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

Deep Herschel PACS/SPIRE imaging and (CO)-C-12(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 (2007, ApJ, 657, 810) models. Combined with an empirical prescription for the dependence of the gas-to-dust ratio on metallicity (delta(GDR)(mu(0))), the CO luminosities and M-dust values are used to derive for each galaxy the CO-to-H-2 conversion factor, alpha(CO). Like in the local Universe, the value of alpha(CO) is a factor of similar to 5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We additionally uncover a relation between alpha(CO) and dust temperature (T-dust; alpha CO decreasing with increasing T-dust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the alpha(CO)(T-dust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of M-dust, delta(GDR)(mu(0)) or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of alpha(CO) with the dust temperature, we can unambiguously conclude that in galaxies of near-solar metallicity, a critical value of T-dust = 30 K can be used to determine whether the appropriate alpha(CO) is closer to the starburst value (1.0 M-circle dot (K km s(-1) pc(2))(-1), when T-dust > 30K) or closer to the Galactic value (4.35 M-circle dot (K km s(-1) pc(2))(-1), when T-dust < 30 K). 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 alpha(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., Delta log (SSFR)(MS) = log[SSFR(galaxy)/SSFRMS(M-*, z)]) can be used to identify galaxies requiring the use of an alpha(CO) conversion factor lower than the Galactic value (i.e., when Delta log (SSFR)(MS) greater than or similar to 0.3 dex).