Publ. Astron. Soc. Japan (2018) 00(0), 1–23 1 doi: 10.1093/pasj/xxx000 “Big Three Dragons”: a z =7.15 Lyman Break Galaxy Detected in [Oiii] 88 µm, [Cii] 158 µm, and Dust Continuum with ALMA Takuya Hashimoto1,2,3 Akio K. Inoue1,2, Ken Mawatari2,4, Yoichi Tamura5, Hiroshi Matsuo3,6, Hisanori Furusawa3, Yuichi Harikane4,7, Takatoshi Shibuya8, Kirsten K. Knudsen9, Kotaro Kohno10,11, Yoshiaki Ono4, Erik Zackrisson12, Takashi Okamoto13, Nobunari Kashikawa3,6,7, Pascal A. Oesch14, Masami Ouchi4,15, Kazuaki Ota16, Ikkoh Shimizu17, Yoshiaki Taniguchi18, Hideki Umehata18,19, and Darach Watson20. 1Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan 2Department of Environmental Science and Technology, Faculty of Design Technology, Osaka Sangyo University, 3-1-1, Nagaito, Daito, Osaka 574-8530, Japan 3National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 4Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan 5Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya 6Department of Astronomical Science, School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan 7Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan 8Department of Computer Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan 9Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden 10Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan 11Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 12Observational Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden 13 arXiv:1806.00486v2 [astro-ph.GA] 8 Apr 2019 Department of Cosmosciences, Graduates School of Science, Hokakido University, N10 W8, Kitaku, Sapporo 060-0810, Japan 14Geneva Observatory, University of Geneva, Ch. des Maillettes, 51, 1290 Versoix, Switzerland 15Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan 16Kyoto University Research Administration Office, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501 Japan 17Theoretical Astrophysics, Department of Earth & Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan 18The Open University of Japan, 2-11 Wakaba, Mihama-ku, Chiba 261-8586, Japan 19RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan c 2018. Astronomical Society of Japan. 2 Publications of the Astronomical Society of Japan, (2018), Vol. 00, No. 0 20Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Denmark ∗E-mail: [email protected] Received h02-Jun-2018i; Accepted h08-Apr-2019i Abstract We present new ALMA observations and physical properties of a Lyman Break Galaxy at z = 7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, MUV ≈−22.4, and has been spectroscopically identified in Lyα with a small rest-frame equivalent width of ≈ 4 A.˚ Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [Oiii] 88 µm, [Cii] 158 µm, and their underlying dust continuum emission. In the whole system of B14- 65666, the [Oiii] and [Cii] lines have consistent redshifts of 7.1520 ± 0.0003, and the [Oiii] 8 luminosity, (34.4 ± 4.1) × 10 L⊙, is about three times higher than the [Cii] luminosity, (11.0 ± 8 1.4) × 10 L⊙. With our two continuum flux densities, the dust temperature is constrained to be Td ≈ 50 − 60 K under the assumption of the dust emissivity index of βd =2.0 − 1.5, leading 12 to a large total infrared luminosity of LTIR ≈ 1 × 10 L⊙. Owing to our high spatial resolution data, we show that the [Oiii] and [Cii] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by ≈ 200 km s−1. We also find these two clumps have comparable UV, infrared, [Oiii], and [Cii] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation +119 −1 rate (defined as the star-formation rate per unit stellar mass), sSFR = 260−57 Gyr , inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [Oiii]-to-[Cii] luminosity ratio. Key words: galaxies: formation — galaxies: high-redshift — galaxies: ISM 1 Introduction However, based on a compiled sample with [Cii] obser- > vations at z ∼ 5, Harikane et al. (2018) and Carniani et al. Understanding properties of galaxies during reionization, (2018b) have revealed that [Cii] may be weak for galaxies > at redshift z ∼ 6 − 7, is important. While a large num- with strong Lyα emission, so-called Lyα emitters (LAEs; ber of galaxy candidates are selected with a dropout tech- > ˚ rest-frame Lyα equivalent widths EW0(Lyα) ∼ 20−30 A). > nique at z ∼ 7 (e.g., Ellis et al. 2013; Bouwens et al. Harikane et al. (2018) have interpreted the trend with pho- 2014; Oesch et al. 2018), the spectroscopic identifications toionization models of CLOUDY (Ferland et al. 2013) im- > at z ∼ 7 remain difficult (e.g., Stark et al. 2017 and refer- plemented in spectral energy distribution (SED) models ences therein). This is mainly due to the fact that the most of BEAGLE (Chevallard & Charlot 2016). The authors prominent hydrogen Lyα line is significantly attenuated by show that low metallicity or high ionization states in LAEs the intergalactic medium (IGM). lead to weak [Cii]. Theoretical studies also show that such ii With the advent of the Atacama Large ISM conditions lead to the decrease in the [C ] luminos- Millimeter/Submillimeter Array (ALMA) telescope, ity (Vallini et al. 2015, 2017; Olsen et al. 2017; Lagache > it has become possible to detect rest-frame far-infrared et al. 2018). If we assume that z ∼ 7 galaxies in general ii (FIR) fine structure lines in star-forming galaxies at z> 5 have low metallicity or high ionization states, [C ] may not > (e.g., Capak et al. 2015; Maiolino et al. 2015). A most be the best line to spectroscopically confirm z ∼ 7 galaxies. ii commonly used line is [Cii] 158 µm, which is one of the Indeed, a number of null-detections of [C ] are reported at > brightest lines in local galaxies (e.g., Malhotra et al. z ∼ 7 (e.g., Ota et al. 2014; Schaerer et al. 2015; Maiolino 1997; Brauher et al. 2008). To date, more than 21 [Cii] et al. 2015; Inoue et al. 2016). < < detections are reported at 5 ∼ z ∼ 7 (Carniani et al. 2018b In fact, based on Herschel spectroscopy for local dwarf and references therein; Pentericci et al. 2016; Matthee galaxies, Cormier et al. (2015) have demonstrated that et al. 2017; Smit et al. 2018). [Oiii] 88 µm becomes brighter than [Cii] at low metallic- Publications of the Astronomical Society of Japan, (2018), Vol. 00, No. 0 3 ity (see also Malhotra et al. 2001). Based on calculations from the UV luminosity could be naturally explained if of CLOUDY, Inoue et al. (2014b) also theoretically predict the system is a merger-induced starburst. More recently, that the [Oiii] line at high-z should be bright enough to be with ALMA Band 6 observations, Bowler et al. (2018) have detected with ALMA. detected dust continuum emission at the peak significance Motivated by these backgrounds, we are conducting level of 5.2σ, which is the third detection of dust emission follow-up observations of the [Oiii] 88 µm line for z > 6 in normal star-forming galaxies at z> 7 (cf., Watson et al. galaxies with ALMA. After the first detection of [Oiii] 2015; Laporte et al. 2017 see also Knudsen et al. 2017). in the reionization epoch in Inoue et al. (2016) at z = In ALMA Cycle 4, we have performed high spatial reso- 7.21, the number of [Oiii] detections is rapidly increasing. lution follow-up observations of B14-65666 with beam size There are currently ten objects with [Oiii] detections at of ≈0′′.3×0′′.2 (0′′.3×0′′.3) in Band 6 (Band 8). In ALMA z ≈ 6−9 (Carniani et al. 2017; Laporte et al. 2017; Marrone Cycle 5, we have also obtained deeper Band 8 data with a et al. 2018; Hashimoto et al. 2018a; Tamura et al. 2018, slightly larger beam size of ≈ 0′′.4 × 0′′.4. We successfully Hashimoto et al. 2018b; Walter et al. 2018). Remarkably, detect spatially resolved [Cii], [Oiii], and dust continuum Hashimoto et al. (2018a) have detected [Oiii] in a z = 9.11 emission in two bands, making B14-65666 the first object > 1 galaxy with a high significance level of 7.4σ. Importantly, at z ∼ 6 with a complete set of these three features . The [Oiii] is detected from all the targeted galaxies (six detec- spatially resolved data enable us to investigate the velocity tions out of six objects) from our team, i.e., the success gradients of the [Cii] and [Oiii] lines. These emission lines rate is currently 100% . These results clearly demonstrate also allow us to investigate the Lyα velocity offset with that [Oiii] is a powerful tool to confirm z> 6 galaxies. respect to the systemic, ∆vLyα, which is an important pa- Inoue et al.