Observational evidence of a slow downfall of star formation efficiency
  in massive galaxies during the last 10 Gyr
  In this paper we study the causes of the reported mass-dependence of the
slope of SFR-M* relation, the so-called "Main Sequence" of star-forming
galaxies, and discuss its implication on the physical processes that shaped the
star formation history of massive galaxies over cosmic time. We use the CANDELS
near-IR imaging from the Hubble Space Telescope to perform the bulge-to-disk
decomposition of distant galaxies and measure for the first time the slope of
the SFR-Mdisk relation at z=1. We find that this relation follows very closely
the shape of the SFR-M* correlation, still with a pronounced flattening at the
high-mass end. This is clearly excluding, at least at z=1, the secular growth
of quiescent bulges in star-forming galaxies as the main driver for the change
of slope of the Main Sequence. Then, by stacking the Herschel data available in
the CANDELS field, we estimate the total gas mass and the star formation
efficiency at different positions on the SFR-M* relation. We find that the
relatively low SFRs observed in massive galaxies (M* > 5e10 Msun) are caused by
a decreased star formation efficiency, by up to a factor of 3 as compared to
lower stellar mass galaxies, and not by a reduced gas content. The trend at the
lowest masses is likely linked to the dominance of atomic over molecular gas.
We argue that this stellar-mass-dependent SFE can explain the varying slope of
the Main Sequence since z=1.5, hence over 70% of the Hubble time. The drop of
SFE occurs at lower masses in the local Universe (M* > 2e10 Msun) and is not
present at z=2. Altogether this provides evidence for a slow downfall of the
star formation efficiency in massive Main Sequence galaxies. The resulting loss
of star formation is found to be rising starting from z=2 to reach a level
comparable to the mass growth of the quiescent population by z=1. We finally
discuss the possible physical origin of this phenomenon.
