AKARI detections of hot dust in luminous infrared galaxies

Search for dusty active galactic nuclei

¹ Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
e-mail: oyabu@u.phys.nagoya-u.ac.jp
² Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yosinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan
⁴ Academia Sinica, Institute of Astronomy and Astrophysics, Taiwan
⁵ Department of Space and Astronautical Science, the Graduate University for Advanced Studies (Sokendai), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan
⁶ Department of Astronomy, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

Received: 9 February 2010
Accepted: 4 March 2011

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(K_S) > 2 where K_S 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.