Stellar populations and interstellar medium in nearby galaxies with MEGARA@GTC

The evolution of galaxies is the result of a large number of very complex interactions that take place between the different components that make them up. This means that unravelling the mysteries that remain unknown requires a huge amount of data covering as many observables as possible. The aim of this thesis is to deepen our knowledge of the characteristics of stellar populations and the interstellar medium in nearby galaxies with the help of the latest tools developed in the field of astrophysics.

MEGARA (Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía) is one of the most recent advances in astronomical instrumentation. This spectrograph, installed on the 10.4- metre Gran Telescopio de Canarias (GTC), has two observing modes. The LCB mode (Large Compact Bundle) uses an integral field unit (IFU) to obtain spectra of connected regions of the sky covering an area of 12.5×11.3 arcsec^2 using 567 fibres coupled to hexagonal microlenses (with 56 extra fibres to take sky background measurements) with a spatial sampling size of 0.62 arcsec. The MOS (Multi-Object) mode is capable of simultaneously taking spectra from 92 different sources over a field of 3.5×3.5arcmin2. Both modes have 18 Volume Phase Holographic gratings (VPH) that enable it to observe at resolutions of R ∼ 6,000, 12,000 and 20,000 (low, medium and high resolution, respectively) covering a spectral range from 3653 to 9700 Å.

MEGADES (MEgara GAlaxy Discs Evolution Survey) is the legacy science project whose goals guided the design features of MEGARA. The main goal of this project is to study the role played in the evolution of disc galaxies by the various secular and rapid processes involved in shaping their current properties, e.g. in-situ star formation, stellar migration, mergers or feedback. As expected, MEGADES is a very ambitious project that will take several years to complete. In this thesis we present the kick-off of the project by addressing some of the questions included within MEGADES in the inner regions of the galaxies in the sample.

Since this thesis relies on the first scientific results obtained from observations made with MEGARA, the first efforts are devoted to conduct pilot studies on observations taken during the commissioning period of the instrument. These studies, in addition to obtaining results of high scientific value, serve to familiarise us with the data, their reduction process and to develop the software necessary for their analysis.

We first focused on studying the inner regions of the disc galaxy NGC 7025 to find out the nature of its bulge. To do so, we used photometric observations of the Sloan (SDSS) i-band , with which we performed one- and two-dimensional photometric structural decompositions, and high (R ∼ 20,000) and low (R ∼ 6,000) resolution MEGARA observations to measure the kinematic properties of the bulge stars. This study allowed us to analyse the dynamical state of these stars and whether the assembly mechanism of the galaxy bulge was internal (pseudo-bulge) or external (classical bulge).

Another goal of MEGADES is to study the age and metallicity properties of the stellar populations of the galaxies in the sample. Therefore, our next study, again using the observations made on the central regions of NGC 7025, focuses on the ability of MEGARA to estimate these properties. For this purpose we perform analyses based on both spectral fitting tools and index measurements of different absorption lines. We also carry out different tests to estimate the errors and possible biases that we may have when studying stellar populations and derive their characteristics as a function of parameters such as the signal-to-noise ratio, the velocity dispersion of the stars (σ), the spectral range covered by the observations, or the ages of the stars. With all this information we are able to establish a possible scenario for the evolution of the galaxy.

With the tools in place to extend these studies to a larger number of objects, we present the first observations of the MEGADES sample of galaxies and perform an analysis that encompasses many different aspects. First, we perform a kinematic and dynamical analysis that allows us to know the dynamical state of the central regions of the galaxies in the sample. We also derive stellar age and metallicity gradients that provide valuable information about the evolution of these galaxies and we analyse the effect and relation between the dynamical state of the stars and these gradients. Finally, we perform a detailed diagnosis of the ionised gas present in the observations. We use a dynamical criterion based on the ratio of the velocity dispersion of the [NII]λ6584 and Hα emission lines to separate the emission lines according to their excitation mechanisms. To check the effectiveness of this new criterion, we compare the distribution of the points obtained with the so-called BPT diagrams. This analysis allows us to measure more precisely the chemical abundances in the ionised gas and compare them with the abundances measured in the stars. The properties of the stellar populations and the ionised gas are finally related to the dynamical state of the stars.

This thesis shows the variety of kinematic and stellar properties of the galaxies in the sample by finding different age and metallicity gradients, shock zones, AGN zones, dynamically cold regions, and dynamically hot regions in the sample. It also shows how they are related to each other. The dynamically cooler zones, and therefore with higher rotation levels, follow the Strömberg asymmetric drift formula better, and also have more confined metallicity gradients (less gradient dispersion). A correlation between gradients and central metallicity is also observed (the steeper the gradient, the higher the central metallicity, which can be interpreted as due to radial dilution effects of abundances over a universal starting gradient).