[OII] as a proxy for star formation in AGN host galaxies: beware of extended emission line regions

TL;DR

This paper investigates how extended emission line regions (EELRs) in AGN host galaxies can contaminate [OII] emission measurements, leading to inaccurate star formation rate estimates, and proposes using [NeV] as an EELR indicator.

Contribution

It introduces a method to identify EELRs using [NeV] emission and demonstrates the impact of EELRs on [OII]-based star formation estimates in AGN hosts.

Findings

[OII] flux is contaminated by EELRs in AGN spectra.

Removing EELR contamination reveals higher [OII] flux in radio-loud AGN.

EELRs can significantly bias star formation rate measurements.

Abstract

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.