X-ray observations of luminous dusty quasars at z > 2

Lansbury, G.B., Banerji, M., Fabian, A.C. and Temple, M.J. (2020) X-ray observations of luminous dusty quasars at z > 2. Monthly Notices Of The Royal Astronomical Society, 495 (3), 2652-2663. (doi:10.1093/mnras/staa1220).

Abstract

We present new X-ray observations of luminous heavily dust-reddened quasars (HRQs) selected from infrared (IR) sky surveys. HRQs appear to be a dominant population at high redshifts and the highest luminosities, and may be associated with a transitional ‘blowout’ phase of black hole and galaxy co-evolution models. Despite this, their high-energy properties have been poorly known. We use the overall sample of 10 objects with XMM–Newton coverage to study the high-energy properties of HRQs at ⟨L_bol⟩=10^47.5 erg s⁻¹ and ⟨z⟩=2.5. For seven sources with strong X-ray detections, we perform spectral analyses. These find a median X-ray luminosity of ⟨L_2−10keV⟩=10^45.1erg s⁻¹, comparable to the most powerful X-ray quasars known. The gas column densities are NH = (1–8) × 10²² cm⁻², in agreement with the amount of dust extinction observed. The dust-to-gas ratios are sub-Galactic, but are higher than found in local AGN. The intrinsic X-ray luminosities of HRQs are weak compared to the mid-IR (⁠L_6μm⁠) and bolometric luminosities (L_bol), in agreement with findings for other luminous quasar samples. For instance, the X-ray to bolometric corrections range from κ_bol ≈ 50 to 3000. The moderate absorption levels and accretion rates close to the Eddington limit (⁠⟨λ_Edd⟩=1.06) are in agreement with a quasar blowout phase. Indeed, we find that the HRQs lie in the forbidden region of the N_H–λ_Edd plane, and therefore that radiation pressure feedback on the dusty interstellar medium may be driving a phase of blowout that has been ongoing for a few 10⁵ yr. The wider properties, including [O III] narrow-line region kinematics, broadly agree with this interpretation.

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Contributors

Author: M. Banerji

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