[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, similar to 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.