Dense gas in nearby galaxies - XII. A survey for CO J=3-2 emission

The J = 3 - 2 sub-mm line of (CO)-C-12 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, I-32, are compar ed to 1 - 0 and 2 - 1 fluxes measured with the IRAM 30-m telescope. Model c alculations show that the I-32/I-10 ratio can be used as a measure of the a verage H-2 density. For most sources observed, I-32/I-10 is in the range 0. 2-0.7, which is predicted if kinetic temperatures are < 50 K and/or H-2 den sities < 1000 cm(-3). Our measurements of the J = 1 - 0, 2 - 1, and 3 - 2 lines toward M 82 do no t 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 rat io is close to unity. Unlike single dish data from the lower excited CO lin es, the CO 3 - 2 profile toward Arp 220 shows two velocity components, poss ibly arising from the edges of its molecular disk.