  PKS 1138-262 is a massive radio galaxy at z = 2.16 surrounded by
overdensities of Lya emitters, Ha emitters, EROs 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 et al. (2002) 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 AGN) 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 AGN, and three are
M-dwarf stars.

Astrophysics