|Annu. Rev. Astron. Astrophys. 1978. 16: Copyright © 1978 by .
All rights reserved 3.2. Rotation Curves in the Outer Parts of
Spiral Galaxies Radio measurements of the HI 21-cm line
velocities in galaxies have considerably increased the radial
distance to which rotation curves can be obtained; optically
derived rotation curves rarely extend beyond ~ 1/3RHo, while in
HI they often reach beyond RHo. Before such observations were
available it was usually assumed that at large radii the
rotation curves eventually became Keplerian, Vrot R-1/2. In fact
actual rotation curves are now found to decline much more slowly
or even not at all (Roberts 1975b, Huchtmeier 1975, Combes et
al. 1977). Bosma (1978a, b) has recently analyzed HI velocity
fields in a sample of about 20 galaxies, based on data from new
observations with the WSRT and on results available in the
literature. His results can be summarized as follows: The
rotation curves are almost always either flat or slowly
declining in the outer parts. (3) Constructio