Evolution of the dust emission of massive galaxies up to z=4 and
  constraints on their dominant mode of star formation
  We aim to measure the average dust and molecular gas content of massive
star-forming galaxies ($\rm > 3 \times 10^{10}\,M_\odot$) up to z=4 in the
COSMOS field to determine if the intense star formation observed at high
redshift is induced by major mergers or caused by large gas reservoirs.
Firstly, we measured the evolution of the average spectral energy distributions
as a function of redshift using a stacking analysis of Spitzer, Herschel,
LABOCA, and AzTEC data for two samples of galaxies: normal star-forming objects
and strong starbursts, as defined by their distance to the main sequence. We
found that the mean intensity of the radiation field $< U >$ heating the dust
(strongly correlated with dust temperature) increases with increasing redshift
up to z$\sim$4 in main-sequence galaxies. We can reproduce this evolution with
simple models that account for the decrease of the gas metallicity with
redshift. No evolution of $< U >$ with redshift is found in strong starbursts.
We then deduced the evolution of the molecular gas fraction (defined here as
$\rm M_{\rm mol}/(M_{\rm mol}+M_\star)$) with redshift and found a similar,
steeply increasing trend for both samples. At z$\sim$4, this fraction reaches
$\sim$60%. The average position of the main-sequence galaxies is on the locus
of the local, normal star-forming disks in the integrated Schmidt-Kennicutt
diagram (star formation rate versus mass of molecular gas), suggesting that the
bulk of the star formation up to z=4 is dominated by secular processes.
