2010ApJ...711..757P

We report new single dish CO J = 6-5 line observations for the archetypal Ultra Luminous Infrared Galaxy (ULIRG) Arp 220 with the James Clerk Maxwell Telescope atop Mauna Kea in Hawaii. The J = 6-5 line is found to be faint, with brightness temperature ratios (6-5)/(1-0), (6-5)/(3-2) of R_65/10 = 0.080±0.017 and R_65/32= 0.082±0.019, suggesting very low excitation conditions that cannot be reconciled with the warm and very dense molecular gas present in one of the most extreme starbursts in the local universe. We find that an optically thick dust continuum, with τ(ν ≳ 350 GHz) ≳ 1 for the bulk of the warm dust and gas in Arp 220, submerges this line to an almost black body curve, reducing its flux, and affecting its CO spectral line energy distribution at high frequencies. This also resolves the C⁺ line deficiency in this object, first observed by Infrared Space Observatory: the near absence of that line is a dust optical depth effect, not a dense photodissociation region phenomenon. Finally, we briefly comment on the possibility of such extreme interstellar medium (ISM) states in other ULIRGs in the distant universe, and their consequences for the diagnostic utility of high frequency molecular and atomic ISM lines in such systems. In the case of Arp 220, we anticipate that the now spaceborne Herschel Space Observatory will find faint high-J CO lines at ν ≳ 690 GHz that would appear as sub-thermally excited with respect to the low-J ones as a result of the effects of dust absorption.