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Deep Herschel imaging and 12CO(2-1) line luminosities from the IRAM PdBI arecombined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sampleincludes galaxies both on and above the main sequence (MS) traced bystar-forming galaxies in the SFR-M* plane. The far-infrared data are used toderive dust masses, Mdust. Combined with an empirical prescription for thedependence of the gas-to-dust ratio on metallicity (GDR), the CO luminositiesand Mdust values are used to derive for each galaxy the CO-to-H2 conversionfactor, alpha_co. Like in the local Universe, the value of alpha_co is a factorof ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). Wealso uncover a relation between alpha_co and dust temperature (Tdust; alpha_codecreasing with increasing Tdust) as obtained from modified blackbody fits tothe far-infrared data. While the absolute normalization of the alpha_co(Tdust)relation is uncertain, the global trend is robust against possible systematicbiases in the determination of Mdust, GDR or metallicity. Although we cannotformally distinguish between a step and a smooth evolution of alpha_co with thedust temperature, we can conclude that in galaxies of near-solar metallicity, acritical value of Tdust=30K can be used to determine whether the appropriatealpha_co is closer to the starburst value (1.0 Msun(K kms pc^2)^-1, ifTdust>30K) or closer to the Galactic value (4.35 Msun (K kms pc^2)^-1, ifTdust<30K). This indicator has the great advantage of being less subjectivethan visual morphological classifications of mergers/SFGs, which can bedifficult at high z because of the clumpy nature of SFGs. In the absence offar-infrared data, the offset of a galaxy from the main sequence (i.e.,log[SSFR(galaxy)/SSFR_MS(M*,z)]) can be used to identify galaxies requiring theuse of an alpha_co conversion factor lower than the Galactic value.

Dust temperature and CO-to-H2 conversion factor variations in the SFR-M* plane