A 21-CENTIMETER Study of the Spiral Galaxy Messier 33

The distribution of neutral atomic hydrogen and the kinematic properties of the late-type spiral galaxy M33 are derived from observations obtained at 21-cm wavelength with a multichannel radiometer attached to the 300-foot paraboloidal transit telescope of the National Radio Astronomy Observatory. The effective half-power beam- and bandwidths of the telescope system are 10,0 in &, 11 `.4 in , and 21 km Sr' in radial velocity. The assumed distance to M33 is 720 kpc. The galaxy contains H = 1.6 X 10 of neutral hydrogen distributed in a broad ring of radius 18' plus an underlying uniform distribution of radius 35' and two outlying "wings" or companions. The companions contain 10 of the total hydrogen content. The centers of the H I ring and of the distributions of Hii regions and luminous stars in M33 are offset by a few minutes of arc from the optical center of the galaxy. The systemic radial velocity is S = -180 km s ' (heliocentric). The rotation curve peaks at V = S + 85 km l, 30' from the galactic center. For an inclination of 550, this leads to a total mass of = 4.9 X 1010 o , of which 2.3 X 1010 is contained within 60' of the center (roughly the extent of the observations). An asymmetry of the rotation curves for the two halves of the major axis leads to a mass ratio of N/ 5 = 1.2. Noncircular motions are small in the main body of the galaxy. Velocities in the wings deviate by 40-50 km s ' from those predicted by the rotation curve. The data are consistent with a picture of the wings as stable gaseous companions of M33, gravitationally bound to the galaxy but not physically connected to the galactic plane. The global parameters of M33- li ,/ t = 0.03, H ,/L, = 0.38, and t17L5 = 11.5 (solar units)-are not significantly different from average values of the corresponding parameters for galaxies of similar structural type.