The nature of X-ray absorbed QSOs

TL;DR

This study investigates X-ray absorbed QSOs at z~2, revealing they are embedded in starburst galaxies and exhibit ionized absorption, supporting their role as a transitional phase in galaxy evolution.

Contribution

The paper provides detailed X-ray and ultraviolet spectra of X-ray absorbed QSOs, demonstrating ionized absorption as the best explanation and linking these objects to galaxy evolution models.

Findings

All five QSOs show strong C IV absorption lines.

X-ray spectra are best modeled with ionized absorbers.

The outflow kinetic power is about 4% of the radiated power.

Abstract

There exists a significant population of broad line, z~2 QSOs which have heavily absorbed X-ray spectra. Follow up observations in the submillimetre show that these QSOs are embedded in ultraluminous starburst galaxies, unlike most unabsorbed QSOs at the same redshifts and luminosities. Here we present X-ray spectra from XMM-Newton for a sample of 5 such X-ray absorbed QSOs that have been detected at submillimetre wavelengths. We also present spectra in the restframe ultraviolet from ground based telescopes. All 5 QSOs are found to exhibit strong C IV absorption lines in their ultraviolet spectra with equivalent width > 5 Angstroms. The X-ray spectra are inconsistent with the hypothesis that these objects show normal QSO continua absorbed by low-ionization gas. Instead, the spectra can be modelled successfully with ionized absorbers, or with cold absorbers if they posess unusually flat X-ray continuum shapes and unusual optical to X-ray spectral energy distributions. We show that the ionized absorber model provides the simplest, most self-consistent explanation for their observed properties. We estimate that the fraction of radiated power that is converted into kinetic luminosity of the outflowing winds is typically ~4 per cent, in agreement with recent estimates for the kinetic feedback from QSOs required to produce the M - sigma relation, and consistent with the hypothesis that the X-ray absorbed QSOs represent the transition phase between obscured accretion and the luminous QSO phase in the evolution of massive galaxies.