Star Formation in Ultraluminous Infrared Galaxies Probed with AKARI Near-infrared Spectroscopy

Abstract

We conducted systematic observations of the H I Brα line (4.05 μm) and the polycyclic aromatic hydrocarbon (PAH) feature (3.3 μm) in 50 nearby (z\lt 0.3) ultraluminous infrared galaxies (ULIRGs) with AKARI. The Brα line is predicted to be the brightest among the H I lines under conditions of high dust extinction ({A}_V\gt 15 mag). The Brα line traces ionizing photons from OB stars and so is used as an indicator of star formation on the assumption of the initial mass function. We detected the Brα line in 33 ULIRGs. The luminosity of the line ({L}_{Brα }) correlates well with that of the 3.3 μm PAH emission ({L}_3.3). Thus we utilize {L}_3.3 as an indicator of star formation in fainter objects where the Brα line is undetected. The mean {L}_{Brα }/{L}_{IR} ratio in LINERs/Seyferts is significantly lower than that in H II galaxies. This difference is reconfirmed with the {L}_3.3/{L}_{IR} ratio in the larger sample (46 galaxies). Using the ratios, we estimate that the contribution of starburst in LINERs/Seyferts is ∼ 67 % , and active galactic nuclei contribute the remaining ∼ 33 % . However, comparing the number of ionizing photons, {Q}_{Brα }, derived from {L}_{Brα } with that, Q _IR, expected from the star formation rate required to explain {L}_{IR}, we find that the mean {Q}_{Brα }/{Q}_{IR} ratio is only (55.5 ± 7.5)% even in H II galaxies, which are thought to be energized by pure starburst. This deficit of ionizing photons traced by the Brα line is significant even taking heavy dust extinction into consideration. We propose that dust within H II regions absorbs a significant fraction of ionizing photons.