Star Formation in Ultraluminous Infrared Galaxies Probed with AKARI Near-Infrared Spectroscopy
Abstract
We conducted systematic observations of the HI Br-alpha line (4.05 micron) and the polycyclic aromatic hydrocarbon (PAH) feature (3.3 micron) in 50 nearby (z<0.3) ultraluminous infrared galaxies (ULIRGs) with AKARI. The Br-alpha line is predicted to be the brightest among the HI lines under high dust-extinction conditions (AV>15 mag). The Br-alpha 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 Br-alpha line in 33 ULIRGs. The luminosity of the line (LBrA) correlates well with that of the 3.3 micron PAH emission (L3.3). Thus we utilize L3.3 as an indicator of star formation in fainter objects where the Br-alpha line is undetected. The mean LBrA/LIR ratio in LINERs/Seyferts is significantly lower than that in HII galaxies. This difference is reconfirmed with the L3.3/LIR 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 to the remaining ~33%. However, comparing the number of ionizing photons, QBrA, derived from LBrA with that, QIR, expected from star formation rate required to explain LIR, we find that the mean QBrA/QIR ratio is only 55.5+/-7.5% even in HII galaxies which are thought to be energized by pure starburst. This deficit of ionizing photons traced by the Br-alpha line is significant even taking heavy dust extinction into consideration. We propose that dust within HII regions absorbs a significant fraction of ionizing photons.
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