[Ne V ] Emission in Optically Classified Starbursts

Detecting active galactic nuclei (AGNs) 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 4+ , infrared [Ne V] emission lines are thought to be excellent AGN diagnostics. However, stellar evolution models predict that 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 reproduces the [Ne V] observations, yet leaves the optical spectrum looking like a starburst. We also find that infrared emission-line diagnostics are much more sensitive to the AGNs than optical diagnostics, particularly for weak AGNs. We apply our model to the optically classified, yet [Ne V] emitting, starburst galaxy NGC 3621. We find, when taking the infrared and optical spectrum into account, ~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.