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

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 $\langle$Lbol$\rangle$ =1047.5 erg s−1 and $\langle$z$\rangle$ =2.5. For seven sources with strong X-ray detections, we perform spectral analyses. These find a median X-ray luminosity of $\left\langle L_{\rm 2\!-\!10\, keV} \right\rangle = 10^{45.1}$ erg 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 ($L_{\rm 6\, \mu m}$) and bolometric luminosities (Lbol), in agreement with findings for other luminous quasar samples. For instance, the X-ray to bolometric corrections range from κbol ≈ 50 to 3000. The moderate absorption levels and accretion rates close to the Eddington limit ($\langle$λEdd$\rangle$ =1.06) are in agreement with a quasar blowout phase. Indeed, we find that the HRQs lie in the forbidden region of the NH–λEdd 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.

The multiwavelength properties of red QSOs: Evidence for dusty winds as the origin of QSO reddening

“…We remark that the log N H values quoted here are not inferred from the X-rays but from the infrared alone, using the log N H term to parametrize the torus templates by Silva et al (2004). These estimates of N H are, however, overall consistent with measures from X-rays spectroscopic studies of SDSS QSOs (Wilkes et al 2002;Urrutia et al 2005), although these are found to be significantly higher for red QSOs selected in the near-infrared (Goulding et al 2018;Lansbury et al 2020).…”

“…We find we can reject this hypothesis with a confidence level of 99.9% (p-value = 2×10 −5 ) and therefore conclude this difference is statistically significant. The tentative observation, supported by the KS-test, that red QSOs have lower reprocessing efficiency would hint towards a slight difference in the torus composition, possibly related to larger gas to dust fractions and a more diffuse obscuring medium (Lansbury et al 2020).…”

AGN-driven galactic outflows: comparing models to observations

“…Interestingly, most of the sources with both UFO and molecular outflow detections are located in the forbidden region of the N H -λ plane (Section 5). Similarly, highly dust-reddened quasars at high redshifts (z ∼ 2), as well as other populations of luminous reddened quasars, are observed to lie in the forbidden region, suggesting that they are indeed in a blowout phase (Lansbury et al 2020). On the other hand, dust obscured galaxies may be located just at the boundary between the forbidden and long-lived regions (Zou et al 2020), and could potentially represent sources at a slightly earlier evolutionary phase.…”

A highly accreting low-mass black hole hidden in the dust: Suzaku and NuSTAR observations of the NLS1 Mrk 1239

“…Some exceptional cases are, however, found in very luminous dusty quasars at higher redshifts (e.g. > 0.2, Lansbury et al 2020).…”

“…Some exceptional dusty quasars at higher redshifts, e.g. > 0.2, are found to be located in the forbidden region(Lansbury et al 2020). The black circle shows the column density of the torus in Mrk 1239.…”