Characterisation of the stellar populations in the outer parts of spiral galaxies

Resumen

Galaxies, the building blocks of the Universe, are gravitationally linked systems made up of baryonic (stars, gas, and dust) and dark matter. How they form and evolve is one of the most studied, yet not understood, topics in modern astrophysics. To unveil their formation and evolution, the analysis of the stars populating these systems has been proven a powerful tool as the different stages in the evolution of a galaxy are imprinted in its stellar content. In particular, this analysis up to the outer parts of the galaxies is essential. As in these faint regions gravity is lower and the dynamical time-scales longer, fossil records from the time of galaxy assembly and formation are retained. In this thesis we use high-quality, spectroscopic (CALIFA) and photometric (SDSS) data along with cosmological simulations (RaDES) to characterise the stellar content in spiral galaxies, paying special attention to their outer regions. We define a sample of 88 disc galaxies from the CALIFA survey, for which we study their light distribution and obtain their stellar age and metallicity radial profiles. The classification of their Surface Brightness (SB) profiles in types I, II, and III is made on a two-dimensional basis applying the GASP2D code to the SDSS science frames. We obtain that different decomposition methods arise different SB classifications but we stick to this more realistic 2D approach. The stellar content of the analysed galaxies is obtained by applying a set of full-spectrum fitting codes (pPXF, GANDALF, and STECKMAP) to the CALIFA data. This method successfully replicates the stellar content recovered from more reliable, resolved stellar populations analysis in a case example, i.e. a region in the Large Magellanic Cloud bar. The outer parts of the CALIFA galaxies reveal ``U-shape'' light-weighted age profiles for type I and II galaxies, suggesting that age and SB profiles are not directly coupled and thus, different mechanisms shape both properties. This age upturn disappears when we compute mass-weighted quantities, pointing to an important role in creating such age profile of the most recent star formation. This outer ageing is not found in all the analysed galaxies. Although this can be interpreted as a lack of universality, it can be a consequence of the limited radial coverage of the CALIFA data. Stellar radial migration has been regarded as a mechanism affecting the SB and other stellar population parameter profiles. The study of the inner stellar content discloses no differences between galaxies displaying the diverse outer SB profiles, setting limits on the importance of radial migration. Based on these results we cannot conclude whether migration is the main factor shaping the SB profiles or not. However, we can claim that the fraction of the migrated stellar mass is not significant enough as to affect the inner galaxy stellar content. In this thesis we also assess the effect of the cosmological assembly history on the chemical and dynamical properties of the discs of the RaDES spiral galaxies up to their outskirts. We find that assembly histories can be divided into three phases according to the number of satellites surrounding the host discs. A merger dominated phase is followed by a quieter one before a ``secular'' epoch settles the discs. The stars born during the third phase and found in the most external regions of the simulated discs display inverted Age-Metallicity Relation as a consequence of the joint effect of radial motions and star formation in satellites temporarily located in these outer parts. In addition, ``U-shape'' age profiles are found in all the cases even if radial motions are not taken into account, suggesting that the outer age upturn could be a universal feature. The study carried out throughout this thesis suggests that the outer parts of spiral galaxies seem to display an age upturn as a frequent feature (supported by observations and simulations). This upturn is neither linked to a particular SB profile nor the result of an unique mechanism such as radial migration. This shape seems to be a consequence of inside-out growth of the disc, radial motions of disc stars (inwards and outwards), accretion of old stars from satellites, and the presence of an early in situ star formation along the entire disc of the galaxy. The fact that this outer ageing disappears in the mass-weighted age profiles suggests the importance of recent star formation in shaping these features. Although we cannot rule out radial migration as the main driver shaping the SB profiles, according to our findings it does not seem to redistribute a large amount of disc stellar mass as the inner gradients of stellar properties for different galaxies do not seem to differ.