AKARI mid-infrared slitless spectroscopic survey of star-forming galaxies at z ≲ 0.5

¹ Academia Sinica, Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU, No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617 Taiwan, ROC
e-mail: ohyama@asiaa.sinica.edu.tw
² Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
³ Japan Space Forum, Shin-Ochanomizu Urban Trinity Bldg. 2F 3-2-1, Kandasurugadai, Chiyoda-ku, Tokyo 101-0062, Japan
⁴ Division of Astronomy and Astrophysics, University of California, Los Angeles, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA
⁵ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁶ National Tsing hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan, ROC
⁷ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
⁸ Astronomy Program, Department of Physics and Astronomy, Seoul National University, Shillim-Dong, Kwanak-Gu, Seoul, 151-742, Republic of Korea
⁹ Subaru Telescope, National Astronomical Observatory of Japan, 650 North Aòhoku Place, Hilo, HI 96720, USA
¹⁰ RAL Space, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
¹¹ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹² Oxford Astrophysics, Denys Wilkinson Building, University of Oxford, Keble Rd, Oxford OX1 3RH, UK
¹³ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁴ CRAL, Observatoire de Lyon, 9, avenue Charles André, 69561 Saint-Genis-Laval, France
¹⁵ Graduate School of Science, Nagoya University, Furo-cho, Chikusaku, Nagoya, Aichi 464-8602, Japan
¹⁶ Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁷ Center for Planetary Science, Graduate School of Science, Kobe University, 7-1-48 Minatojima-Minamimachi, Chuo-Ku, Kobe 650-0047, Japan
¹⁸ Laboratoire d’Astrophysique de Marseille, Pôle de l’Étoile Site de Château-Gombert 38, rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁰ Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
²¹ Instituto de Astronomía Campus Ensenada, Universidad Nacional Autónoma de México, Km. 103 Carretera Tijuana-Ensenada, Ensenada, Baja California 22860, Mexico
²² Physics Section, Faculty of Humanities and Social Sciences, Iwate University, Morioka 020-8550, Japan
²³ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan

Received: 29 June 2017
Accepted: 9 August 2018

Abstract

Context. Deep mid-infrared (MIR) surveys have revealed numerous strongly star-forming galaxies at redshift z ≲ 2. Their MIR fluxes are produced by a combination of continuum and polycyclic aromatic hydrocarbon (PAH) emission features. The PAH features can dominate the total MIR flux, but are difficult to measure without spectroscopy.

Aims. We aim to study star-forming galaxies by using a blind spectroscopic survey at MIR wavelengths to understand evolution of their star formation rate (SFR) and specific SFR (SFR per stellar mass) up to z ≃ 0.5, by paying particular attention to their PAH properties.

Methods. We conducted a low-resolution (R ≃ 50) slitless spectroscopic survey at 5–13 μm of 9 μm flux-selected sources (>0.3 mJy) around the north ecliptic pole with the infrared camera (IRC) onboard AKARI. After removing 11 AGN candidates by using the IRC photometry, we identify 48 PAH galaxies with PAH 6.2, 7.7, and 8.6 μm features at z < 0.5. The rest-frame optical–MIR spectral energy distributions (SEDs) based on CFHT and IRC imaging covering 0.37–18 μm were produced, and analysed in conjunction with the PAH spectroscopy. We defined the PAH enhancement by using the luminosity ratio of the 7.7 μm PAH feature over the 3.5 μm stellar component of the SEDs.

Results. The rest-frame SEDs of all PAH galaxies have a universal shape with stellar and 7.7 μm bumps, except that the PAH enhancement significantly varies as a function of the PAH luminosities. We identify a PAH-enhanced population at z ≳ 0.35, whose SEDs and luminosities are typical of luminous infrared galaxies. They show particularly larger PAH enhancement at high luminosity, implying that they are vigorous star-forming galaxies with elevated specific SFR. Our composite starburst model that combines a very young and optically very thick starburst with a very old population can successfully reproduce most of their SED characteristics, although we cannot confirm this optically think component from our spectral analysis.