Star formation in ultraluminous infrared galaxies probed with AKARI near-infrared spectroscopy
- Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
- Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210 (Japan)
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<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{sub V}>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{sub Brα}) correlates well with that of the 3.3 μm PAH emission (L{sub 3.3}). Thus we utilize L{sub 3.3} as an indicator of star formation in fainter objects where the Brα line is undetected. The mean L{sub Brα}/L{sub IR} ratio in LINERs/Seyferts is significantly lower than that in H ii galaxies. This difference is reconfirmed with the L{sub 3.3}/L{sub 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{sub Brα}, derived from L{sub Brα} with that, Q {sub IR}, expected from the star formation rate required to explain L{sub IR}, we find that the mean Q{sub Brα}/Q{sub 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.
- OSTI ID:
- 22869621
- Journal Information:
- Astrophysical Journal, Vol. 833, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
AKARI IRC 2.5-5 μm spectroscopy of infrared galaxies over a wide luminosity range
Mid-infrared properties of luminous infrared galaxies. II. Probing the dust and gas physics of the goals sample