Active Galactic Nuclei in the Extremely Overdense Galaxy Region BOSS 1441: A Chandra Observation

We present a Chandra/ACIS-I study of X-ray sources in BOSS 1441, a protocluster at z = 2.32 ± 0.02 that exhibits a prominent overdensity of Lyα emitters (LAEs). Using a 45 ks observation, we identify seven X-ray sources spatially coincident with LAE density peaks. The average X-ray photon index for the seven sources, derived from an absorbed power-law model with Galactic absorption fixed, is 1.49 (ranging from −0.68 to 2.51), corresponding to an average luminosity of 6.85 × 10⁴⁴erg s⁻¹ in the rest-frame 2-33 keV band, with individual luminosities spanning (3.57-13.96) × 10⁴⁴ erg s⁻¹. Three sources exhibit relatively flat spectral slopes. Two are associated with the MAMMOTH-1 nebula, while the third, located at the edge of BOSS 1441 with a > 5 ′ offset from the LAE density peak, resides in a region with a high submillimeter-band density. We estimate the fraction of X-ray-detected active galactic nuclei (AGNs) among the LAEs to be 11 . 5 − 4.6 + 3.8 % , approximately double that of previously studied LAEs. This elevated fraction suggests BOSS 1441 is in a mature evolutionary stage, with even higher AGN fractions expected in massive LAEs such as PKS 1138-262. In contrast, the submillimeter galaxy population shows a lower AGN fraction ( 6 . 9 − 4.5 + 6.9 % ), consistent with their typically obscured nature. These results indicate that the protocluster’s massive galaxies are evolving into the bright red sequence galaxies observed in local clusters, where AGNs likely play a critical role in quenching their star formation.