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

We propose that the emission of the high excitation [O IV] 25.9 mu m line o bserved with ISO in NGC 5253 and II Zw 40 is due to the presence of hot Wol f-Rayet (WR) stars in these objects. We construct a consistent evolutionary synthesis and photoionization model which successfully reproduces the cons traints on their massive star content and the relevant optical and IR emiss ion Lines including [O Iv] 25.9 mu m. Our explanation for the origin of [O Iv] is supported empirically by: I) the simultaneous presence of nebular He II and [O Iv] in these objects, and 2) the close relation between nebular He Ir and WR stars in extragalactic HII regions. Photoionization by hot WR stars is mainly expected to be of importance in young low metallicity galax ies. Alternate mechanisms are likely at the origin of [O Iv] 25.9 mu m emis sion in other objects.