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

Abstract

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 deg2 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 for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes. Conclusions. Our results indicate that the population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect ~90 X-ray luminous AGNs at redshifts 5.7 < z < 6.4 in the full-sky survey (De+RU). While subject to different jet physics, both high-redshift quasars detected by eROSITA so far are radio-loud; a hint at the great potential of combined X-ray and radio surveys for the search of luminous high-redshift quasars.

Simulating infrared spectro-photometric surveys with a Spritz

“…The bright-end of the X-ray LF of the simulation is in agreement with previous observational results up to z = 6 (Aird et al 2010;Aird et al 2015;Miyaji et al 2015;Vito et al 2018;Wolf et al 2021), above which no observations are at the moment available. The faint-end slope, on the contrary, is slightly overestimated and the difference between our simulation and observations increases with redshift, from a factor of four at z ∼ 0.5 to a factor of ∼15 at z ∼ 5, but still within the uncertainties.…”

The LOFAR Two-metre Sky Survey

“…Meanwhile, valuable synergies are being established such as those with the LOFAR Magnetism Key Science Project 2 , APERTIF imaging surveys, Extended Baryon Oscillation Spectroscopic Survey (eBOSS; Dawson et al 2016), extended ROentgen Survey with an Imaging Telescope Array (eROSITA; Predehl et al 2021) and the William Herschel Telescope Enhanced Area Velocity Explorer survey of LOFAR selected sources (WEAVE-LOFAR; Smith et al 2016) which are each enabling new scientific studies (e.g. Ghirardini et al 2021, Morganti et al 2021, O'Sullivan et al 2020, Wolf et al 2021. Finally, the LOFAR surveys are also having a large technical impact with studies of calibration and imaging techniques (e.g.…”

The SRG X-ray orbital observatory, its telescopes and first scientific results

“…Particularly interesting is the quasar CFHQS J142952+544717 at z = 6.2 (corresponding to the age of the Universe of 900 million years), which has been detected in X-rays for the first time by eROSITA and proved to have the highest X-ray luminosity (∼ 3 × 10 46 erg/s, or ∼ 10 13 bolometric luminosities of the Sun) among quasars at z > 6 (Medvedev et al 2020(Medvedev et al , 2021. During the CalPV phase and in the course of scanning the entire sky, SRG/eROSITA has discovered a number of extremely luminous quasars at redshift z > 5 and a noticeable number at z > 4 (Khorunzhev et al 2020;Dodin et al 2020;Bikmaev et al 2020;Wolf et al 2021). In Fig.…”