Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs)

XXII. Chandra observations of narrow-line quasar candidates at z ≥ 6

¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1 08028 Barcelona, Spain
² ICREA Pg. Lluís Companys 23 08010 Barcelona, Spain
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Gobetti 93/3 I-40129 Bologna, Italy
⁴ Research Center for Space and Cosmic Evolution, Ehime University 2-5 Bunkyo-cho Matsuyama Ehime 790-8577, Japan
⁵ Waseda Institute for Advanced Study (WIAS), Waseda University, 1-21-1 Nishi-Waseda Shinjuku Tokyo 169-0051, Japan
⁶ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), UTIAS, Tokyo Institutes for Advanced Study, University of Tokyo Chiba 277-8583, Japan
⁷ Department of Astrophysical Sciences, Princeton University 4 Ivy Lane Princeton NJ 08544, USA
⁸ Department of Astronomy, School of Science, The University of Tokyo 7-3-1 Hongo Bunkyo Tokyo 113-0033, Japan
⁹ Department of Physical Sciences, Ritsumeikan University Kusatsu Shiga 525-8577, Japan
¹⁰ Academia Sinica Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, AS/NTU, No. 1, Section 4 Roosevelt Road Taipei 10617, Taiwan
¹¹ Kavli Institute for Astronomy and Astrophysics, Peking University Beijing 100871, China
¹² Department of Physics, Graduate School of Science Tokyo Metropolitan University Hachioji-shi Tokyo 192-0397, Japan
¹³ National Astronomical Observatory of Japan, Osawa Mitaka Tokyo 181-8588, Japan
¹⁴ Institute of Astronomy, Graduate School of Science, The University of Tokyo 2-21-1 Osawa Mitaka Tokyo 181-0015, Japan
¹⁵ Research Center for the Early Universe, Graduate School of Science, The University of Tokyo Hongo Tokyo 113-0033, Japan
¹⁶ Department of Astronomy, Kyoto University Sakyo-ku Kyoto, Japan

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Received: 6 May 2025
Accepted: 17 December 2025

Abstract

We report on Chandra X-ray observations of four narrow-line quasar candidates at z ∼ 6, selected from the Subaru high-z exploration of low-luminosity quasars (SHELLQs) project, based on the Subaru Hyper Suprime-Cam survey. These objects are characterised by narrow (FWHM ≤ 310 km s⁻¹), luminous (> 10⁴⁴ erg s⁻¹) Lyα, and faint UV continuum (M₁₄₅₀ = −22 to −21), prompting us to examine whether they are obscured luminous active galactic nuclei (AGNs) at the epoch of reionization. However, none of these objects were detected by Chandra, giving an upper limit to their rest-frame 2−10 keV luminosity (L_X) of 2 × 10⁴⁴ erg s⁻¹ (2σ), assuming a spectral slope of Γ = 2. Subsequent rest-frame optical spectroscopy of these objects by the James Webb Space Telescope, presented in a companion paper, shows weak broad Balmer emission at the base of narrow cores. With the scaling relation for low-redshift AGNs, the observed strong [O III]λ5007 flux of these sources would predict L_X to be around 10⁴⁵ erg s⁻¹, which is well above the Chandra upper limits. These optical spectra and X-ray quietness are reminiscent of JWST-selected broad-line AGNs. We attribute the weak broad Balmer emission to the broad-line regions hidden partially by optically thick obscuring matter that also hides the optical and X-ray continuum emission from the accretion disc. Compton-thick obscuration, which would strongly suppress X-ray emission, could be due to a dense interstellar medium that is often present in galaxies at high redshifts. Alternatively, the same effect could be obtained from an inflated disc at the innermost radii in a supercritical accretion flow, when the disc is viewed at inclined angles.