Timing the formation and assembly of early-type galaxies via spatially
  resolved stellar populations analysis
  To investigate star formation and assembly processes of massive galaxies, we
present here a spatially-resolved stellar populations analysis of a sample of
45 elliptical galaxies (Es) selected from the CALIFA survey. We find rather
flat age and [Mg/Fe] radial gradients, weakly dependent on the effective
velocity dispersion of the galaxy within half-light radius. However, our
analysis shows that metallicity gradients become steeper with increasing galaxy
velocity dispersion. In addition, we have homogeneously compared the stellar
populations gradients of our sample of Es to a sample of nearby relic galaxies,
i.e., local remnants of the high-z population of red nuggets. This comparison
indicates that, first, the cores of present-day massive galaxies were likely
formed in gas-rich, rapid star formation events at high redshift (z>2). This
led to radial metallicity variations steeper than observed in the local
Universe, and positive [Mg/Fe] gradients. Second, our analysis also suggests
that a later sequence of minor dry mergers, populating the outskirts of
early-type galaxies (ETGs), flattened the pristine [Mg/Fe] and metallicity
gradients. Finally, we find a tight age-[Mg/Fe] relation, supporting that the
duration of the star formation is the main driver of the [Mg/Fe] enhancement in
massive ETGs. However, the star formation time-scale alone is not able to fully
explain our [Mg/Fe] measurements. Interestingly, our results match the expected
effect that a variable stellar initial mass function would have on the [Mg/Fe]
ratio.
