A deep X-ray look to the most obscured quasar at <i>z</i>   ∼  3.6 and its environment

Context. The most luminous and obscured quasars (QSOs) detected through sensitive infrared all-sky surveys are thought to represent a key co-evolutionary phase from nuclear to circumgalactic (CG) scales in the formation of massive galaxies. In this context, hot dust obscured galaxies (hot DOGs) in the redshift interval z ∼ 2 − 4 (the so-called cosmic noon) provide unique opportunities to investigate the relation between cosmic mass assembly and the nuclear accretion processes of luminous QSOs and galaxies at high- z . W0410−0913 (hereafter W0410−09) is a luminous ( L bol ∼ 6.4 × 10 47 erg s −1 ) and obscured QSO at z = 3.631 that is characterized by a 30 kpc CG Ly α nebula (CGLAN), which is rather small when the ∼ 100 kpc Ly α nebulae around the unobscured QSO is compared to the Type I QSO peers, and by an exceptional overdense environment of Ly α emitters (LAEs), with ∼19 of them located in the CG region with a radius of 300 kpc at a distance of ±200 km s −1 from the Hot DOG. Aims. Our aim is to detect and characterize active nuclear accretion in the Hot DOG W0410−09 and its environment. Methods. We carried out this study by exploiting a deep proprietary ∼280 ks Chandra X-ray Observatory observation. We employed a set of empirical models suited for obscured sources and physically motivated spectroscopic models to account for a toroidal X-ray obscurer and the reprocessing of the X-ray radiation. Results. The source W0410−09 consistently exhibits nuclear obscuration levels from mild to high star formation; it is Compton-thick (CT) and has a hydrogen column density of N H &gt; 10 24 cm −2 (and up to N H ∼ 10 25 cm −2 ) and an intrinsic luminosity of L 2 − 10 &gt; 10 45 erg s −1 . W0410−09 is therefore one of the most luminous and obscured QSO at z &gt; 3.5 discovered so far. This level of obscuration and the highly accreting nature of the Hot DOGs suggest that W0410−09 is undergoing a blow-out phase. This phase is predicted by models of merger-driven QSO formation scenarios, where strong winds begin to clear the dusty obscuring medium from the nuclear surroundings. We speculate that this heavy nuclear obscuration limits the amount of UV disk emission that powers its CGLAN, which in turn likely explains its small nebula size. Except for W0410−09, we detected no X-ray emission from any of the 19 LAEs. We analyzed their combined emission in several bands and only found a significant signal at the 3 σ level in the 6−7 keV rest-frame energy band. We interpret this as caused by the Fe K α line. This strongly suggests heavily obscured but so far undetected active galactic nuclei (AGN) emission in several LAEs. Considering W0410−09, we estimate an AGN fraction of f AGN LAE = 5 −4 +12 %. This value can reach ∼35% when we account for unresolved obscured AGN, as suggested by the detection of the Fe K α line. Conclusions. W0410−09 is powered by an intrinsically luminous CT quasar. Its high obscuration likely explains the limited extent of its CGLAN. Our analysis suggests that this object is in a crucial transitional blow-out phase, during which powerful QSO-driven outflows will sweep out the nuclear obscuration to pave the way for an unobscured bright quasar.