[Ne V] Emission in Optically Classified Starbursts

TL;DR

This study investigates whether [Ne V] emission in starburst galaxies can originate from star formation alone or indicates the presence of an AGN, using spectral modeling and applying it to NGC 3621.

Contribution

The paper introduces models that demonstrate [Ne V] emission can arise from star formation, challenging its use as an exclusive AGN indicator, and quantifies AGN contribution in starburst galaxies.

Findings

[Ne V] can be produced by star formation under certain conditions.

Infrared diagnostics are more sensitive to AGN than optical diagnostics.

Approximately 30-50% of NGC 3621's luminosity is due to an AGN.

Abstract

Detecting Active Galactic Nuclei (AGN) in galaxies dominated by powerful nuclear star formation and extinction effects poses a unique challenge. Due to the longer wavelength emission and the ionization potential of Ne^3+, infrared [Ne V] emission lines are thought to be excellent AGN diagnostics. However, stellar evolution models predict Wolf-Rayet stars in young stellar clusters emit significant numbers of photons capable of creating Ne^4+. Recent observations of [Ne V] emission in optically classified starburst galaxies require us to investigate whether [Ne V] can arise from star-formation activity and not an AGN. In this work, we calculate the optical and IR spectrum of gas exposed to a young starburst and AGN SED. We find: 1) a range of parameters where [Ne V] emission can be explained solely by star-formation and 2) a range of relative AGN to starburst luminosities that reproduce the [Ne V] observations, yet leaves the optical spectrum looking like a starburst. We also find infrared emission-line diagnostics are much more sensitive to the AGN than optical diagnostics, particularly for weak AGN. We apply our model to the optically classified, yet [Ne V] emitting, starburst galaxy NGC 3621. We find, when taking the IR and optical spectrum into account, about 30 - 50% of the galaxy's total luminosity is due to an AGN. Our calculations show that [Ne V] emission is almost always the result of AGN activity. The models presented in this work can be used to determine the AGN contribution to a galaxy's power output.