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Lyα\alpha imaging around the hyperluminous dust-obscured quasar W2246−-0526 at z=4.6z=4.6

Yibin Luo （罗毅彬） CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Lulu Fan (范璐璐) CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Deep Space Exploration Laboratory, Hefei 230088, China Yongming Liang （梁永明） Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan Weida Hu （胡维达） Department of Physics and Astronomy, Texas A&M University, College Station, TX, 77843-4242 USA George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, 77843-4242 USA Junxian Wang （王俊贤） CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Zhen-ya Zheng （郑振亚） CAS Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Shanghai 200030, China Zheyu Lin （林哲宇） CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Bojun Tao （陶柏钧 CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Zesen Lin （林泽森） Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong S.A.R., China minxuan Cai CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China mengqiu Huang （黄梦秋） CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Zhen Wan （宛振） CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Yongling Tang （唐永灵 CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
Abstract

Hot dust-obscured galaxies (Hot DOGs) are a population of hyperluminous, heavily obscured quasars discovered by the Wide-field Infrared Survey Explorer (WISE) all-sky survey at high redshift. Observations suggested the growth of these galaxies may be driven by mergers. Previous environmental studies have statistically shown Hot DOGs may reside in dense regions. Here we use the Very Large Telescope (VLT) narrowband and broadband imaging to search for Lyα\alpha emitters (LAEs) in the 6.8′×6.8′6.8\arcmin\times 6.8\arcmin field of the Hot DOG W2246−-0526 at z=4.6z=4.6. W2246−-0526 is the most distant Hot DOG. We find that there is an overdensity of LAEs in W2246−-0526 field compared with the blank fields. This is the direct evidence that this most distant Hot DOG is in an overdense environment on the Mpc scale, and the result relates to the merger origin of Hot DOGs.

galaxies: active – galaxies: formation - galaxies: evolution - galaxies: high redshift - galaxies: clusters
††facilities: VLT(FORS2)

1 Introduction

Based on “W1W2-dropout” method, a new population of hyperluminous, hot dust-obscured galaxies were discovered using the WISE and were called as Hot DOGs (Eisenhardt2012; Wu2012). These galaxies are prominent in the WISE 12 μm\mu m (W3) and 22 μm\mu m (W4) bands, but are very faint or undetected in the 3.4 μm\mu m (W1) and 4.6 μm\mu m (W2) bands. Previous studies have found that Hot DOGs are extremely luminous Lbol>1013L⊙L_{bol}>10^{13}L_{\odot}, heavily dust-obscured quasars at high redshift, and represent a transition phase between starburst-dominated phase and optically bright quasars phase (Assef2015; Fan2016a; Fan2018; Fan2020; Piconcelli2015; Stern2014; Sun2024; Aranda2024).

Models predict that the growth of these galaxies may be driven by mergers (Hopkins2006; Matteo2008). Galaxy mergers remove gas angular momentum and drive it directly into the center, fueling intense starbursts and central SMBH accretion. Multiwavelength observations of Hot DOGs also suggested that Hot DOGs are likely triggered by galaxy mergers. Fan2016b found a high merger fraction (62±14%62\pm 14\%) in a sample of 18 Hot DOGs using Hubble Space Telescope (HST) WFC3 imaging. Atacama Large Millimeter Array (ALMA) observations of a sample of 7 Hot DOGs suggested that there may be multiple merger events at the stage of Hot DOGs (Diaz2021). High merger fraction may be associated with high-density regions. Although some statistical studies have found overdensities of mid-IR-selected and sub-millimeter-selected galaxies around Hot DOGs (Assef2015; Jones2014; Jones2017; Fan2017), and two case studies have found overdensities of distant red galaxies (DRGs) (Luo2022) and Lyman Break Galaxies (LBGs) (Zewdie2023) around Hot DOGs, direct observations such as spectroscopically confirmed companion galaxies or narrowband observations are still very rare. Ginolfi2022 found an overdensity of Lyman-alpha emitters (LAEs) around a z=3.6z=3.6 Hot DOG using the VLT/MUSE. Diaz2018 found a z=4.6z=4.6 Hot DOG (W2246−-0526, which is the most distant Hot DOG) in a multiple merger system using ALMA observations, with at least three spectroscopically confirmed companion galaxies within a few tens of kpc. Due to the limitations of the field of view of ALMA, the study of environment was restricted to scales of a few tens of kpc. Considering previous environment studies suggested that Hot DOGs may be a good tracer for overdense regions such as protoclusters, it is important to further study the environments of Hot DOGs in clusters scales of Mpc.

A commonly used technique to identify high-redshift galaxies is to search for sources with a prominent Lyα\alpha emission using specific narrowband filters (Δλ∼100\Delta\lambda\sim 100Å). These sources are called as Lyα\alpha emitters (LAEs). The covered redshift range of this technique is narrow (Δz∼0.1\Delta z\sim 0.1), which can reduce the impact of projection effects on environment studies. Numerous studies have used this technique to study the large scale environment (Venemans2002; Venemans2005; Venemans2007; Cai2017; Mazzucchelli2017; Kikuta2017; Zheng2017; Garc2019; Hu2019; Liang2021; Hu2021).

In this work, we use narrowband and broadband images obtained from VLT FOcal Reducer and low dispersion Spectrograph 2 (FORS2, appenzeller1992) to study the environment of the Hot DOG W2246−-0526 (hereafter, W2246) by searching LAEs in this field. The redshift of W2246 derived from ALMA [Cii] line is z=4.601z=4.601 (Diaz2016). Among the Hot DOGs with spectroscopic redshift, W2246 is the most distant one so far. In addition, W2246 is the most luminous one with bolometric luminosity Lbol>1014L⊙L_{bol}>10^{14}L_{\odot} (Tsai2015). Strong AGN feedback was found in W2246 (Diaz2016). Fan2018 found that most of its IR luminosity come from AGN torus, suggesting the rapid growth of the central SMBH. Tsai2018 measured the SMBH mass of about 1010M⊙10^{10}M_{\odot}, and the Eddington ratio of 2.8.

The paper is structured as follows. We present the observation and data reduction in Section LABEL:sec:obs. We describe the color criteria and LAE sample in Section LABEL:sec:selection. Results and discussions are described in Section LABEL:sec:results and Section LABEL:sec:discussion, respectively. Finally, we give the summary and conclusion in Section LABEL:sec:summary. Throughout this work, we use the AB magnitude system and assume a cosmology with H0=70kms−1Mpc−1H_{0}=\rm{70\ km\ s^{-1}\ Mpc^{-1}}, ΩM=0.27\Omega_{M}=0.27 and ΩΛ=0.73\Omega_{\Lambda}=0.73 (komatsu2011). All magnitudes are corrected for Galactic extinction (SF2011).

Refer to caption
Figure 1: NBNB image of the W2246 field and the effective area is 33.6 arcmin2{}^{2}. Positions of the LAEs are shown using the red circle and the bold red circle marks the position of W2246.