GOODS-Herschel: Gas-to-dust Mass Ratios and CO-to-H₂ Conversion Factors in Normal and Starbursting Galaxies at High-z

Abstract

We explore the gas-to-dust mass ratio (M _gas/M _d) and the CO luminosity-to-M _gas conversion factor (α_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^+0.7 _-0.6 × 10⁹) M _sun for GN20 and (8.6^+0.6 _-0.9 × 10⁸) M _sun for BzK-21000. The addition of mm data reduces the uncertainties of the derived M _dust by a factor of ~2, allowing the use of the local M _gas/M _d versus metallicity relation to place constraints on the α_CO values of the two sources. For GN20 we derive a conversion factor of α_CO < 1.0 M _sun 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, α_CO ~4.0 M _sun pc^-2 (K km s^-1)^-1, in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ~25 L _sun/M _sun for BzK-21000, a factor of ~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.