[OII] as a proxy for star formation in AGN host galaxies: beware of
  extended emission line regions
  The [OII] 3726+3728\AA\ emission line doublet is often used to estimate star
formation rates within the host galaxies of active galactic nuclei (AGN), as it
is known to be strongly excited by star formation, but is only weakly excited
in the broad and narrow line regions of AGN. However, within AGN host galaxies,
[OII] can also be excited in low-density gas located at appreciable distances
from the nucleus, but still ionized by the AGN. These AGN extended emission
line regions (EELRs) can contribute significant flux to integrated spectra,
even in the presence of luminous AGN. Here, we identify EELRs by the presence
of the [NeV] 3426\AA\ emission line, which, like [OII], is not strongly excited
in the inner regions of AGN, but is a prominent emission line in the lower
density EELRs. Critically, unlike [OII], [NeV] is not excited by star
formation. Therefore, when strong [NeV] is present in an AGN spectrum, the flux
from the EELR is not negligible, implying the [OII] flux is contaminated by
emission from the EELR, and is not a good measure of star formation. After
removing objects with EELRs identified by [NeV], the [OII] flux in the host
galaxies of radio-loud AGN is found to be higher than that within radio-quiet
AGN, which could either indicate higher star-formation rates, or the presence
of moderate-velocity shocks. Being mindful of EELRs for upcoming large-area
spectroscopic surveys, particularly those tied to radio continuum surveys, will
be important for determining star formation rates in AGN host galaxies.
