HIGH-DENSITY MOLECULAR GAS IN EXTERNAL GALAXIES - FORMALDEHYDE AND CARBON MONOSULFIDE

Observations in the 211-212 transition of ortho-formaldehyde (H2CO) were made to search for high-density (n > 104 cm-3) gas in extragalactic sources. We detected the line in absorption toward NGC 253 and in emission toward M82. Upper limits were found in the 2 cm line for NGC 3079, NGC 3628, and IC 4553. Combining 211-212 (2 cm) and 110-111 (6 cm) absorption data for the wings of the line in NGC 253 and for Cen A, we obtain densities on the order of 105 cm-3. A comparison of the 2 and 6 cm line profiles in NGC 253 shows that the central part of the 6 cm absorption profile may contain a superposed emission component. This feature is most likely a maser, pumped by the nonthermal radio continuum source associated with the nucleus of the galaxy. In M82 the 6 cm absorption and 2 cm emission indicate that low-density (<104 cm-3) as well as high-density (106 cm3) molecular gas is present. A high-resolution map of M82 in the CS J = 2-1 transition shows two spatially unresolved centers of emission which are part of a molecular ring. Comparisons of the formaldehyde and CS emissions suggest that the formaldehyde emission arises from a region 10″-15″ southwest of the nucleus. Also reported is the detection of the 303-202 transition from M82. This line is the first rotational line of formaldehyde and the first transition of para-H2CO observed in an extragalactic source. The detection of 2 cm emission in M82 is also a first for external galaxies.