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We aim to measure the average dust and molecular gas content of massivestar-forming galaxies ($\rm > 3 \times 10^{10}\,M_\odot$) up to z=4 in theCOSMOS field to determine if the intense star formation observed at highredshift is induced by major mergers or caused by large gas reservoirs.Firstly, we measured the evolution of the average spectral energy distributionsas a function of redshift using a stacking analysis of Spitzer, Herschel,LABOCA, and AzTEC data for two samples of galaxies: normal star-forming objectsand strong starbursts, as defined by their distance to the main sequence. Wefound that the mean intensity of the radiation field $< U >$ heating the dust(strongly correlated with dust temperature) increases with increasing redshiftup to z$\sim$4 in main-sequence galaxies. We can reproduce this evolution withsimple models that account for the decrease of the gas metallicity withredshift. 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 locusof the local, normal star-forming disks in the integrated Schmidt-Kennicuttdiagram (star formation rate versus mass of molecular gas), suggesting that thebulk of the star formation up to z=4 is dominated by secular processes.

Evolution of the dust emission of massive galaxies up to z=4 and constraints on their dominant mode of star formation