The intrinsic characteristics of galaxies on the SFR-stellar mass plane
  at 1.2<z<4: I. the correlation between stellar age, central density and
  position relative to the main sequence
  We use the deep CANDELS observations in the GOODS North and South fields to
revisit the correlations between stellar mass ($M_*$), star--formation rate
(SFR) and morphology, and to introduce a fourth dimension, the mass-weighted
stellar age, in galaxies at $1.2<z<4$. We do this by making new measures of
$M_*$, $SFR$, and stellar age thanks to an improved SED fitting procedure that
allows various star formation history for each galaxy. Like others, we find
that the slope of the Main Sequence (MS) of star formation in the $(M_*;SFR)$
plane bends at high mass. We observe clear morphological differences among
galaxies across the MS, which also correlate with stellar age. At all
redshifts, galaxies that are quenching or quenched, and thus old, have high
$\Sigma_1$ (the projected density within the central 1 kpc), while younger,
star-forming galaxies span a much broader range of $\Sigma_1$, which includes
the high values observed for quenched galaxies, but also extends to much lower
values. As galaxies age and quench, the stellar age and the dispersion of
$\Sigma_1$ for fixed values of $M_{*}$ shows two different regimes, one, at the
low--mass end, where quenching might be driven by causes external to the
galaxies; the other, at the high--mass end, where quenching is driven by
internal causes, very likely the mass given the low scatter of $\Sigma_1$ (mass
quenching). We suggest that the monotonic increase of central density as
galaxies grow is one manifestation of a more general phenomenon of structural
transformation that galaxies undergo as they evolve.
