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

I. Villani, L. Zappacosta, E. Piconcelli, M. Ginolfi, F. Ricci, F. La Franca, F. Arrigoni-Battaia, A. Bongiorno, S. Cantalupo, S. Carniani, F. Civano, A. Comastri, F. Fiore, R. Maiolino, L. Pentericci, C. Ricci, R. Schneider, R. Valiante, C. Vignali, F. Vito

First listed 2025-11-07 | Last updated 2025-11-07

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 ($\rm ~L_{\rm bol} \sim 6.4 \times10^{47} \rm erg\ s^{-1}$) obscured QSO at z = 3.631, with a 30 kpc CG Ly$α$ nebula (CGLAN), smaller than the ~ 100 kpc nebulae around unobscured Type-I QSOs, and an exceptional overdense environment of ~ 19 Ly$α$ emitters (LAEs) within 300 kpc and $\pm$ 200 $\rm km ~s^{-1}$ 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 ($\rm~ N_H > 10^{24} \rm cm^{-2}$) and high intrinsic luminosity ($\rm ~L_{2-10} > 10^{45} \rm erg ~s^{-1}$), 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$σ$ signal in the 6-7 keV rest-frame band, interpreted as Fe K$α$ emission, suggesting the presence of heavily obscured yet undetected AGN emission in several LAEs. Including W0410-09, the estimated AGN fraction is $f_{\rm AGN}^{\rm LAE} = 5^{+12}_{-4}$%, potentially up to ~35% if unresolved obscured AGN are considered as suggested by the Fe K$α$ 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.

Short digest

A 280 ks Chandra view of the Hot DOG quasar W0410-09 (z=3.631) reveals Compton-thick obscuration (N_H > 10^24 cm^-2) and an intrinsic hard X-ray luminosity L_2–10 > 10^45 erg s^-1, making it among the most luminous obscured QSOs beyond z > 3.5. In its overdense environment of 19 Lyα emitters within 300 kpc and ±200 km s^-1, no sources are individually detected in X-rays, but a rest-frame 6–7 keV stack shows a ~3σ Fe Kα signal, implying hidden AGN; the LAE AGN fraction is f_AGN^LAE = 5^{+12}_{-4}% and could reach ~35% if these obscured components are included. The relatively compact 30 kpc Lyα nebula compared to ~100 kpc structures around unobscured QSOs is attributed to heavy nuclear obscuration suppressing ionizing UV, consistent with a transitional blow-out phase. This system links extreme accretion and obscuration from nuclear to circum-galactic scales during early massive-galaxy assembly.

Key figures to inspect

• Figure 1: Inspect the ACIS-S 0.3–7 keV image and extraction regions to see the isolated X-ray point source at W0410-09 and the lack of nearby X-ray detections among LAEs after wavdetect masking.
• Figure 2: Compare empirical fits (power law, absorbed power law, reflection-dominated) to visualize the hard-band curvature and residuals that motivate a heavily obscured/reflection component.
• Figure 3: Examine BORUS/BorSphere torus fits separating transmitted and reflection components; note how CT-level N_H and geometry affect the continuum and reflection strength, setting the intrinsic L_2–10.
• Figure 4: Use MYTorus decomposition (transmitted, Compton-scattered, line components) to evaluate CT obscuration and the Fe Kα line behavior that anchors the inferred column density.