Resumen de Minor merger effects on galaxy evolution
María Trinidad Tapia Peralta
In this thesis we study the effect that minor mergers have on the evolution of galaxies. We focus on three topics: the size evolution of massive elliptical galaxies, metallicity gradients in elliptical galaxies, and the formation of the thick disc. To study if, in a ¿CDM universe, cosmological dry merger histories can generate the observed size evolution of massive elliptical galaxies, we analyzed eight different high-resolution merger histories of massive elliptical galaxies. The mass ratio and orbital parameters of these galaxies were extracted from self- consistent cosmological simulations, and we followed observational relationships to set their sizes. Our progenitor galaxies at z = 2 resemble observed high- redshift galaxies with re ~ 1 kpc with a mass range between 5.21 × 1010M¿ and 2.18 × 1011M¿, and a variety of velocity dispersions from ~ 236 km/s to ~ 486 km/s. We find that the median growth is a factor 4 in size and a factor 2 in mass from z=2 to z=0, with a moderate increase of the velocity dispersion. To study if sequential dry minor mergers induce a gradient of metallicity in the remnant of a high redshift, massive elliptical galaxy, we have estimated the metallicity gradients of a representative subsample of our merger histories from Chapter 3 in order to examine whether we can reproduce the metallicity gradients observed in present-day elliptical galaxies by following observational mass-metallicity relationships for our cosmological-motivated dry mergers remnants. We find that the metallicity profiles are approximately linear with log(r), and that the effect of accreting satellites is to produce a shallower metallicity gradient. The final gradients are consistent with those observed in z = 0 massive ellipticals. A high number or sequential accretions tend to produce a smoother metallicity gradient than fewer merger events, even when the total accreted mass is approximately the same. We also find a large scatter in the observed gradients, which scales with the number of sequential accretions and which is compatible with the large scatter in metallicity gradients observed in massive ellipticals. Within the merger-driven scenarios, there are two qualitatively different corresponding mechanisms. In the first case, thick-disc stars form in external galaxies and are subsequently deposited by accretion events at large scale heights (e.g. Abadi et al. 2003b; Yoachim and Dalcanton 2005). To study the effects of minor mergers on disc thickening, focussing on quantifying the effects of numerical heating, we perform a suite of merger experiments of the single accretion of one satellite onto a disc galaxy, using a range of mass ratios, orbital types and number of particles. Our conclusion is that both mechanisms, the accretion of satellites and the heating of the disc, play a role in the building of the thick disc in the merger-driven scenario. The accretion of a satellite results in the deposition of mass in the outer parts and the injection of kinetic energy into the disc, thus heating the disc and enhancing the settling of thin disc material into greater heights. We stress the necessity of more than one satellite to shape a thick disc by tidal debris of disrupted systems or to produce a big change in the direction of the angular momentum of the disc to generate a slowly rotating or counterrotating thick disc. Also, we find that the single accretion of a low-mass satellite produces a thickening of a factor ~ 2.7-3.5. Nevertheless, it is not sufficient to perturb the disc and to produce the boxy isophotes expected if the thick disc is formed by mergers.
