On the origin of [O iv] emission in Wolf-Rayet galaxies

Abstract

We propose that the emission of the high excitation [O iv] 25.9 mu m line observed with ISO in NGC 5253 and II Zw 40 is due to the presence of hot Wolf-Rayet (WR) stars in these objects. We construct a consistent evolutionary synthesis and photoionization model which successfully reproduces the constraints on their massive star content and the relevant optical and IR emission lines including [O iv] 25.9 mu m. Our explanation for the origin of [O iv] is supported empirically by: 1) the simultaneous presence of nebular He ii\ and [O iv] in these objects, and 2) the close relation between nebular He ii\ and WR stars in extragalactic Hii regions. Photoionization by hot WR stars is mainly expected to be of importance in young low metallicity galaxies. Alternate mechanisms are likely at the origin of [O iv] 25.9 mu m emission in other objects.