Neutral hydrogen distribution and dynamics in galaxies. 2. Study of the spiral galaxy NGC 2403.

In the course of an observing program of hydrogen distributions and dynamics of nearby galaxies, the Sc spiral NGC 2403 was scanned in the 21 cm line with the transit telescope at . Receiving equipment, observing procedures and reduction methods are described in the same issue (preceeding paper). Half-power beamwidths are 4' in azimuth and 33' in elevation at the declination of NGC 2403. Beam efficiency is 0.5. Spectral resolution is 59 km/s. The r.m.s. noise after 5 m integration time is 0.15 0K. Amongst different series of hydrogen distributions investigated, the best fit to observed spectra is obtained for a gaussian distribution of 20' width between half-intensity points. The rotation law is given through a Brandt curve with a turnover point radius of 7' and a maximum rotational velocity of 135 km/s. The shape parameter n of the Brandt rotation curve was found to be 1.5, but its uncertainty is rather large owing to the poor spectral resolution. Within the errors quoted, the hydrogen and velocity models are symmetrical with respect to the center. The ring-shaped neutral hydrogen distribution found by M. S. Roberts at Green-Bank does not correspond with our results. We have computed model-proffles for this ring-distribution. Our measurements are inconsistent with the results of this computation and seem to rule out any annular distribution for this galaxy. To fit the annular model, our measurements should be at least twice noisier than they are. This conclusion stands whatever the velocity distribution may be. The total hydrogen mass is 2.5 x 10 M0 at a distance of 2.8 Mpc. The evaluated correction for finite optical depth was too small to be applied. The computed total mass (5.5 X 1010 M0.) is subject to a considerable error in connection with the uncertainty on the n parameter of the Brandt rotation curve. The morphological type of M 33 is very close to that of NGC 2403. However, hydrogen distributions of both galaxies are very different. This suggests an independance of hydrogen distribution with spiral structure. Key words: Galaxies - hydrogen La galaxie NGC 2403 (type Sc+) a 6 observee dans la raic 21 cm de l' l'aide du de (lobe de 4' X 33' a' mi-puissance a' 650 de ). Les distributions de vitesse de rotation, de densite' d' neutre atomique et de densite' de masse ont e't des observations par ca]cul de modeles. La masse d' neutre est de 2.5 X 10 M0.. La masse totale de 5.5 X 1010 M0.. La distribution d'hydroge'ne est diff6rente du type de distribution observe' affleurs dans les galaxies spirales: elle ne pas de minimum au centre. La comparaison avec une galaxie du me me type morphologique, M 33, une certaine entre la distribution d' et la structure spirale.