Extragalactic carbon monoxide

We report observations of 2.6 mm carbon monoxide emission in five galaxies: M82, NGC 253, M51, NGC 5236, and NGC 1068. An extensive map of the emission from M82 is presented, along with less extensive maps of NGC 253 and M51. Models for the CO distributions have been fitted to the mapping results for M82 and M51. Comparisons of OH, H2CO, and CO observations toward M82 and NGC 253 show that relative molecular abundances are similar to their Galactic values. The central CO profile in M82 closely resembles an averaged Hα emission profile for the same region. The estimated mass in molecular clouds is found to be greater than that in atomic hydrogen for most of the regions studied. Total CO column densities are estimated for the five galaxies, and a possible correlation is noted between these column densities and the nuclear 10 μm luminosities. Despite sensitive searches, no 13CO emission is observed toward NGC 253 and M82. These negative results imply either upper limits approx.10 for the optical depth of the 12CO features or lower limits to the [12C]/[13C] ratios approx.15 and approx.40 in M82 and NGC 253, respectively. Negative results for CO emission are reported for 24 other galaxies, including M31 and M63