Abundance gradient slopes versus mass in spheroids: predictions by
  monolithic models
  We investigate whether it is possible to explain the wide range of observed
gradients in early type galaxies in the framework of monolithic models. To do
so, we extend the set of hydrodynamical simulations by Pipino et al. (2008a) by
including low-mass ellipticals and spiral (true) bulges. These models satisfy
the mass-metallicity and the mass-[alpha/Fe] relations. The typical metallicity
gradients predicted by our models have a slope of -0.3 dex per decade variation
in radius, consistent with the mean values of several observational samples.
However, we also find a few quite massive galaxies in which this slope is -0.5
dex per decade, in agreement with some recent data. In particular, we find a
mild dependence from the mass tracers when we transform the stellar abundance
gradients into radial variations of the Mg_2 line-strength index, but not in
the Mg_b. We conclude that, rather than a mass- slope relation, is more
appropriate to speak of an increase in the scatter of the gradient slope with
the galactic mass. We can explain such a behaviour with different efficiencies
of star formation in the framework of the revised monolithic formation
scenario, hence the scatter in the observed gradients should not be used as an
evidence of the need of mergers. Indeed, model galaxies that exhibit the
steepest gradient slopes are preferentially those with the highest star
formation efficiency at that given mass.
