Low-excitation gas in HR 10: Possible implications for estimates of metal-rich H-2 mass at high redshifts

We examine the physical conditions of the molecular gas in the extremely red object HR 10 (J164502+4626.4, or [HR94] 10) at z=1.44, as constrained by the recent detection of CO J+1 -->M J=1, 4 lines. The line ratios of such widely spaced CO transitions are very sensitive probes of gas excitation; in the case of HR 10, they provide a rare opportunity to study the state of H-2 gas in a gas-rich starburst at a cosmologically significant distance. Contrary to earlier claims, we find the CO J = 5-4 transition to be of low excitation with a brightness temperature ratio of (5-4)/(2-1)similar to0.16. Depending upon how often such conditions prevail in intense starbursts, this may have serious consequences for molecular gas-mass estimates and the detectability of high-J CO lines from similar objects at high redshifts (zgreater than or similar to3).