Comparative internal kinematics of the H II regions in interacting and isolated galaxies : implications for massive star formation modes

We have observed 12 interacting galaxy pairs using the Fabry-Perot interferometer GH alpha FaS (Galaxy H alpha Fabry-Perot system) on the 4.2-m William Herschel Telescope at the Observatorio del Roque de los Muchachos, La Palma. We present here the H alpha surface brightness, velocity and velocity dispersion maps for the 10 systems we have not previously observed using this technique, as well as the physical properties (sizes, H alpha luminosities and velocity dispersion) of 1259 H II regions from the full sample. We also derive the physical properties of 1054 H II regions in a sample of 28 isolated galaxies observed with the same instrument in order to compare the two populations of H II regions. We find a population of the brightest H II regions for which the scaling relations, for example the relation between the H alpha luminosity and the radius, are clearly distinct from the relations for the regions of lower luminosity. The regions in this bright population are more frequent in the interacting galaxies. We find that the turbulence, and also the star formation rate (SFR), are enhanced in the H II regions in the interacting galaxies. We have also extracted the H alpha equivalent widths for the H II regions of both samples, and we have found that the distribution of H II region ages coincides for the two samples of galaxies. We suggest that the SFR enhancement is brought about by gas flows induced by the interactions, which give rise to gravitationally bound gas clouds which grow further by accretion from the flowing gas, producing conditions favourable to star formation.