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

TL;DR

This study investigates the origin of [OII] emission in AGN host galaxies at z~0.9, revealing that AGN activity, not star formation, predominantly causes the emission, thus challenging its use as a star formation indicator.

Contribution

It demonstrates that [OII] emission in these AGN hosts is mainly due to AGN activity, not star formation, highlighting the need to reconsider [OII] as a star formation tracer in such galaxies.

Findings

Most [OII] emission is AGN-driven, not star formation.

[OII]-based star formation rates overestimate actual activity by five times.

High fraction of post-starburst AGN hosts suggests AGN feedback suppresses star formation.

Abstract

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.