The extended rotation curve and the dark matter halo of M33

We present the 21-cm rotation curve of the nearby galaxy M33 out to a galac tocentric distance of 16 kpc (13 disc scalelengths). The rotation curve kee ps rising out to the last measured point and implies a dark halo mass great er than or similar to 5 x 10(10) M-circle dot. The stellar and gaseous disc s provide virtually equal contributions to the galaxy gravitational potenti al at large galactocentric radii, but no obvious correlation is found betwe en the radial distribution of dark matter and the distribution of stars or gas. Results of the best fit to the mass distribution in M33 picture a dark halo which controls the gravitational potential from 3 kpc outward, with a matt er density which decreases radially as R-1.3. The density profile is consis tent with the theoretical predictions for structure formation in hierarchic al clustering cold dark matter (CDM) models, and favours lower mass concent rations than those expected in the standard cosmogony.