A Study of the Distribution of Star-forming Regions in Luminous Infrared Galaxies by Means of H Imaging Observations

We performed H-alpha imaging observations of 22 luminous infrared galaxies toinvestigate how the distribution of star-forming regions in these galaxies isrelated to galaxy interactions. Based on correlation diagrams between H-alphaflux and continuum emission for individual galaxies, a sequence for thedistribution of star-forming regions was found: very compact (~100 pc) nuclearstarbursts with almost no star-forming activity in the outer regions (type 1),dominant nuclear starbursts < 1 kpc in size and a negligible contribution fromthe outer regions (type 2), nuclear starbursts > 1 kpc in size and asignificant contribution from the outer regions (type 3), and extendedstarbursts with relatively faint nuclei (type 4). These classes of star-formingregion were found to be strongly related to global star-forming properties suchas star-formation efficiency, far-infrared color, and dust extinction. Therewas a clear tendency for the objects with more compact distributions ofstar-forming regions to show a higher star-formation efficiency and hotterfar-infrared color. An appreciable fraction of the sample objects weredominated by extended starbursts (type 4), which is unexpected in the standardscenario of interaction-induced starburst galaxies. We also found that thedistribution of star-forming regions was weakly but clearly related to galaxymorphology: severely disturbed objects had a more concentrated distribution ofstar-forming regions. This suggests that the properties of galaxy interactions,such as dynamical phase and orbital parameters, play a more important role thanthe internal properties of progenitor galaxies, such as dynamical structure orgas mass fraction. We also discuss the evolution of the distribution ofstar-forming regions in interacting galaxies.