Low-Excitation Gas in HR 10: Possible Implications for Estimates of Metal-rich H2 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 → J, 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 H2 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) ~ 0.16. 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 (z ≳ 3).