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

TL;DR

This work presents a deep $\sim$280 ks Chandra study of the highly luminous, Compton-thick QSO W0410-09 at $z\approx3.63$ and its overdense Ly$\alpha$-emitting environment. Through empirical and geometry-aware torus models (Borus and MYTorus), the nucleus is shown to be heavily obscured with $N_{H}\sim(0.9-1.0)\times10^{24}\ \mathrm{cm^{-2}}$ and intrinsic $L_{2-10}$ exceeding $10^{45}\ \mathrm{erg\ s^{-1}}$, indicating a luminous source in a blow-out phase. The CO-observed environment hosts 19 LAEs; while no X-ray LAEs are detected, a $\sim3\sigma$ Fe K region signal in the LAEs suggests a substantial population of heavily obscured AGN within the overdensity, potentially up to $\sim35\%$ when unresolved sources are included. The small $\sim30$ kpc CG Ly$\alpha$ nebula relative to unobscured QSOs supports the idea that heavy nuclear obscuration and outflows suppress ionizing flux to CG scales, aligning with merger-driven QSO growth scenarios. Overall, the paper highlights the power of deep X-ray observations to probe SMBH growth and feedback in the most luminous, obscured high-$z$ systems and sets the stage for future X-ray facilities.

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.

Figures (10)

• Figure 1: Left panel: ACIS-S image of W0410 $-$ 09 in the 0.3 $-$ 7 keV energy band. The red circle shows the 2 arcsec wide region used. Right panel: the circular region used from the extraction of the background. Sources detected by wavedetect and W0410 $-$ 09, and reported as crossed out circular regions, are not considered.
• Figure 2: Chandra/ACIS-S spectrum of W0410 $-$ 09. Empirical models are reported in different colors: Pow model in red, CabsPow model in blue and ReflDom model in orange. The spectrum has been slightly rebinned for better visualization. In the lower panel, the residuals for each model are shown in the same color as their respective model.
• Figure 3: Left panel: Chandra/ACIS-S spectrum with Borus model and residuals. The red solid line represents the best-fit model, the dotted line represents the reflection component and the dashed line represents the heavily-absorbed power-law continuum with exponential cut-off. The spectra have been slightly rebinned for better visualization. Right panel: Chandra/ACIS-S spectrum with BorSphere model and residuals. The red solid line represents the best-fit model, the dotted line represents the Compton reflection component, the dashed line indicates the absorbed cut-off power-law component.
• Figure 4: Chandra/ACIS-S spectrum with MYTorus model and residuals. The red solid line represents the best-fit model, the dashed line indicates the absorbed component, the dotted line represents the Compton-scattered component and the dot-dashed line denotes the lines component. The spectrum has been slightly rebinned for better visualization.
• Figure 5: ACIS-S image in the 0.3 $-$ 7 keV energy band showing the 24 LAEs. The 19 companions associated with the Hot DOG are highlighted with green crosses, while those not associated are indicated with black squares. The green circles mark the regions used for the companions' photometry. The X-ray sources detected using the wavdetect algorithm are shown in blue. The dashed circles indicate the regions excluded from the companions' photometry, while the Hot DOG is shown in red.