SIMBAD references

We use some of the maps of the catalogue presented in Paper I to provide some evidence for global conditions that must be fulfilled by the galaxies to have extended hydrogen. For this purpose, we tried to find possible connections between the HI gas extension and other properties of the galaxies (morphological type, surface brightness, gas density, etc.). With isophotal hydrogen diameters of a large sample, we could observe that optically smaller galaxies seem to have greater relative HI extensions. By means of the relation with the apparent HI surface density, we found an expression that should provide a rough estimate of the gas extension. With respect to the dependence on morphological type, we could not find any significant correlation either for the real HI surface density or the relative gas extension. Nevertheless, whereas for spiral and irregular galaxies the real HI surface density exhibits a broad range of values, the values are rather lower for elliptical and S0 galaxies.