The eROSITA Final Equatorial-Depth Survey (eFEDS). The first archetypal quasar in the feedback phase discovered by eROSITA

Theoretical models of the co-evolution of galaxies and active galactic nuclei (AGNs) ascribe an important role in the feedback process to a short, luminous, obscured, and dust-enshrouded phase during which the accretion rate of the supermassive black hole is expected to be at its maximum and the associated AGN-driven winds are also predicted to be maximally developed. To test this scenario, we have isolated a textbook candidate from the eROSITA Final Equatorial-Depth Survey (eFEDS) obtained within the performance and verification program of the eROSITA telescope on board the Spectrum Rontgen Gamma mission. From an initial catalogue of 246 hard X-ray selected sources that are matched with the photometric and spectroscopic information available within the eROSITA and Hyper Suprime-Cam consortia, three candidates quasars in the feedback phase have been isolated applying a diagnostic proposed previously. Only one source (eFEDS J091157.4+014327) has a spectrum already available (from SDSS-DR16, z = 0.603) and it unambiguously shows a broad component (full width at half maximum similar to 1650 km s(-1)) in the [OIII]5007 line. The associated observed L-[OIII] is similar to 2.6 x 10(42) erg s(-1), one to two orders of magnitude higher than that observed in local Seyfert galaxies and comparable to those observed in a sample of z similar to 0.5 type 1 quasars. From the multi-wavelength data available, we derive an Eddington ratio (L-b(ol)/L-Edd) of similar to 0.25 and a bolometric correction in the hard X-ray band of k(bol) similar to 10, which is lower than the corrections observed for objects at similar bolometric luminosity. These properties, along with the outflow, the high X-ray luminosity, the moderate X-ray obscuration (L-X similar to 10(44.8) erg s(-1), N-H similar to 2.7 x 10(22) cm(-2)), and the red optical colour, all match the prediction of quasars in the feedback phase from merger-driven models. Forecasting to the full eROSITA all-sky survey with its spectroscopic follow-up, we predict that by the end of 2024, we will have a sample of few hundred such objects at z = 0.5-2.