Observations of CO J = 1–0 near z = 2.5: constraints on the ISM of high-redshift galaxies

¹ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
² Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Rd., Piscataway, NJ 08854, USA

Abstract

We discuss observations of the CO J =  1–0 line from massive galaxies in the early Universe. In the submillimeter galaxies (SMGs) we have observed, the CO line intensity ratios rule out low-J emission dominated by subthermally excited gas. This implies that an interpretation of subthermally excited gas from the 2–1/1–0 line ratio, the basis for the mass density conversion factor derived for ULIRGs in the local universe, is unlikely to apply to SMGs. Our observations can be more physically explained by interstellar media with different filling factors for different gas components.