2025A&A...694A.165T

Context. Exploring supermassive black hole (SMBH) populations in protoclusters offers valuable insights into how environment affects SMBH growth. However, research on active galactic nuclei (AGN) within these areas is still limited by the small number of protoclusters known at high redshift and by the availability of associated deep X-ray observations.
Aims. In order to understand how different environments affect AGN triggering and growth at high redshift, we investigated the X-ray AGN population in the field of the MUSE Quasar Nebula 01 (MQN01) protocluster at z ≃ 3.25. This field is known for hosting one of the largest Ly α nebulae and overdensities of UV continuum selected and sub-millimetre galaxies found so far at this redshift.
Methods. We conducted an ultra-deep Chandra X-ray survey (634 ks) observation of the MQN01 field and produced a comparative analyses of the properties of the X-ray AGN detected in MQN01 against those observed in other selected protoclusters, such as Spiderweb and SSA22.
Results. By combining the X-ray observations with deep MUSE and ALMA data of the same field, we identified six X-ray AGN within a volume of 16 comoving Mpc² and ±1000 km s^–1, corresponding to an X-ray AGN overdensity of δ ≈ 1000. This overdensity increases at the bright end (log(L_2#x2212;10 keV_/erg s^–1) ≳ 44.5), exceeding what was observed in the Spiderweb and SSA22 within similar volumes. The AGN fraction measured in MQN01 is significantly higher (> 20%) than in the field and increases with stellar masses, reaching a value of 100% for log(M_*/M_⊙) > 10.5. Lastly, we observe that the average specific accretion rate (λ_sBHAR) for SMBH populations in MQN01 is higher than in the field and other protoclusters, generally increasing as one moves toward the centre of the overdense structures.
Conclusions. Our results, especially the large fraction of highly accreting SMBHs in the inner regions of the MQN01 overdensity, suggest that protocluster environments offer ideal physical conditions for SMBH triggering and growth.