The Westerbork HI survey of spiral and irregular galaxies III. HI observations of early-type disk galaxies

We present H observations of 68 early-type disk galaxies from the WHISP survey. They have morphological types between S0 and Sab and absolute B-band magnitudes between –14 and –22. These galaxies form the massive, high surface-brightness extreme of the disk galaxy population, few of which have been imaged in H before. The H properties of the galaxies in our sample span a large range; the average values of MHI/LB and DHI/D25 are comparable to the ones found in later-type spirals, but the dispersions around the mean are larger. No significant differences are found between the S0/ S0a and the Sa/Sab galaxies. Our early-type disk galaxies follow the same H mass-diameter relation as later-type spiral galaxies, but their effective H surface densities are slightly lower than those found in later-type systems. In some galaxies, distinct rings of H emission coincide with regions of enhanced star formation, even though the average gas densities are far below the threshold of star formation derived by Kennicutt (1989, ApJ, 344, 685). Apparently, additional mechanisms, as yet unknown, regulate star formation at low surface densities. Many of the galaxies in our sample have lopsided gas morphologies; in most cases this can be linked to recent or ongoing interactions or merger events. Asymmetries are rare in quiescent galaxies. Kinematic lopsidedness is rare, both in interacting and isolated systems. In the appendix, we present an atlas of the H observations: for all galaxies we show H surface density maps, global profiles, velocity fields and radial surface density profiles.