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

Abstract

We examine the physical conditions of the molecular gas in the extremely red object (ERO), HR10 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 HR10, 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).