A

Abstract

The total mass and total extent of galaxies (including their dark halos) are fundamental parameters that are completely unknown for all galaxies. The best estimates we have for spiral and dwarf irregular galaxies come from detailed mass models using extended HI rotation curves. But, in every galaxy studied so far, such analysis only succeeded to derive lower limits of the total mass and total extent of their dark halo out to the last measured velocity point of the rotation curves which are still flat or even rising, implying that more dark mass is present at larger radii. While it is conceivable that the total mass of galaxies could be de-termined by dynamical methods (satellite galaxies, galaxies in pairs or in groups), the main hope to derive the total extent is that, at least in some galaxies, the HI extends all the way to the edge of the halo (and still be detectable) so that the kinematics show the expected Keplerian decline if all the mass has been encountered.