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 (G/D) and the CO luminosity-to-Mgas conversion factor (aco) of two well studied galaxies in the GOODS-N 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 mm interferometry (1.1-6.6mm) along with Herschel, PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li (2007) models, as well as a modified black body function, we measure the dust mass (Md) of the sources and find 2.0+0.7-0.6 x 109 Msun for GN20 and 8.6+0.6-0.9 x 108 Msun for BzK-21000. The addition of mm data reduces the uncertainties of the derived Md by a factor of ~2, allowing the use of the local G vs metallicity relation to place constraints on the aco values of the two sources. For GN20 we derive a conversion factor of aco < 1.0 Msun pc-2(K km s-1)-1, consistent with that of local ULIRGs, while for BzK-21000 we find a considerably higher value, aco ~4.0 Msun pc-2(K km s-1)-1, in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ~25 Lsun/Msun 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
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