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Comprehensive Broadband X-Ray and Multiwavelength Study of Active Galactic Nuclei in Local 57 Ultra/Luminous Infrared Galaxies Observed with Nustar And/Or Swift/BAT

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Comprehensive Broadband X-Ray and Multiwavelength Study of Active Galactic Nuclei in Local 57 Ultra/Luminous Infrared Galaxies Observed with Nustar And/Or Swift/BAT Draft version July 26, 2021 Typeset using LATEX twocolumn style in AASTeX631 Comprehensive Broadband X-ray and Multiwavelength Study of Active Galactic Nuclei in Local 57 Ultra/luminous Infrared Galaxies Observed with NuSTAR and/or Swift/BAT Satoshi Yamada ,1 Yoshihiro Ueda ,1 Atsushi Tanimoto ,2 Masatoshi Imanishi ,3, 4 Yoshiki Toba ,1, 5 Claudio Ricci ,6, 7, 8 and George C. Privon 9 1Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan 2Department of Physics, The University of Tokyo, Tokyo 113-0033, Japan 3National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan 4Department of Astronomical Science, Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 5Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan 6N´ucleo de Astronom´ıade la Facultad de Ingenier´ıa,Universidad Diego Portales, Av. Ej´ercito Libertador 441, Santiago, Chile 7Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China 8George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA 9National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA (Received April 13, 2021; Revised June 11, 2021; Accepted Jul, 2021) ABSTRACT We perform a systematic X-ray spectroscopic analysis of 57 local ultra/luminous infrared galaxy systems (containing 84 individual galaxies) observed with Nuclear Spectroscopic Telescope Array and/or Swift/BAT. Combining soft X-ray data obtained with Chandra, XMM-Newton, Suzaku and/or Swift/XRT, we identify 40 hard (>10 keV) X-ray detected active galactic nuclei (AGNs) and con- strain their torus parameters with the X-ray clumpy torus model XCLUMPY (Tanimoto et al. 2019). Among the AGNs at z < 0:03, for which sample biases are minimized, the fraction of Compton-thick 24 −2 +14 +12 (NH ≥ 10 cm ) AGNs reaches 64−15% (6/9 sources) in late mergers, while 24−10% (3/14 sources) in early mergers, consistent with the tendency reported by Ricci et al.(2017a). We find that the bolomet- ric AGN luminosities derived from the infrared data increase, but the X-ray to bolometric luminosity ratios decrease, with merger stage. The X-ray weak AGNs in late mergers ubiquitously show massive 46 −1 outflows at sub-pc to kpc scales. Among them, the most luminous AGNs (Lbol;AGN ∼ 10 erg s ) 23 −2 have relatively small column densities of .10 cm and almost super-Eddington ratios (λEdd ∼ 1.0). (22) Their torus covering factors (CT ∼ 0:6) are larger than those of Swift/BAT selected AGNs with similarly high Eddington ratios. These results suggest a scenario that, in the final stage of mergers, multiphase strong outflows are produced due to chaotic quasi-spherical inflows and the AGN becomes extremely X-ray weak and deeply buried due to obscuration by inflowing and/or outflowing material. Keywords: Black hole physics (159); Active galactic nuclei (16); X-ray active galactic nuclei (2035); arXiv:2107.10855v1 [astro-ph.GA] 22 Jul 2021 Infrared galaxies (790); Supermassive black holes (1663); Observational astronomy (1145) 1. INTRODUCTION nomenon. Many theoretical studies (e.g., Hopkins et al. For understanding the coevolution of galaxy and su- 2006) suggest that major mergers trigger both intense permassive black hole (SMBH), gas-rich galaxy merg- star formation in the galaxies and mass accretion onto ers (i.e., major mergers) are considered as a key phe- the SMBHs, being enshrouded by a huge amount of gas and dust. This makes them appear as luminous and ultraluminous infrared galaxies (U/LIRGs; Sanders & Corresponding author: Satoshi Yamada [email protected] 2 Yamada et al. Mirabel 1996 for a review)1 containing obscured active in the AGN structure between mergers and nonmergers galactic nuclei (AGNs). ULIRGs may be in a prephase of may provide us with a clue. Yamada et al.(2020) apply quasi stellar objects (quasars) where star forming activ- the XCLUMPY model to the spectra of two \nonmerg- ities become relatively weak. Thus, studies of ULIRGs ing LIRGs" and find that their AGNs are not buried, are indispensable to reveal the whole history of star for- unlike those in late mergers. This implies that accreting mation and SMBH growth triggered by major mergers. environments are different between major mergers and Recent numerical simulations of galaxy mergers (e.g., secular processes. From a statistical study of Swift/BAT Kawaguchi et al. 2020) predict that the mass accretion selected AGNs, whose majority are nonmergers, Ricci rates can exceed the Eddington limits as the separa- et al.(2017b) suggest that the torus structure is regu- tion of the two SMBHs decreases. However, the nuclear lated by radiation pressure from the AGN. Thus, stud- environments of accreting SMBHs in U/LIRGs have ies of AGNs in U/LIRGs will enable us to answer if the been still veiled in mystery. This is because the cen- same physics works or not in deeply buried AGNs in tral regions in the final phase of mergers become deeply major mergers. \buried" with large covering factors, where even the di- Comparison of X-ray luminosities3 with bolometric rection of the lowest dust column-density can be opaque ones inferred from infrared observations also provide us to the ionizing UV photons (e.g., Imanishi et al. 2006, with information on the AGN physics in U/LIRGs. Teng 2008; Yamada et al. 2019). To shed light on the proper- et al.(2015) report that the bolometric-to-X-ray lumi- ties of these hidden AGNs, hard X-rays (>10 keV) are nosity ratio (κbol;X) of the AGN in an ULIRG is much 2 4 useful thanks to their high penetrating power against ob- larger (i.e., X-ray weak), κbol;X ∼ 10 {10 , than that of 2 scuration. In fact, Ricci et al.(2017a) identity 25 AGNs a normal AGN, for which typically κbol;X ∼ 20 (e.g., Va- in 30 interacting U/LIRGs with the Nuclear Spectro- sudevan & Fabian 2007). The reason for the difference is scopic Telescope Array (NuSTAR; Harrison et al. 2013) not clear. To answer this question, comprehensive mul- and/or Swift/BAT, which cover the 3{79 keV and 14{ tiwavelength investigation of a large number of AGNs in 195 keV bands, respectively. They also report that the U/LIRGs is required. fraction of Compton-thick (CT; with hydrogen column In this work, we carry out a systematic X-ray spectral 24 −2 densities NH ≥ 10 cm ) AGNs is significantly larger study of a sample of 57 local U/LIRG systems (con- +12 +13 (65−13%) in late mergers than in early ones (35−12%). taining 84 individual galaxies) observed with NuSTAR This indicates that the AGNs in late stage mergers are and/or Swift/BAT by combining the available soft X- indeed deeply buried by CT material. ray data of Chandra, XMM-Newton, Suzaku, and/or To best constrain the properties of obscuring mate- Swift/XRT. The paper is structured as follows. Sec- rial (\torus") in AGNs through X-ray observations, con- tion2 describes our sample and merger classification. struction of spectral models based on the realistic ge- Details of the X-ray observations and data reduction ometries of AGN tori is crucial. Since many studies in- are presented in Section3. Section4 describes our X- dicate that the torus structure is clumpy (e.g., Krolik & ray spectral analysis. In Section5 we compare our X-ray Begelman 1988; Wada & Norman 2002; H¨onig& Beckert results with those of previous multiwavelength observa- 2007; Laha et al. 2020), Tanimoto et al.(2019) develop tions. In Section6 we discuss the origin of the X-ray the X-ray clumpy torus model called XCLUMPY, which weakness, torus structure, and the connection between adopts the same geometry as that of the infrared (IR) AGN and starburst activities. Section7 summarizes our CLUMPY model (Nenkova et al. 2008a,b). The model is main conclusions. In two companion papers (Ricci et al. applied to the broadband X-ray spectra of nearby AGNs 2021a; C. Ricci et al., in preparation) we will discuss: i) to constrain their torus covering factors (e.g., Miyaji the evolution of obscuration along the merger sequence, et al. 2019; Tanimoto et al. 2020, 2021; Toba et al. 2020; and ii) the relation between X-ray emission and multi- Ogawa et al. 2021; Uematsu et al. 2021). wavelength proxies of AGN activity. The cosmological −1 −1 To understand the connection between mass accretion parameters adopted are H0 = 70 km s Mpc ,ΩM = and obscuration in mergers, investigation of differences 3 In this paper, we define \absorption-corrected X-ray luminosi- 1 LIRGs and ULIRGs are defined by using the total 8{1000 µm ties" of an AGN in the 2{10 keV and 10{50 keV bands, L2−10 11 12 12 and L10−50, as those corrected for line-of-sight absorption that luminosities of LIR = 10 {10 L and >10 L , respectively. is recognized in the X-ray spectra. It remains possible, how- 2 Considering the relation between V -band extinction and hydro- ever, that a part of the X-ray emission from the innermost region gen column density of the Galactic interstellar medium, N /A H V is completely blocked by optically thick material and hence the = 2 × 1021 cm−2 mag−1 (Draine 2003), the penetration depth of true X-ray luminosity could be underestimated, as discussed in 10 keV X-rays corresponds to at least A ∼ 500 mag. V Section 6.1.6. 3 0.3, and ΩΛ = 0.7. All uncertainties are quoted at the it is reported that they show different properties of star 90% confidence level unless otherwise stated.
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