1999A&A...341..256M

The J=3-2 sub-mm line of ¹²CO has been observed with the Heinrich-Hertz-Telescope (HHT) toward a sample of 28 nearby galaxies, and toward Arp 220. All sources are detected. This is the largest extragalactic sample of CO 3-2 spectra published so far. The 3-2 line fluxes, I₃₂, are compared to 1-0 and 2-1 fluxes measured with the IRAM 30-m telescope. Model calculations show that the I₃₂/I₁₀ ratio can be used as a measure of the average H₂ density. For most sources observed, I₃₂/I₁₀ is in the range 0.2-0.7, which is predicted if kinetic temperatures are <50K and/or H₂ densities <1000cm^–3. Our measurements of the J=1-0, 2-1, and 3-2 lines toward M 82 do not support earlier claims of 2-1/1-0 line ratios much larger than 1. As in other active galaxies (NGC 253, Arp 220) the measured line intensity ratio is close to unity. Unlike single dish data from the lower excited CO lines, the CO 3-2 profile toward Arp 220 shows two velocity components, possibly arising from the edges of its molecular disk.