We performed systematic observations of the Hi Brα line (4.05 μm) in 51 nearby (z<0.3) ultraluminous infrared galaxies (ULIRGs), using AKARI near-infrared spectroscopy. The Brα line is predicted to be the brightest among the Hi recombination lines in ULIRGs with visual extinction higher than 15 mag. We detected the Brα line in 33 ULIRGs. In these galaxies, the relative contribution of starburst to the total infrared luminosity (LIR) is estimated on the basis of the ratio of the Brα line luminosity (LBrα) to LIR. The mean LBrα/LIR ratio in LINERs or Seyferts is significantly lower (~50%) than that in Hii galaxies. This result indicates that active galactic nuclei contribute signicantly (~50%) to LIR in LINERs, as well as Seyferts. We also estimate the absolute contribution of starburst to LIR using the ratio of star formation rates (SFRs) derived from LBrα (SFRBrα) and those needed to explain LIR (SFRIR). The mean SFRBrα/SFRIR ratio is only 0.33 even in Hii galaxies, where starburst is supposed to dominate the luminosity. We attribute this apparently low SFRBrα/SFRIR ratio to the absorption of ionizing photons by dust within Hii regions.

ABSTRACT
1. INTRODUCTION
2. OBSERVATION AND RESULTS
3. DISCUSSION
    3.1. Effectiveness of IR Observations
    3.2. Relative Contribution of Starburst to LIR
    3.3. Absolute Contribution of Starburst to LIR
REFERENCES

• Kenichi Yano(Department of Physics, Graduate School of Science, The University of Tokyo/Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• Takao Nakagawa(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• Naoki Isobe(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• Mai Shirahata(National Astronomical Observatory of Japan)