THE ORIGIN OF [OII] IN POST-STARBURST AND RED-SEQUENCE GALAXIES IN HIGH-REDSHIFT CLUSTERS

We present the first results from a near-IR spectroscopic campaign of the Cl1604 supercluster at z similar to 0.9 and the cluster RX J1821.6+6827 at z similar to 0.82 to investigate the nature of [OII]lambda 3727 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 [OII] 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 [OII] 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 [OII]-emitting cluster members that exhibited signs of ongoing star formation activity. Nearly half (similar to 47%) of the 19 [OII]-emitting, absorption-line-dominated galaxies exhibit [OII] to H alpha 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 (similar to 68%) of these 19 galaxies are classified as LINER/Seyfert based primarily on the emission-line ratio of [NII]lambda 6584 and H alpha. The fraction of LINER/Seyferts increases to similar to 85% for red [OII]emitting, absorption-line-dominated galaxies. The LINER/Seyfert galaxies in our Cl1604 sample exhibit average L([OII])/L(H alpha) ratios that are significantly higher than that observed in populations of star-forming galaxies, suggesting that [OII] is a poor indicator of star formation in a significant fraction of high-redshift cluster members. From the prevalence of [OII]-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 similar to 20% of galaxies in high-redshift clusters with M-circle dot > 10(10)-10(10.5) M-circle dot 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.