X-ray emission from a rapidly accreting narrow-line Seyfert 1 galaxy at z = 6.56

Abstract

Context. The space density of X-ray-luminous, blindly selected active galactic nuclei (AGN) traces the population of rapidly accreting super-massive black holes through cosmic time. It is encoded in the X-ray luminosity function, whose bright end remains poorly constrained in the first billion years after the Big Bang as X-ray surveys have thus far lacked the required cosmological volume. With the eROSITA Final Equatorial-Depth Survey (eFEDS), the largest contiguous and homogeneous X-ray survey to date, X-ray AGN population studies can now be extended to new regions of the luminosity-redshift space (L 2−10 keV > 10 45 erg s −1 and z > 6). Aims. The current study aims at identifying luminous quasars at z > 5.7 among X-ray-selected sources in the eFEDS field in order to place a lower limit on black hole accretion well into the epoch of re-ionisation. A secondary goal is the characterisation of the physical properties of these extreme coronal emitters at high redshifts. Methods. Cross-matching eFEDS catalogue sources to optical counterparts from the DESI Legacy Imaging Surveys, we confirm the low significance detection with eROSITA of a previously known, optically faint z = 6.56 quasar from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey. We obtained a pointed follow-up observation of the source with the Chandra X-ray telescope in order to confirm the low-significance eROSITA detection. Using new near-infrared spectroscopy, we derived the physical properties of the super-massive black hole. Finally, we used this detection to infer a lower limit on the black hole accretion density rate at z > 6.Results. The Chandra observation confirms the eFEDS source as the most distant blind X-ray detection to date. The derived X-ray luminosity is high with respect to the rest-frame optical emission of the quasar. With a narrow Mgii line, low derived black hole mass, and high Eddington ratio, as well as its steep photon index, the source shows properties that are similar to local narrow-line Seyfert 1 galaxies, which are thought to be powered by young super-massive black holes. In combination with a previous high-redshift quasar detection in the field, we show that quasars with L 2−10 keV > 10 45 erg s −1 dominate accretion onto super-massive black holes at z ∼ 6.

On the Cosmic Evolution of AGN Obscuration and the X-Ray Luminosity Function: XMM-Newton and Chandra Spectral Analysis of the 31.3 deg2 Stripe 82X

“…These models are similar to the A19 curve, although slightly different in shape, and they lie well above our curve, particularly for z  2. The discrepancy between X-rays and theoretical simulations is known in the literature (e.g., Vito et al 2018;Barchiesi et al 2021;Wolf et al 2023), and it is generally explained with the challenges in efficiently detecting heavily obscured, Compton-thick sources in X-ray surveys (e.g., Hickox & Alexander 2018). These tensions suggest that Compton-thick AGNs may have a substantial role in the SMBHs accretion history, especially at high redshift, even though their contribution is still not well understood.…”

Steep-spectrum AGN in eROSITA Final Equatorial-Depth Survey (eFEDS): Their host galaxies and multi-wavelength properties

“…2.2, the steep part of the X-ray spectrum of AGN with high accretion rates would need to be limited below the rest-frame 2 keV. However, the spectra of bright NLS1s are generally steeper than those of normal Seyferts at energies even above 2 keV, which is the case for the eROSITAdetected z ∼ 6.5 quasar HSC J0921+0007 (Wolf et al 2023), for example, which has Γ ∼ 3 in the rest-frame energies above ∼5 keV. This discrepancy needs to be further investigated using a larger sample such as that being gathered by the eROSITA All-Sky Survey (Merloni et al 2012;Predehl et al 2021).…”

“…This means that, as well as objects similar to local NLS1s, more luminous counterparts at higher redshifts can also be detected in the eROSITA survey. An illustrative example has already been discovered: one of the Subaru high-z exploration of lowluminosity quasars (SHELLQs), HSC J0921+0007 at z = 6.56 (Matsuoka et al 2018), was detected by eROSITA and the subsequent Chandra observation revealed its steep spectrum of Γ 3.2 (Wolf et al 2023).…”