The spatially-resolved star formation histories of CALIFA galaxies:
  Implications for galaxy formation
  This paper presents the spatially resolved star formation history (SFH) of
nearby galaxies with the aim of furthering our understanding of the different
processes involved in the formation and evolution of galaxies. To this end, we
apply the fossil record method of stellar population synthesis to a rich and
diverse data set of 436 galaxies observed with integral field spectroscopy in
the CALIFA survey. The sample covers a wide range of Hubble types, with stellar
masses ranging from $M_\star \sim 10^9$ to $7 \times 10^{11} M_\odot$. Spectral
synthesis techniques are applied to the datacubes to retrieve the spatially
resolved time evolution of the star formation rate (SFR), its intensity
($\Sigma_{\rm SFR}$), and other descriptors of the 2D-SFH in seven bins of
galaxy morphology (E, S0, Sa, Sb, Sbc, Sc, and Sd), and five bins of stellar
mass. Our main results are: a) Galaxies form very fast independently of their
current stellar mass, with the peak of star formation at high redshift ($z >
2$). Subsequent star formation is driven by $M_\star$ and morphology, with less
massive and later type spirals showing more prolonged periods of star
formation. b) At any epoch in the past the SFR is proportional to $M_\star$,
with most massive galaxies having the highest absolute (but lowest specific)
SFRs. c) While nowadays $\Sigma_{\rm SFR}$ is similar for all spirals, and
significantly lower in early type galaxies (ETG), in the past $\Sigma_{\rm
SFR}$ scales well with morphology. The central regions of today's ETGs are
where $\Sigma_{\rm SFR}$ reached the highest values ($> 10^3
\,M_\odot\,$Gyr$^{-1}\,$pc$^{-2}$), similar to those measured in high redshift
star forming galaxies. d) The evolution of $\Sigma_{\rm SFR}$ in Sbc systems
matches that of models for Milky-Way-like galaxies, suggesting that the
formation of a thick disk may be a common phase in spirals at early epochs.
