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 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 - 6.6 mm ) 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 ( M _ { dust } ) of the sources and find ( 2.0 ^ { +0.7 } _ { -0.6 } \times 10 ^ { 9 } ) M _ { \odot } for GN20 and ( 8.6 ^ { +0.6 } _ { -0.9 } \times 10 ^ { 8 } ) M _ { \odot } for BzK-21000 .
The addition of mm data reduces the uncertainties of the derived M _ { dust } by a factor of \sim 2 , allowing the use of the local M _ { gas } / M _ { d } vs 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 _ { \odot } pc ^ { -2 } ( K km s ^ { -1 } ) ^ { -1 } , consistent with that of local ULIRGs , while for BzK-21000 we find a considerably higher value , \alpha _ { CO } \sim 4.0 M _ { \odot } pc ^ { -2 } ( K km s ^ { -1 } ) ^ { -1 } , in agreement with an independent kinematic derivation reported previously .
The implied star formation efficiency is \sim 25 L _ { \odot } /M _ { \odot } for BzK-21000 , a factor of \sim 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 .