Faint [O iv] emission from starburst galaxies

Abstract

We report the detection of faint emission in the high-excitation [O iv] 25.90mu m line in a number of starburst galaxies, from observations obtained with the Short Wavelength Spectrometer (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 measurable quantities in H ii 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 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 M 82, shocks are the most plausible source of [O iv] emission. Based on observations with ISO, an ESA project with instruments funded by ESA Member States and with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.