FAINT O IV EMISSION FROM STARBURST GALAXIES

We report the detection of faint emission in the high- excitation (Oiv) 25.90m line in a number of starburst galaxies, from observations obtained with the Short Wavelength Spec- trometer (SWS) on board ISO. Further observations of M 82 spatially resolve the (O iv) emitting region. Detection of this line in starbursts is surprising since it is not produced in measur- able 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. (O iv) that is spa- tially 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 M 82, shocks are the most plausible source of (Oiv) emission.