1204.0516 (Jesus Zavala et al.)

The growth of galactic bulges through mergers in LCDM haloes revisited. I. Present-day properties    [PDF]

Jesus Zavala, Vladimir Avila-Reese, Claudio Firmani, Michael Boylan-Kolchin

(Abridged) We use the combined data-sets of the Millennium I and II N-body cosmological simulations to revisit the impact of mergers in the growth of bulges in central galaxies in the LCDM scenario. To do so, we seed galaxies within the growing CDM haloes at each epoch using empirical relations to assign stellar and gaseous masses, and an analytical treatment to estimate the transfer of stellar mass to the bulge after a galaxy merger. Our results show that this model roughly reproduces the observed correlation between the bulge-to-total (B/T) mass ratio and stellar mass in present-day central galaxies as well as their observed demographics, although low-mass B/T < 0.1 (bulgeless) galaxies might be scarce relative to the observed abundance. In our merger-driven scenario, bulges have a composite stellar population made of (i) stars acquired from infalling satellites, (ii) stars transferred from the primary disc due to the strong merger-induced perturbations, and (iii) newly formed stars in starbursts triggered by mergers. We find that the first two are the main channels of mass assembly, with the first (second) one being dominant for massive (low- and intermediate mass) galaxies and creating large (small) bulges with a different (similar) stellar population to that of the inner disc. We associate the dominion of the first (second) channel to classical (pseudo) bulges, and compare the predicted fractions of these types to observations. We emphasize that our treatment does not include intrinsic secular processes in the disc as a mechanism of bulge formation. Interestingly, we find that the evolution of the stellar and gaseous contents of the satellite as it spirals towards the central galaxy is a key ingredient in setting the morphology of the remnant, and that a good match to observations of the morphological mixture occurs when this evolution proceeds closely to that of the central galaxy.

View original: http://arxiv.org/abs/1204.0516