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

Abstract 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 Ly α 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 × 10 44 erg s −1 in the rest-frame 2–33 keV band, with individual luminosities spanning (3.57–13.96) × 10 44 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>&gt;</mml:mo> <mml:mn>5</mml:mn> <mml:mspace width="0.1em"/> <mml:mo accent="false">′</mml:mo> <mml:mspace width="0.1em"/> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>11</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3.8</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> </mml:math> , 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>6</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>6.9</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> </mml:math> ), 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.