[Ne V] Emission in Optically Classified Starbursts

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.