Evidence for Intrinsic X-Ray Weakness among Red Quasars at Cosmic Noon

Abstract

Quasar feedback is a key ingredient in shaping galaxy evolution. A rare population of extremely red quasars (ERQs) at z = 2−3 are often associated with high-velocity [O iii ] λ 5008 outflows and may represent sites of strong feedback. In this paper, we present an X-ray study of 50 ERQs to investigate the link between the X-ray and outflow properties of these intriguing objects. Using hardness ratio analysis, we confirm that the ERQs are heavily obscured systems with gas column density reaching N _H = 10 ^23−24 cm ^−2 . We identify 20 X-ray-nondetected ERQs at high mid-infrared (MIR) luminosities of ν L _ν _,6 _ μ _m ≳ 3 × 10 ^46 erg s ^−1 . By stacking the X-ray observations, we find that the nondetected ERQs are on average underluminous in X-rays by a factor of ∼10 for their MIR luminosities. We consider such X-ray weakness to be due to both heavy gas absorption and intrinsic factors. Moreover, we find that the X-ray-weak sources also display higher-velocity outflows. One option to explain this trend is that weaker X-rays facilitate more vigorous line-driven winds, which then accelerate the [O iii ]-emitting gas to kiloparsec scales. Alternatively, super-Eddington accretion could also lead to intrinsic X-ray weakness and more powerful continuum-driven outflow.

Keywords

• Quasars
• Supermassive black holes
• X-ray quasars