A possible solution to the [alpha/Fe]-sigma problem in early type galaxies within a hierarchical galaxy formation model

Abstract

The most massive elliptical galaxies apparently formed the fastest, because the ratio of alpha elements (such as oxygen) to iron is the smallest. In fact, iron is mainly produced from type Ia supernovae on a timescale of ~ 0.1-1 billion years, while the alpha elements come from massive stars on timescales of a few tens of million years (Matteucci 1994). Reproducing such a alpha/Fe correlation has long been a severe problem for cosmological theories of galaxy formation, which envisage massive galaxies to assemble gradually from smaller progenitors, and to be characterized by a star formation history too much extended towards late cosmic times. While it has recently become clear that feedback from Active Galactic Nuclei (AGN) activity play a role in the late quenching of star formation (e.g. Cattaneo et al. 2009), and that early star formation history in the galaxy progenitors affect the alpha/Fe ratio (Calura & Menci 2009), major mergers alone cannot enhance the star formation in the high-redshift progenitors to the levels required to match the steepness of the observed alpha/Fe correlation (Spolaor et al. 2010). Here we report that the inclusion of the effects of fly-by 'harassments', that trigger lower level starbursts, combined with the AGN quenching of the starburst activity, considerably enhances the capability to account for the observed alpha/Fe ratio in ellipticals within cosmological galaxy formation models . The critical difference between the earlier work and the present result is the effect of starbursts driven by fly-by encounters that would have been very common amongst the high-redshift progenitors of massive galaxies and which would have boosted star formation in the first 2 billion years after the Big Bang, combined with quenching of the burst activity within the first 3-4 Gyr.