Abundance gradient slopes versus mass in spheroids: predictions by monolithic models

Abstract

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. by including low-mass ellipticals and spiral (true) bulges. These models satisfy the mass–metallicity and the mass–[α/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 Mg2 line-strength index, but not in the Mgb. 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.