Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) XXII. Chandra observations of narrow-line quasar candidates at z>6

Abstract

We report on Chandra X-ray observations of four narrow-line quasar candidates at z~6, selected from the SHELLQs project, based on the Subaru Hyper Suprime-Cam survey. These objects are characterised by narrow (FWHM<310 km/s), luminous (>1e44 erg/s) Lya and faint UV continuum (M_1450 = -22 - -21), prompting us to examine whether they are obscured luminous AGN 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 (Lx) of 2e44 erg/s (2 sigma), assuming a spectral slope Gamma=2. Subsequent rest-frame optical spectroscopy of these objects by the JWST-NIRSpec, presented in a companion paper, show weak broad Balmer emission at the base of narrow cores. With the scaling relation for low-redshift AGN, the observed strong [OIII]5007 flux of these sources would predict Lx to be around 1e45 erg/s, which is well above the Chandra upper limits. These optical spectra and X-ray quietness are reminiscent of JWST-selected broad-line AGN. We attribute the weak broad Balmer emission to the broad-line regions hidden partially by optically-thick obscuring matter which 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 inter-stellar 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.

Figures (8)

• Figure 1: Line widths (FWHM) and rest-frame equivalent width (EWs) of Ly$\alpha$ of SHELLQs quasars. Left panel: CDF of FWHM(Ly$\alpha$) in units of km s$^{-1}$. Seven objects show unresolved lines that have an upper limit on FWHM of 230 km s$^{-1}$. Dotted line indicates the dividing line at 500 km s$^{-1}$ below which narrow-line objects are defined. Right panel: CDF of EW(Ly$\alpha$). Dotted line indicates 250 Å, above which five outliers in Fig. \ref{['fig:LM_diagram']}, including the four Chandra targets are found.
• Figure 2: UV absolute magnitude $M_{1450}$ as a function of Ly$\alpha$ luminosity of SHELLQs quasars. Broad-line (FWHM(Ly$\alpha$)$\geq 500$ km s$^{-1}$) and narrow-line (FWHM(Ly$\alpha$)$<500$ km s$^{-1}$) objects are plotted with blue circles and red squares, respectively. The four Chandra targets are circled in red. Dashed line indicates the best linear fit to both broad- and narrow-line objects. The two dotted lines show the 95% compatible intervals. Data for LAEs at $z\sim 6$ detected in the Subaru HSC survey from shibuya2018 are also plotted in grey for comparison. Crosses are LAEs for which UV continuum emission was detected while triangles are those for which only upper limits of UV emission were obtained.
• Figure 3: Mean Ly$\alpha$ profile of the four Chandra targets.
• Figure 4: Chandra 1-5 keV images of the four narrow-line SHELLQs quasars. Each panel is a 15$\times$15 field of view centred on the target. The image orientation is North up, East to the left. The circle indicates the optical position with a 1 arcsec radius.
• Figure 5: Upper limit of intrinsic $L_{\rm X}$ corresponding to 3 counts (the 95% upper limit obtained from a 60 ks exposure) for different assumed spectral slope $\Gamma$. $L_{\rm X}$ is the unabsorbed, rest-frame 2-10 keV luminosity. Examples for three values of photon index $\Gamma =1.6$ (dotted line), 2.0 (dashed line) or 2.4 (dot-dashed line) as a function of $N_{\rm H}$ are shown.