An optical study of the galaxy M101. Derivation of a mass model from the kinematic of the gas.

Results are presented for observations of M101 with a Fabry-Perot interferometer in the H-alpha line and with a long-slit spectrograph in the H-alpha line, the N II lines at 6548 and 6584 A, and the S II lines at 6717 and 6731 A. Radial-velocity measurements are described, along with a derivation of the observed rotation curve. Potential gradients computed for standard disk and bulge populations are fitted to the velocity data, and the model rotation curve is compared with radio results. Masses, luminosities, and M/L ratios are estimated for the disk and the bulge component. The results obtained indicate a systematic velocity of about 238 km/s, a disk mass of the order of 100 billion solar masses, a bulge mass of approximately 2.7 billion solar masses, and a bulge M/L ratio (blue light) in the range from 1 to 9