The Origin of [OII] Emission in Recently Quenched AGN Hosts
  We have employed emission-line diagnostics derived from DEIMOS and NIRSPEC
spectroscopy to determine the origin of the [OII] emission line observed in six
AGN hosts at z~0.9. These galaxies are a subsample of AGN hosts detected in the
Cl1604 supercluster that exhibit strong Balmer absorption lines in their
spectra and appear to be in a post-starburst or post-quenched phase, if not for
their [OII] emission. Examining the flux ratio of the [NII] to Halpha lines, we
find that in five of the six hosts the dominant source of ionizing flux is AGN
continuum emission. Furthermore, we find that four of the six galaxies have
over twice the [OII] line luminosity that could be generated by star formation
processes alone given their Halpha line luminosities. This strongly suggests
that AGN-excited narrow-line emission is contaminating the [OII] line flux. A
comparison of star formation rates calculated from extinction-corrected [OII]
and Halpha line luminosities indicates that the former yields a five-fold
overestimate of current activity in these galaxies. Our findings reveal the
[OII] line to be a poor indicator of star formation activity in a majority of
these moderate-luminosity Seyferts. This result bolsters our previous findings
that an increased fraction of AGN at high redshifts are hosted by galaxies in a
post-starburst phase. The relatively high fraction of AGN hosts in the Cl1604
supercluster that show signs of recently truncated star formation activity
suggest AGN feedback may play an increasingly important role in suppressing
ongoing activity in large-scale structures at high redshift.
