Hardness ratio between the 2–8 keV (hard) and 0.5–2 keV (soft) bands (i.e., X-ray hardness) as a function of redshift. The dashed lines are simulated tracks for an absorbed power-law with photon index fixed at 1.8. The red star is the X-ray hardness for GH500.30. For comparison, we also plot in gray dots the hardness ratios for SMGs in Wang et al. (2013), which include the most distant Compton-thick AGN, LESS J033229.4-275619 at z = 4.76 (red circle).
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