Atomic Carbon in ARP 220

Abstract

We present detections of the ³P₁-³P₀ fine-structure line of neutral carbon and of the J=3-2 rotational line of carbon monoxide in the ultraluminous infrared galaxy Arp 220, obtained at the Caltech Submillimeter Observatory. C I emission in Arp 220 is found to be at the expected level in relation to the strong far-infrared (FIR) dust emission and to the CO emission, whereas the C II emission seen by the Infrared Space Observatory long-wavelength spectrometer is very weak. The C I data confirm the presence of a large mass of molecular gas in the nucleus of Arp 220, corresponding to a high extinction in the visible (A_v=1000). For this galaxy, the total cooling due to atomic carbon is comparable to the total cooling due to carbon monoxide. We use the observed C I/CO (1-0) and CO (3-2)/CO (1-0) emissivity ratios, together with the photon-dominated region models, to infer an average gas density of n(H₂)~10⁴ cm^-3. We estimate the UV radiation field in this galaxy from the FIR continuum, and independently from hydrogen recombination lines, to be about a factor of 40,000 larger than in the solar vicinity (G₀=4×10⁴). The relative weakness of the C II fine-structure line is explained by the large gas density, the exceptionally strong UV field, and the large dust opacity. C II emission from high-redshift analogs of Arp 220 may not be as easy to detect as had been anticipated. However, both CO and C I lines will still be strong emitters.