Spatially-Resolved Star Formation Main Sequence of Galaxies in the
  CALIFA Survey
  The "main sequence of galaxies" $-$ defined in terms of the total star
formation rate $\psi$ vs. the total stellar mass $M_*$ $-$ is a well-studied
tight relation that has been observed at several wavelengths and at different
redshifts. All earlier studies have derived this relation from integrated
properties of galaxies. We recover the same relation from an analysis of
spatially-resolved properties, with integral field spectroscopic (IFS)
observations of 306 galaxies from the CALIFA survey. We consider the SFR
surface density in units of log(M$_{\odot}$ yr$^{-1}$ Kpc$^{-2}$) and the
stellar mass surface density in units of log(M$_{\odot}$ Kpc$^{-2}$) in
individual spaxels which probe spatial scales of 0.5-1.5 Kpc. This local
relation exhibits a high degree of correlation with small scatter ($\sigma =
0.23$ dex), irrespective of the dominant ionisation source of the host galaxy
or its integrated stellar mass. We highlight: $(i)$ the integrated star
formation main sequence formed by galaxies whose dominant ionisation process is
related to star formation, for which we find a slope of 0.81 $\pm 0.02$; (ii)
the spatially-resolved relation obtained with the spaxel analysis, we find a
slope of 0.72 $\pm 0.04$; (iii) for the integrated main sequence we identified
also a sequence formed by galaxies that are dominated by an old stellar
population, which we have called the retired galaxies sequence.
