Massive Star Formation in Luminous Infrared Galaxies: Giant H II Regions and Their Relation to Super-Star Clusters^*

We have used Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer broadband (at 1.6 μm) and narrowband Paα (λ_rest = 1.87 μm) images to identify, respectively, star clusters and H II regions in a sample of eight luminous infrared galaxies (LIRGs). These observations have revealed the presence of a large population of super–star clusters and bright H II regions. A significant fraction of the H II regions shows Hα luminosities above that of 30 Doradus, the prototypical giant H II region. The excess of extremely luminous H II regions in LIRGs has been confirmed by comparison with normal galaxies observed at similar spatial resolutions. Despite the large numbers of identified star clusters and H II regions in LIRGs, we find only a small fraction of coincidences, between 4% and 30% of the total number of detected sources. Using evolutionary synthesis models we have reproduced the relative fractions of young H II regions and intermediate and old star clusters observed in Arp 299 and the central region NGC 3256 by using a Salpeter initial mass function and instantaneous star formation. H II regions with no detected near-infrared cluster counterpart (25%–39% of the detected sources) represent the youngest sites of star formation, with ages up to approximately 5 Myr and mostly intermediate mass (≃10⁵ M_⊙) ionizing clusters. For these two galaxies, within the present detection threshold we can detect only coincidences (4%–10% of the detected sources) between an H II region and a near-infrared star cluster for the most massive star clusters (≃10⁶ M_⊙) during the first 7 Myr of their evolution. If there is significant extinction during the first million years, we may not detect the youngest star-forming regions, and hence the observed fractions of H II regions and coincidences will be lower limits. The identified near-infrared super–star clusters with no detectable Paα emission represent the "old" population (53%–66% of the detected sources), with ages between 7 and 20–40 Myr. Older clusters possibly created in this or previous episodes of star formation are likely to exist in these systems but cannot be identified with the present detection threshold. Our study demonstrates that Paα narrowband imaging of LIRGs and interacting galaxies identifies the youngest sites of star formation that could be otherwise missed by near-infrared broadband continuum surveys.