On the Keplerian Rotation Curves of Galaxies

We propose a criterion for examining whether or not the uncertainty of theouter rotation curves is sufficiently small to distinguish a Keplerian rotationcurve from flat ones. We have applied this criterion to both Keplerian andnon-Keplerian rotation curves so far obtained, and investigated their relativefraction. We also studied the minimum extent of the dark halos indicated bynon-Keplerian rotation curves using the criterion. We have found that onecannot rule out the possibility that a significant fraction of rotation curvesbecome Keplerian within 10-times the disk scale length. If the Keplerianrotation curves so far observed trace the mass truncation, several galaxies mayhave rather small halos, the extent of which is not larger than twice that ofthe optical disk.Comment: Latex, 6 pages and 3 figures, to appear in PASJ Vol.49 No.