Low-excitation CO in HR10: implications for measuring metal-rich H2 mass at high redshifts

We examine the physical conditions of the molecular gas in the extremely red object (ERO), HR10 (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 extremely sensitive probes of gas excitation; in the case of the HR 10, they provide a rare opportunity to study the state of the molecular 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. The brightness temperature ratio of (5–4)/(2–1) ∼ 0.15. This has serious consequences for estimates of H2 gas mass and the detectability of high-J CO lines from similar objects at high redshifts (z > ∼ 3).