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.
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