2012A&A...548A..22M

Deep Herschel PACS/SPIRE imaging and ¹²CO(2-1) line luminosities from the IRAM Plateau de Bure Interferometer are combined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sample includes galaxies both on and above the main sequence (MS) traced by star-forming galaxies in the SFR-M_* plane. The far-infrared data are used to derive dust masses, M_dust, following the Draine & Li (2007ApJ...657..810D) models. Combined with an empirical prescription for the dependence of the gas-to-dust ratio on metallicity (δ_GDR(µ₀)), the CO luminosities and M_dust values are used to derive for each galaxy the CO-to-H₂ conversion factor, α_CO. Like in the local Universe, the value of α_CO is a factor of ∼5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We additionally uncover a relation between α_CO and dust temperature (T_dust; α_CO decreasing with increasing T_dust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the α_CO(T_dust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of M_dust, δ_GDR(µ₀) or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of α_CO with the dust temperature, we can unambiguously conclude that in galaxies of near-solar metallicity, a critical value of T_dust=30K can be used to determine whether the appropriate α_CO is closer to the ``starburst'' value (1.0M_☉/(K.km/s/pc2), when T_dust>30K) or closer to the Galactic value (4.35M_☉/(K.km/s/pc2), when T_dust<30K). This indicator has the great advantage of being less subjective than visual morphological classifications of mergers/SFGs, which can be difficult at high z because of the clumpy nature of SFGs. Using T_dust to select the appropriate α_CO is also more indicative of ISM conditions than a fixed L_IR criterion. In the absence of far-infrared data, the offset of a galaxy from the star formation main sequence (i.e., Δlog(SSFR)_MS=log[SSFR(galaxy)/SSFR_MS(M_*,z)]) can be used to identify galaxies requiring the use of an α_CO conversion factor lower than the Galactic value (i.e., when Δlog(SSFR)_MS>0.3dex).