Galactic mass determinations from incomplete rotation curves

Given an incomplete rotation curve of a spiral galaxy, various assumptions about the Galaxy beyond the last observed point are made: (A) the force falls off as 1/r2, (B) the mass density is zero, and (C) the mass density falls of as 1/r3. The mass distributions obtained from each of these assumptions are all well behaved, and it is possible to choose the correct curve from considerations of the resulting mass distributions alone. The correct mass distribution in the disk system of a galaxy cannot be deduced from an incomplete rotation curve. (Auth.)