2004ApJ...615L..29P

We present new sensitive wide-band measurements of the fine-structure line ³P₁⟶³P₀(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-H₂concomitance. We find excellent agreement between the H₂gas mass estimated with this method and the standard methods using ¹²CO. This may provide a new way to measure H₂gas mass in galaxies and one that may be very valuable in ULIRGs since in such systems the bright ¹²CO emission is known to systematically overestimate the gas mass while their ¹³CO 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 H₂gas, fueling starburst events in the distant universe.