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The Filamentary Large-Scale Structure around the z = 2.16 Radio Galaxy PKS 1138−262

PKS 1138-262 is a massive radio galaxy at z = 2.16 surrounded by overdensities of Lyα emitters, Hα emitters, extremely red objects, and X-ray emitters. Numerous lines of evidence exist that it is located in a forming cluster. We report on Keck spectroscopy of candidate members of this protocluster, including nine of the 18 X-ray sources detected by Pentericci and coworkers in this field. Two of these X-ray sources (not counting PKS 1138-262 itself) were previously confirmed to be members of the protocluster; we have discovered that an additional two (both active galactic nuclei [AGNs]) are members of a filamentary structure at least 3.5 Mpc in projection aligned with the radio jet axis, the 150 kpc–sized emission-line halo, and the extended X-ray emission around the radio galaxy. Three of the nine X-ray sources observed are lower redshift AGNs, and three are M dwarf stars.