We present the first results from a near-IR spectroscopic campaign of the Cl1604 supercluster at z ~ 0.9 and the cluster RX J1821.6+6827 at z ~ 0.82 to investigate the nature of [O II] λ3727 emission in cluster galaxies at high redshift. Of the 401 members in Cl1604 and RX J1821+6827 confirmed using the Keck II/DEIMOS spectrograph, 131 galaxies have detectable [O II] emission with no other signs of current star formation activity, as well as strong absorption features indicative of a well-established older stellar population. The combination of these features suggests that the primary source of [O II] emission in these galaxies is not a result of star formation processes, but rather due to the presence of a low-ionization nuclear emission-line region (LINER) or Seyfert component. Using the NIRSPEC spectrograph on the Keck II 10 m telescope, 19 such galaxies were targeted, as well as 6 additional [O II]-emitting cluster members that exhibited signs of ongoing star formation activity. Nearly half (~47%) of the 19 [O II]-emitting, absorption-line-dominated galaxies exhibit [O II] to Hα equivalent width (EW) ratios higher than unity, the typical observed value for star-forming galaxies, with an EW distribution similar to that observed for LINERs at low redshift. A majority (~68%) of these 19 galaxies are classified as LINER/Seyfert based primarily on the emission-line ratio of [N II] λ6584 and Hα. The fraction of LINER/Seyferts increases to ~85% for red [O II]-emitting, absorption-line-dominated galaxies. The LINER/Seyfert galaxies in our Cl1604 sample exhibit average L([O II])/L(Hα) ratios that are significantly higher than that observed in populations of star-forming galaxies, suggesting that [O II] is a poor indicator of star formation in a significant fraction of high-redshift cluster members. From the prevalence of [O II]-emitting, absorption-line-dominated galaxies in both systems and the fraction of such galaxies that are classified as LINER/Seyfert, we estimate that at least ~20% of galaxies in high-redshift clusters with M 1010-1010.5 M ☉ contain a LINER/Seyfert component that can be revealed with line ratios. We also investigate the effect such a population has on the global star formation rate of cluster galaxies and the post-starburst fraction, concluding that LINER/Seyferts must be accounted for if these quantities are to be physically meaningful.
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