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

Abstract

We performed Hα imaging observations of 22 luminous infrared galaxies to investigate how the distribution of star-forming regions in these galaxies is related to galaxy interactions. Based on correlation diagrams between Ha flux and continuum emission for individual galaxies, a sequence for the distribution of star-forming regions was found: very compact (∼ 100 pc) nuclear starbursts with almost no star-forming activity in the outer regions (type 1), dominant nuclear starbursts ≲ 1 kpc in size with a negligible contribution from the outer regions (type 2), nuclear starbursts ≳ 1 kpc in size with a significant contribution from the outer regions (type 3), and extended starbursts with relatively faint nuclei (type 4). These classes of star-forming regions were found to be strongly related to global star-forming properties, such as star formation efficiency, far-infrared color, and dust extinction. There was a clear tendency for the objects with more compact distributions of star-forming regions to show a higher star formation efficiency and hotter far-infrared color. An appreciable fraction of the sample objects were dominated by extended starbursts (type 4), which is unexpected in the standard scenario of interaction-induced starburst galaxies. We also found that the distribution of star-forming regions was weakly but clearly related to galaxy morphology: severely disturbed objects had a more concentrated distribution of star-forming regions. This suggests that the properties of galaxy interactions, such as dynamical phase and orbital parameters, play a more important role than the internal properties of progenitor galaxies, such as dynamical structure or gas mass fraction. We also discuss the evolution of the distribution of star-forming regions in interacting galaxies.