Polycyclic aromatic hydrocarbon (PAH) luminous galaxies at z ~ 1

T. Takagi¹, Y. Ohyama², T. Goto^3,4, H. Matsuhara¹, S. Oyabu¹, T. Wada¹, C. P. Pearson⁵, H. M. Lee⁶, M. Im⁶, M. G. Lee⁶, H. Shim⁶, H. Hanami⁷, T. Ishigaki⁸, K. Imai⁹, G. J. White^5,10, S. Serjeant¹⁰ and M. Malkan¹¹

¹ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 229-8510, Japan e-mail: takagi@ir.isas.jaxa.jp
² Academia Sinica, Institute of Astronomy and Astrophysics, Taiwan
³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822, USA
⁴ National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
⁵ Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
⁶ Department of Physics and Astronomy, FPRD, Seoul National University, Shilim-Dong, Kwanak-Gu, Seoul 151-742, Korea
⁷ Physics Section, Faculty of Humanities and Social Sciences, Iwate University, Morioka 020-8550, Japan
⁸ Asahikawa National College of Technology, Asahikawa, Hokkaido 071-8124, Japan
⁹ TOME R&D Inc. Kawasaki, Kanagawa 213-0012, Japan
¹⁰ Astrophysics Group, Department of Physics, The Open University, Milton Keynes, MK7 6AA, UK
¹¹ Department of Physics and Astronomy, UCLA, Los Angeles, CA, USA

Received: 14 October 2009
Accepted: 17 February 2010

Abstract

Aims. The NEP-deep survey, an extragalactic AKARI survey towards the north ecliptic pole (NEP), provides a comprehensive wavelength coverage from 2 to 24 μm using all 9 photometric bands of the infrared camera (IRC). It allows us to photometrically identify galaxies whose mid-IR emission is clearly dominated by PAHs.

Methods. We propose a single-colour selection method to identify such galaxies, using two mid-IR flux ratios at 11-to-7 μm and 15-to-9 μm (PAH-to-continuum flux ratio in the rest frame), which are useful for identifying starburst galaxies at z ~ 0.5 and 1, respectively. We perform a fitting of the spectral energy distributions (SEDs) from optical to mid-IR wavelengths, using an evolutionary starburst model with a proper treatment of radiative transfer (SBURT), in order to investigate their nature.

Results. The SBURT model reproduces observed optical-to-mid-IR SEDs of more than a half of the PAH-selected galaxies. Based on the 8 μm luminosity, we find ultra luminous infrared galaxies (ULIRGs) among PAH-selected galaxies. Their PAH luminosity is higher than local ULIRGs with a similar luminosity, and the PAH-to-total IR luminosity ratio is consistent with that of less luminous starburst galaxies. They are a unique galaxy population at high redshifts, and we call these PAH-selected ULIRGs “PAH-luminous” galaxies. Although they are not as massive as submillimetre galaxies at z ~ 2, they have the stellar mass of > 3 × 10¹⁰  and therefore are moderately massive.