  The overabundance of Mg relative to Fe, observed in the nuclei of bright
ellipticals, and its increase with galactic mass, poses a serious problem for
all current models of galaxy formation. Here we improve on the one-zone
chemical evolution models for elliptical galaxies by taking into account
positive feedback produced in the early stages of super-massive central black
hole growth. We can account for both the observed correlation and the scatter
if the observed anti-hierarchical behaviour of the AGN population couples to
galaxy assembly and results in an enhancement of the star formation efficiency
which is proportional to galactic mass. At low and intermediate galactic
masses, however, a slower mode for star formation suffices to account for the
observational properties.

Astrophysics