Near- to mid-infrared spectroscopy of the heavily obscured AGN LEDA 1712304 with AKARI/IRC

¹ Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
e-mail: tsuchikawa@u.phys.nagoya-u.ac.jp
² Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
³ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁴ Academia Sinica Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU, No.1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
⁵ Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan

Received: 18 March 2019
Accepted: 5 May 2019

Abstract

Context. Although heavily obscured active galactic nuclei (AGNs) have been found by many observational studies, the properties of the surrounding dust are poorly understood. Using AKARI/IRC spectroscopy, we discovered a new heavily obscured AGN in LEDA 1712304 which shows a deep spectral absorption feature due to silicate dust.

Aims. We study the infrared (IR) spectral properties of circumnuclear silicate dust in LEDA 1712304.

Methods. We performed IR spectral fitting, considering silicate dust properties such as composition, porosity, size, and crystallinity. Spectral energy distribution fitting was also performed on the flux densities in the UV to submillimeter range to investigate the global spectral properties.

Results. The best-fit model indicates 0.1 μm-sized porous amorphous olivine (Mg_2xFe_2−2xSiO₄; x = 0.4) with 4% crystalline pyroxene. The optical depth is τ_sil ∼ 2.3, while the total IR luminosity and stellar mass are estimated to be L_IR = (5 ± 1)×10¹⁰ L_⊙ and M_star = (2.7 ± 0.8)×10⁹ M_⊙, respectively. In such low L_IR and M_star ranges, there are few galaxies that show such a large τ_sil.

Conclusion. The silicate dust in the AGN torus of LEDA 1712304 has properties that are notably similar to those in other AGNs overall, but slightly different in the wing shape of the absorption profile. The porosity of the silicate dust suggests dust coagulation or processing in the circumnuclear environments, while the crystallinity suggests that the silicate dust is relatively fresh.