THE NEUTRAL HYDROGEN CONTENT OF EARLY TYPE DISK GALAXIES

This paper presents the results of a sensitive 21 cm survey of massive early type galaxies made with the Arecibo radio telescope 2. Of the 81 galaxies observed, the detections comprise 48% of the SO's, 73% of the SOa's, and 96% of the Sa's. Only a few of the detections would have been missed by previous surveys due to extraordinarily wide spectral line profiles, so it is unlikely that a significant population of massive galaxies have gone undetected due to this instrumental selection effect. The relatively high detection rate enabled the large range in neutral hydrogen content found for SO galaxies to be investigated. The values of the hybrid, distance-independent H I surface densities of the SO galaxies in our sample ranged continuously from amounts comparable to the most gas-rich Sa galaxies to low estimated upper limits of the H I content. CCD images of most of the gas-rich SO's revealed either faint spiral features or patchy structure in the disks. While no firm correlation between H I content and environmental density is apparent for the galaxies in our sample, two-sample statistics suggest a difference between the highest and the lowest density bins. Early type disk galaxies within low density environments tend to have higher gas surface densities than those within high density environments.