First constraints on the AGN X-ray luminosity function at z ~ 6 from an eROSITA-detected quasar

• Published in: Astronomy and astrophysics (Berlin) 2021-03, Vol.647, p.A5
• Main Authors: Wolf, J., Nandra, K., Salvato, M., Liu, T., Buchner, J., Brusa, M., Hoang, D. N., Moss, V., Arcodia, R., Brüggen, M., Comparat, J., de Gasperin, F., Georgakakis, A., Hotan, A., Lamer, G., Merloni, A., Rau, A., Rottgering, H. J. A., Shimwell, T. W., Urrutia, T., Whiting, M., Williams, W. L.
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Color; Deposition; Evolution; LOFAR; Luminosity; Morphology; Population; Quasars; Radio; Radio sources (astronomy); Red shift; Sky surveys (astronomy); Soft x rays; Spectroscopy; X ray sources; X-ray astronomy; X-rays
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202039724

Context. High-redshift quasars signpost the early accretion history of the Universe. The penetrating nature of X-rays enables a less absorption-biased census of the population of these luminous and persistent sources compared to optical/near-infrared colour selection. The ongoing SRG/eROSITA X-ray all-sky survey offers a unique opportunity to uncover the bright end of the high- z quasar population and probe new regions of colour parameter space. Aims. We searched for high- z quasars within the X-ray source population detected in the contiguous ~140 deg 2 field observed by eROSITA during the performance verification phase. With the purpose of demonstrating the unique survey science capabilities of eROSITA, this field was observed at the depth of the final all-sky survey. The blind X-ray selection of high-redshift sources in a large contiguous, near-uniform survey with a well-understood selection function can be directly translated into constraints on the X-ray luminosity function (XLF), which encodes the luminosity-dependent evolution of accretion through cosmic time. Methods. We collected the available spectroscopic information in the eFEDS field, including the sample of all currently known optically selected z > 5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample. Results. We report the X-ray detection of eFEDSU J083644.0+005459, an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 ( z = 5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous Chandra observations. The detection of SDSS J083643.85+005453.3 allows us to place the first constraints on the XLF at z > 5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at z ~ 6 beyond L * , the knee in the luminosity function. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that SDSS J083643.85+005453.3 could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 indicate that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications f