AKARI detections of hot dust in luminous infrared galaxies. Search for dusty active galactic nuclei
Ohyama, Y.; Matsuhara, H.; Oyabu, S.; Wada, T.; Malkan, M.; Nakagawa, T.; Toba, Y.; Onaka, T.; Yamamura, I.; Ishihara, D.; Kataza, H.; Takita, S.; Shirahata, M.
Japan, United States, Taiwan
Abstract
Aims: We present a new sample of active galactic nuclei (AGNs) identified using the catalog of the AKARI Mid-infrared (MIR) All-Sky Survey. Our MIR search has the advantage of detecting AGNs that are obscured at optical wavelengths by extinction.
Methods: We first selected AKARI 9 μm excess sources with F(9 μm)/F(KS) > 2 where KS magnitudes were taken from the Two Micron All Sky Survey. We then obtained follow-up near-infrared spectroscopy with the AKARI/IRC to confirm that the excess is caused by hot dust. We also obtained optical spectroscopy with the Kast Double Spectrograph on the Shane 3-m telescope at Lick Observatory.
Results: On the basis of these observations, we detected hot dust with a characteristic temperature of ≳ 500 K in two luminous infrared galaxies. The hot dust is suspected to be associated with AGNs that exhibit their nonstellar activity not in the optical, but in the near- and mid-infrared bands, i.e., they harbor buried AGNs. The host galaxy stellar masses of ~4-6 × 109 M⊙ are small compared with the hosts in optically-selected AGN populations. These objects were missed by previous surveys, demonstrating the power of the AKARI MIR All-Sky Survey to widen AGN searches to include more heavily obscured objects. The existence of multiple dusty star clusters with massive stars cannot be completely ruled out with our current data.