Faint [O IV] emission from starburst galaxies

We report the detection of faint emission in the high-excitation [OIV] 25.90um line in a number of starburst galaxies, from observations obtained with the Short Wavelength Spectrometer (SWS) on board ISO. Further observations of M82 spatially resolve the [OIV] emitting region. Detection of this line in starbursts is surprising since it is not produced in measurable quantities in HII regions around hot main-sequence stars, the dominant energy source of starburst galaxies. We discuss various models for the formation of this line. [OIV] that is spatially resolved by ISO cannot originate in a weak AGN and must be due to very hot stars or ionizing shocks related to the starburst activity. For low-excitation starbursts like M82, shocks are the most plausible source of [OIV] emission.