CI emission in ultraluminous infrared galaxies as a molecular gas mass tracer

We present new sensitive wide-band measurements of the fine-structure line P-3(1) --> P-3(0) (J = 1-0, 492 GHz) of neutral atomic carbon (C I) in the two typical ultraluminous infrared galaxies (ULIRGs) NGC 6240 and Arp 220. We then use them along with several other C I measurements in similar objects found in the literature to estimate their global molecular gas content under the assumption of a full C I-H-2 concomitance. We find excellent agreement between the H-2 gas mass estimated with this method and the standard methods using CO. This may provide a new way to measure H-2 gas mass in galaxies and one that may be very valuable in ULIRGs since in such systems the bright (CO)-C-12 emission is known to systematically overestimate the gas mass while their (CO)-C-13 emission (an often-used alternative) is usually very weak. At redshifts z greater than or equal to 1 the C I J = 1-0 line shifts to much more favorable atmospheric windows and can become a viable alternative tracer of the H-2 gas, fueling starburst events in the distant universe.