GOODS-HERSCHEL: GAS-TO-DUST MASS RATIOS AND CO-TO-H2 CONVERSION FACTORS IN NORMAL AND STARBURSTING GALAXIES AT HIGH-z

We explore the gas-to-dust mass ratio (M-gas/M-d) and the CO luminosity-to-M-gas conversion factor (alpha(CO)) of two well-studied galaxies in the Great Observatories Origins Deep Survey North field that are expected to have different star-forming modes, the starburst GN20 at z = 4.05 and the normal star-forming galaxy BzK-21000 at z = 1.52. Detailed sampling is available for their Rayleigh-Jeans emission via ground-based millimeter (mm) interferometry (1.1-6.6 mm) along with Herschel PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li models, as well as a modified blackbody function, we measure the dust mass (M-dust) of the sources and find (2.0(-0.6)(+0.7) x 10(9)) M-circle dot for GN20 and (8.6(-0.9)(+0.6) x 10(8)) M-circle dot for BzK-21000. The addition of mm data reduces the uncertainties of the derived M-dust by a factor of similar to 2, allowing the use of the local M-gas/M-d versus metallicity relation to place constraints on the alpha CO values of the two sources. For GN20 we derive a conversion factor of alpha CO < 1.0 M-circle dot pc(-2) (K km s(-1))(-1), consistent with that of local ultra-luminous infrared galaxies, while for BzK-21000 we find a considerably higher value, alpha CO similar to 4.0 M-circle dot pc(-2) (K km s(-1))(-1), in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is similar to 25 L-circle dot/M-circle dot for BzK-21000, a factor of similar to 5-10 lower than that of GN20. The findings for these two sources support the existence of different disk-like and starburst star formation modes in distant galaxies, although a larger sample is required to draw statistically robust results.