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 (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 very sensitive probes of gas excitation; in the case of HR10, 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). Subject headings: galaxies: evolution — galaxies: individual (HR10) — galaxies: ISM — galaxies: starburst — infrared: galaxies