C i Emission in Ultraluminous Infrared Galaxies as a Molecular Gas Mass Tracer

We present new sensitive wide-band measurements of the fine-structure line 3 P 1 → 3 P 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 12 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 12 CO emission is known to systematically overestimate the gas mass while their 13 CO emission (an often-used alternative) is usually very weak. At redshifts z ≥ 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.