On the origin of the dichotomy of early-type galaxies: the role of dry mergers and active galactic nucleus feedback

Abstract

Using a semi-analytical model for galaxy formation, combined with a large N-body simulation, we investigate the origin of the dichotomy among early-type galaxies. In qualitative agreement with previous studies and with numerical simulations, we find that boxy galaxies originate from mergers with a progenitor mass ratio n < 2 and with a combined cold gas mass fraction F-cold < 0.1. Our model accurately reproduces the observed fraction of boxy systems as a function of luminosity and halo mass, for both central galaxies and satellites. After correcting for the stellar mass dependence, the properties of the last major merger of early-type galaxies are independent of their halo mass. This provides theoretical support for the conjecture of Pasquali, van den Bosch & Rix that the stellar mass (or luminosity) of an early-type galaxy is the main parameter that governs its isophotal shape. If wet and dry mergers mainly produce discy and boxy early-types, respectively, the observed dichotomy of early-type galaxies has a natural explanation within the hierarchical framework of structure formation. Contrary to naive expectations, the dichotomy is independent of active galactic nucleus feedback. Rather, we argue that it owes to the fact that more massive systems (i) have more massive progenitors, (ii) assemble later and (iii) have a larger fraction of early-type progenitors. Each of these three trends causes the cold gas mass fraction of the progenitors of more massive early-types to be lower, so that their last major merger involved less cold gas (was more 'dry'). Finally, our model predicts that (i) less than 10 per cent of all early-type galaxies form in major mergers that involve two early-type progenitors, (ii) more than 95 per cent of all boxy early-type galaxies with M-double dagger less than or similar to 2 x 10(10) h(-1) M-circle dot are satellite galaxies and (iii) about 70 per cent of all low-mass early-types do not form a supermassive black hole binary at their last major merger. The latter may help to explain why low-mass early-types have central cusps, while their massive counterparts have cores.