Kinematics and composition of H II regions in spiral galaxies. II. M51, M101, and NGC 2403

We used the MX multi-object spectrometer to measure the velocities of 103 H II regions in M101 and 32 regions in NGC 2403, and to measure the excitations, log([O III]/Hβ), of 80 regions in M101, 30 in NGC 2403, and 15 in M51. For M101 and NGC 2403 we derive inclinations 17° ± 1° and 58° ± 1°, major-axis position angles 35° ± 1° and 126° ± 1°, rotation curves, line-of-sight velocity dispersions 11 ± 3 and 13 ± 4 km s^-1, and mass-to-light ratios in solar units for the optical disks, between 5 and 6 (± 1) for M101 and between 1 and 3 (± 1) for NGC 2403. We demonstrate that late-type spiral galaxies can be divided into two distinct groups based solely on the excitation of their H II regions at a particular fractional isophotal radius. The two groups are analyzed for clues to the cause of the wide variation in excitation. We also discuss excitation variations along the spiral arms of M101 and present evidence that the excitation gradients of M33 and M101 steepen significantly toward the center of the galaxy. Finally, we use the excitation measurements to estimate the magnitude of the abundance gradients in these galaxies. A comparison between our measurement of the abundance gradients in M33, M101, NGC 2403, and NGC 2997 and previous determinations supports our assertion that the abundance gradients obtained by using excitation measurements of more than 30 H II regions per galaxy, are at least as accurate and precise as those presented in the literature. We demonstrate that for late-type spiral galaxies the slope of the abundance gradient correlates with luminosity.