most extreme SFRs observed in high-redshift starbursts would be caused by a SFE boosted induced by major mergers.\
At high redshift, the gas mass is difficult to estimate. Two main methods are used. The first is based on the measurement of the intensity of rotational transitions (generally with J$_{\rm upper}<3$) of $^{12}$CO and an assumed CO-to-H$_2$ conversion factor [@Daddi2008; @Tacconi2010; @Saintonge2013; @Tacconi2013]. The main limitation of this method is the uncertainty on this conversion factor, which is expected to be different from the local calibrations in high-redshift galaxies with strongly sub-solar metallicities [@Bothwell2010; @Engel2010; @Genzel2012; @Tan2013; @Genzel2014]. The second method is based on the estimate of the dust mass, which is then converted into gas mass using the locally-calibrated relation between the gas-to-dust ratio and the gas metallicity [e.g., @Munoz-Mateos2009; @Leroy2011; @Remy2014]. The main weakness of this method is the need of an accurate estimate of the gas metallicity and the possible evolution in normalization and scatter of the relation between gas-to-dust ratio and gas metallicity. This method was applied on individual galaxies at high redshift by @Magdis2011 [@Magdis2012b] and @Scoville2014, but also on mean spectral energy distributions (SEDs) measured through a stacking analysis [@Magdis2012b; @Santini2014]. This method has not 