| Abstract | Summary. The nearby Sc-type galaxy NGC 598 (M33) has been observed within the 21-cm line of neutral atomic hydrogen with the 200-m Nan cay transit telescope of Meudon Observatory. The effective half- power beamwidth and bandwidth of the telescope are respectively 4' in right ascension, 24' in declination and 12.7 km/s in radial velocity. The distribution of neutral hydrogen (H i) and the kinematical properties of the galaxy have been derived. The assumed distance is 720 kpc, and the mass of H i is found to be 1.8 x 10 M0 being 4.5 % of the total mass of 4.0 x 1010 M0. H i could be represented by a large double ring structure overlaid on a uniform disk with a central density depression of 35 %. A more detailed structure could be derived by using a model-fitting procedure. The high-density component of the H 1 distribution of M33 has a mean radius of 32' being located within a large extended component of low density reaching a greatest radius of 55' for 2 % of the H 1 peak. In the level shows a deviation from the optical major axis of the galaxy in the form of wings. The observed beam- averaged column density within this shell of neutral hydrogen is from 1 x 1O' atoms to 20 x 1O' atoms in the wings. The systematic radial velocity is -180 km/s; the maximum circular velocity is found to be 102 km/s at 30' from the centre of the galaxy. A Brandt-type rotation curve with a shape parameter n = 1.4 has been fitted to the observations. Non-circular motions are small in the main body of the galaxy. Velocities in the wings deviate by up to 80 km/s from those predicted by the rotation law. Deviations from circular motions of 12 km/s are found within the western optical arms of the galaxy. A velocity dispersion of 14 + 1 km/s has been found for most parts of the galaxy, growing to 28 km/s in the shell of H i. Key words: galaxies - neutral hydrogen - kinematics north-west and south-east of the galaxy even the 10 % |
