A deep X-ray look to the most obscured quasar at z~3.6 and its environment

Abstract

The most luminous and obscured quasars (QSOs) detected in infrared all-sky surveys could represent a key co-evolutionary phase from nuclear to circum-galactic (CG) scales in the formation of massive galaxies. In this context, Hot Dust Obscured Galaxies (Hot DOGs) at z ~2-4 provide a unique opportunity to study the link between cosmic mass assembly and nuclear accretion in high-z luminous QSOs/galaxies. W0410-0913 (hereafter W0410-09) is a luminous (${\mathrm{L}}_{\mathrm{bol}}\sim 6.4\times 10^{47}\mathrm{erg}{\mathrm{s}}^{-1}$) obscured QSO at z = 3.631, with a 30 kpc CG Ly$\hat{I}\pm$ nebula (CGLAN), smaller than the ~ 100 kpc nebulae around unobscured Type-I QSOs, and an exceptional overdense environment of ~ 19 Ly$\hat{I}\pm$ emitters (LAEs) within 300 kpc and $\pm$ 200 $\mathrm{km}{\mathrm{s}}^{-1}$ of the Hot DOG. We aim to detect and characterize nuclear accretion in W0410-09 and its environment. Exploiting a deep proprietary ~280 ks Chandra observation, using empirical and physically motivated models for obscured sources, we show that W0410-09 exhibits Compton-thick obscuration (${\mathrm{N}}_{\mathrm{H}}>10^{24}\mathrm{c}{\mathrm{m}}^{-2}$) and high intrinsic luminosity (${\mathrm{L}}_{2-10}>10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$), making it one of the most luminous obscured QSOs at z $>$ 3.5. With the exclusion of W0410-09 we do not detect X-ray emission from any of the 19 LAEs, except for a 3$\ddot{I}ƒ$ signal in the 6-7 keV rest-frame band, interpreted as Fe K$\hat{I}\pm$ emission, suggesting the presence of heavily obscured yet undetected AGN emission in several LAEs. Including W0410-09, the estimated AGN fraction is $f_{\mathrm{AGN}}^{\mathrm{LAE}}=5_{-4}^{+12}$%, potentially up to ~35% if unresolved obscured AGN are considered as suggested by the Fe K$\hat{I}\pm$ line detection. We conclude that W0410-09 is in a critical transitional blow-out phase, during which powerful QSO-driven outflows are clearing the nuclear obscuration, ultimately leading to an unobscured luminous quasar.

11 pages, 10 figures, 3 tables. Accepted for publication in A&A