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We have employed emission-line diagnostics derived from DEIMOS and NIRSPECspectroscopy to determine the origin of the [OII] emission line observed in sixAGN hosts at z~0.9. These galaxies are a subsample of AGN hosts detected in theCl1604 supercluster that exhibit strong Balmer absorption lines in theirspectra and appear to be in a post-starburst or post-quenched phase, if not fortheir [OII] emission. Examining the flux ratio of the [NII] to Halpha lines, wefind that in five of the six hosts the dominant source of ionizing flux is AGNcontinuum emission. Furthermore, we find that four of the six galaxies haveover twice the [OII] line luminosity that could be generated by star formationprocesses alone given their Halpha line luminosities. This strongly suggeststhat AGN-excited narrow-line emission is contaminating the [OII] line flux. Acomparison of star formation rates calculated from extinction-corrected [OII]and Halpha line luminosities indicates that the former yields a five-foldoverestimate of current activity in these galaxies. Our findings reveal the[OII] line to be a poor indicator of star formation activity in a majority ofthese moderate-luminosity Seyferts. This result bolsters our previous findingsthat an increased fraction of AGN at high redshifts are hosted by galaxies in apost-starburst phase. The relatively high fraction of AGN hosts in the Cl1604supercluster that show signs of recently truncated star formation activitysuggest AGN feedback may play an increasingly important role in suppressingongoing activity in large-scale structures at high redshift.

The Origin of [OII] Emission in Recently Quenched AGN Hosts