Rejuvenated galaxies with very old bulges at the origin of the bending
  of the main sequence and of the "green valley"
  We investigate the nature of star-forming galaxies with reduced specific star
formation rate (sSFR) and high stellar masses, those `green valley' objects
that seemingly cause a reported bending, or flattening, of the star-forming
main sequence. The fact that such objects host large bulges recently led some
to suggest that the internal formation of bulges was a late event that induced
the sSFRs of massive galaxies to drop in a slow downfall, and thus the main
sequence to bend. We have studied in detail a sample of 10 galaxies at
$0.45<z<1$ with secure SFR from Herschel, deep Keck optical spectroscopy, and
HST imaging from CANDELS allowing us to perform multi-wavelength bulge to disc
decomposition, and to derive star formation histories for the separated bulge
and disc components. We find that the bulges hosted in these systems below main
sequence are virtually all maximally old, with ages approaching the age of the
Universe at the time of observation, while discs are young ($\langle$
T$_{50}\rangle \sim 1.5$ Gyr). We conclude that, at least based on our sample,
the bending of the main sequence is, for a major part, due to rejuvenation, and
we disfavour mechanisms that postulate the internal formation of bulges at late
times. The very old stellar ages of our bulges suggest a number density of
Early Type Galaxies at $z=1-3$ higher than actually observed. If confirmed,
this might represent one of the first direct validations of hierarchical
assembly of bulges at high redshifts.
