1 Introduction

The outer galactic HI layer is without doubt an important tracer of the system kinematics and provides crucial evidence for the presence of dark matter in the outer parts of the galaxies. At present, the search for extensive emission of neutral hydrogen in the majority of galaxies shows that the gas does not spread farther than 2 or 3 Holmberg radii. This so for isolated galaxies, for systems in a group or galaxies in interaction (Hewitt et al. 1983; Briggs et al. 1980). However, some galaxies have been found that exhibit very large HI envelopes, like DDO 154 (Krumm & Burstein 1984), NGC 262 (Morris & Wannier 1980), NGC 5236 (Bottinelli & Gouguenheim 1973), NGC 4449 (Bajaja et al. 1994), etc. Some of these extensions may be present as envelopes, warps, tails or bridges. These features can easily be explained when the galaxy has a clear interaction with another one, but it is difficult when there are no neighbouring galaxies.

The search for relations between the gas extension and other properties of galaxies has not been fruitful as yet. Particularly, it is not clear if the ratio of HI to optical diameter of a galaxy depends on its morphological type. With respect to this dependence, the following results have been found by different authors: the relative gas extension shows a slight variation along the whole morphological sequence (Fouqué 1982, 1983); the dependence is only for spiral galaxies (Bottinelli 1971) or only for early-type galaxies (Balkowski et al. 1972; Balkowski 1979; Krumm & Salpeter 1980); the relative gas extension does not depend at all on the morphological type (Krumm & Salpeter 1979; Bosma 1981; Hewitt et al. 1983). Furthermore, it was found that irregular galaxies are frequently rich in neutral hydrogen gas and some of them have HI distributions extended well beyond their optical boundaries (Huchtmeier et al. 1981; Gallagher & Hunter 1984).

To sum it up, different results so far have shown that the relative size of the gas does not appear to have a clear dependence on any physical properties of the galaxies, such as surface brightness, luminosity, etc. (Haynes et al. 1984; Sancisi 1987; Giovanelli & Haynes 1988). The exception is for the morphology, although the dependence, if real, is very weak and difficult to discern, as discussed above.

With the aim to improve the relationships of these types, we analyse a large sample of galaxies. This will be useful to find the condition of large HI disks.

The sample selected for this work is analysed in Sect. 2. The choice of the distance concerning the parameters used here, is discussed in Sect. 3. Finally the results obtained are presented in Sect. 4 and the conclusion in Sect. 5.