From the relation between the relative HI diameter and the apparent HI surface density, we give useful expression that predicts roughly how extended a galaxy will be in HI.
We find no dependence of the gas extension on morphological type. Instead, there is a broad dispersion within different types. The galaxies with large HI extensions run from S0 to Irr.
The real surface masses of hydrogen in the sample have values between 0.4 and 6.8 1020 at cm-2. There is a broad range of values for Spiral and Irr galaxies, but for Elliptical and S0 galaxies, the real surface masses are generally low.Acknowledgements
We would like to thank J.C. Olano and J.C. Olalde for reading the paper carefully, J.C. Cersosimo and J.C. Testori for valuable material and discussions, and C. Cristina Miguel and M. Fumagalli for computational support.
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