The evolution of galaxies resolved in space and time: an inside-out
  growth view from the CALIFA survey
  The growth of galaxies is one of the key problems in understanding the
structure and evolution of the universe and its constituents. Galaxies can grow
their stellar mass by accretion of halo or intergalactic gas clouds, or by
merging with smaller or similar mass galaxies. The gas available translates
into a rate of star formation, which controls the generation of metals in the
universe. The spatially resolved history of their stellar mass assembly has not
been obtained so far for any given galaxy beyond the Local Group. Here we
demonstrate how massive galaxies grow their stellar mass inside-out. We report
the results from the analysis of the first 105 galaxies of the largest to date
three-dimensional spectroscopic survey of galaxies in the local universe
(CALIFA). We apply the fossil record method of stellar population spectral
synthesis to recover the spatially and time resolved star formation history of
each galaxy. We show, for the first time, that the signal of downsizing is
spatially preserved, with both inner and outer regions growing faster for more
massive galaxies. Further, we show that the relative growth rate of the
spheroidal component, nucleus and inner galaxy, that happened 5-7 Gyr ago,
shows a maximum at a critical stellar mass ~10^10 Msun. We also find that
galaxies less massive than ~10^10 Msun show a transition to outside-in growth,
thus connecting with results from resolved studies of the growth of low mass
galaxies.
