Spiral structure of M 51 : distribution and kinematics of the atomic and

The atomic hydrogen (HI) and the Hα emission lines in the grand-design spiral galaxy M51 have been observed with the Westerbork Synthesis Radio Telescope and the TAURUS Fabry-Perot imaging spectrometer, respectively. Across the inner spiral arms significant tangential and radial velocity gradients are detected in the Hα emission after subtraction of an axi-symmetric component of the velocity field. These gradients are similar to the ones observed recently in CO across the inner northern spiral arm, and resemble streaming motions expected from a density-wave shock: the gas is moving inward and faster compared to pure circular rotation after passing through the spiral arm. The amplitudes of the velocity shifts are very large, reaching 60-90 km s^-1^ in the plane of the galaxy. We discuss more extensively a scenario which we have previously proposed to account for the ~500 pc displacements observed between the spiral arms in HI and the nonthermal radio continuum and/or the dust lanes in M51 and M83. The scenario describes the observations in the framework of density- wave streaming of the interstellar gas. Presuming the nonthermal continuum ridge and/or dust lane trace the locus of maximum density of the interstellar gas, the observations are consistent with the view that most of the HI in the inner spiral arms of M51 and M83 is a product of the dissociation of predominantly molecular gas by the star-formation process. This explains the coincidence observed between the HI arms and the ionized arms. A simple quantitative model of the dissociation process yields the observed HI column densities. We also investigate several alternative explanations involving peculiar geometry and radiative transfer effects, but find them inadequate or unlikely to explain the observed displacement of the HI from the spiral shock.