character- istics to the CALIFA products, based on hydrodynamical simulations. This ongoing project will allow us to explore the optimal strategy to study galaxy ageing from current and forth- coming IFS data, and it will provide the scientific community with a powerful tool to test their analysis pipelines.