1 Introduction

The Magellanic Clouds were the first extragalactic objects observed and mapped in the 21-cm line (Kerr & Hindman 1953; Kerr et al. 1954). Soon, the large angular extension of some nearby objects enabled one to obtain other detailed maps of the HI gas. At present, the density distribution of neutral hydrogen is known for a large number of galaxies.

Since neutral hydrogen is typically found farther out in the disk than optical components, it constitutes an important tool in the study of the dynamics of a galaxy. With respect to the morphology, some high-resolution studies were able of distinguish structures like spiral arms, bars, bubbles, holes of HI, etc. In some galaxies, gas appendages are often seen to extend outward from the disks, like "plumes'', "tails", connecting "bridges" and other such features. Some of these appendages have been explained in terms of tidal interactions amongst neighbouring galaxies, but not all. Other studies have shown that some galaxies exhibit enormously extended HI disks, with important implications. The use of the interstellar gas as a tracer of the gravitational potential produced by the total content of the galaxy, by means of rotation curves or velocity fields, is dynamically important. Only if we know more details of the structure and distribution of the different components of a galaxy, particularly the HI gas, have we an insight into the question of galaxy evolution.

Our initial purpose was to study the gas distribution in the galaxies, to find any possible connection between the extension of the HI gas and other properties of the galaxies (morphological type, size, surface brightness, etc.) using a large sample. To achieve this, we began by taking into account those papers in which it was possible to consider a gaussian distribution for a simpler analysis, as is shown in Paper II. However, during the bibliographical search, we found in the literature many papers related to studies of the HI distribution in galaxies, most of which were not useful for our initial purpose. Nevertheless, and because of the lack of any catalogue providing information about large-scale HI observations in extragalactic systems, we considered that it would be interesting to collect the references, to provide astronomers a guide to the HI maps of galaxies published up to 1995.

The maps collected for this catalogue are described in Sect. 2. The information extracted from the literature and the measurements made on the maps are presented in Sect. 3. Finally, the data are briefly discussed in Sect. 4.