A homogeneous comparison of protocluster AGN populations using Chandra observations of the HS1549 protocluster

Protoclusters at high redshift provide a unique laboratory for studying the co-evolution of galaxies and supermassive black holes in the dense environments that will eventually form galaxy clusters. X-ray surveys of several protoclusters have revealed enhanced active galactic nucleus (AGN) activity relative to the field, but because each was designed around its own target, with different depths, footprints, and ancillary catalogues, obtaining a homogeneous comparison of these systems is challenging. Here, we present 190 ks of Chandra ACIS-I observations of the HS 1549+19 protocluster at redshift z = 2.85, a known overdensity of submillimetre galaxies (SMGs) and Ly α emitters hosting a hyperluminous central quasar and an enormous Ly α nebula. We identify nine X-ray AGN associated with the protocluster, three of them newly identified in this work. Two AGN show radio emission in excess of that expected from star formation, including the central quasar, whose extended radio and X-ray emission provides tentative evidence for jet activity. Additionally, one newly identified AGN is a candidate for merger-driven black hole growth with a nearby SMG. To compare HS 1549+19 directly with other well-studied protoclusters at z ≈ 2–3, we apply a common set of selection criteria to every field: a single X-ray luminosity threshold set by the shallowest survey, fixed physical apertures and comoving volume, spectroscopic membership, and a matched SMG flux limit. Under these cuts, HS 1549+19 shows one of the highest AGN fractions among SMGs in the sample, and a cumulative AGN space density two to four orders of magnitude above the field prediction. At the same time, the protocluster luminosity functions become broadly similar, indicating that some of the apparent field-to-field variation reflects differences in survey design rather than in the AGN populations themselves. These results show that homogeneous comparisons across protocluster X-ray surveys offer a practical way to separate environmental trends from survey-dependent effects.