4 Comments on the catalogue

This catalogue contains information for about 1400 galaxies, with more than 500 references.

For the first twenty years, single-dish telescopes provided the vast majority of the HI data. As can be seen in the catalogue, most of the observations were provided by Green Bank 91-m, Jodrell Bank, Nançay, Effelsberg, Parkes and Arecibo telescopes. Later, with the requirement of higher resolution, the techniques of spectral

line aperture synthesis appeared, developed especially at Cambridge, Green Bank, Owens Valley, Westerbork, NRAO (VLA) and more recently, in Australia (ATCA). With these large telescopes, particular structures such as holes, bubbles, HI concentrations in spiral arms, etc., started to be resolved. More galaxies seem to exhibit large extensions of gas, such as plumes or long tails, that cannot be classified as bridges, because there are no obvious optical companions. With the high spatial resolution of these large telescopes, HI images of uncatalogued small galaxies have also appeared in the observational fields. Suitable deep optical images and/or redshifts are needed to confirm the existence of these galaxies.

Some galaxies have several observations made by different authors and at telescopes. In these cases, we can see a general increase of the HI extension as the sensitivity increases. The extension of gas undoubtedly depends on the sensitivity reached in the observation. When dealing with large extensions of gas, it is necessary to specify the sensitivity level. There is also an apparent increase of the gas extension as the resolution of the telescope decreases. In any case, despite the dependence of the gas extension on the telescope beam and on the observational sensitivity level, a first glance at the catalogue shows that the neutral hydrogen gas is typically detected farther out than the optical component. The exceptions are clear for the galaxies of the Virgo Cluster NGC 4438, NGC 4569 and NGC 4388 which, beyond any doubt, have gas deficiencies.

Acknowledgements

We are grateful to the referee, Dr. W.K. Huchmeier, for valuable suggestions. I also appreciate important remarks on the text by Dr. C. Olano. Thanks also to Dr. E. Bajaja for useful comments on the original manuscript.