Annu. Rev. Astron. Astrophys. 1982. 20: 517-45 Copyright © 1982 by Annual Reviews. All rights reserved

2.1. OH Absorption Surveys

The first observation of an extragalactic molecule was by Weliachew (1971), who used an interferometer to detect OH absorption toward the nuclear continuum sources of NGC 253 and M82. As Weliachew pointed out, OH has the advantage that its detectability depends only on the absorption optical depth (independent of the galaxy' s distance) and on the strength of the background continuum. Searches for OH have thus concentrated on galaxies with strong continuum sources. The principal disadvantage of OH surveys (aside from the obvious selection effect) is that they sample only a limited region in front of the nucleus. In order to get useful information on the distribution of gas in the central regions, one must treat the data statistically.

For some time after the initial work on OH absorption, only Whiteoak & Gardner (1974, 1975a, 1976b) and Gardner & Whiteoak (1975, 1976b) reported further searches. Because of their aperture size, they were limited to a few galaxies with very strong continuum emission. Their work is discussed further in Section 4. More recently, Rickard et al. (1982) used the Arecibo Observatory in a survey of all accessible late-type galaxies whose continuum fluxes are sufficient to ensure detection of OH lines with optical depths of a few percent. The 5 galaxies detected (of 13 searched) are mostly those in which a large fraction of the total continuum flux comes from a compact nuclear source. Together with the line-width data, this indicates the predominance of molecular clouds close to the nucleus. Assuming that the central OH source is a homogeneous, oblate ellipsoid of major-to-minor axis ratio 15 [similar to the Liszt & Burton (1978) model for the nuclear disk of our Galaxy]. Rickard et al. reduce the OH optical depths to equatorial optical depths, _eq. The basic effect controlling the detection rate then seems simply to be the strength of the compact nuclear continuum source; wherever the compact source is strong enough, OH is detected. They also find slight evidence for a bimodal distribution, some galaxies having _eq 0.17 and others having _eq 0.07. The basic implication, however, is that either central molecular sources are nearly universal in spiral galaxies, or else there is a strong correlation between the presence of molecular clouds and the presence of strong radio continuum sources in galactic nuclei.

The distribution of detections among morphological types shows a slight preference for the earlier Hubble types, most likely due to the tendency for such earlier types to have stronger nuclear sources.

Extensions of the survey are still in progress. Using the Nançay telescope to cover higher declinations and higher redshifts. Turner et al. (in preparation) CO Emission Surveys have detected OH absorption in galaxies as distant as NGC 7469 (63 Mpc). The volume of space accessible to molecular studies is thus about 10% of that commonly studied in HI.