X-RAY STUDY OF STARBURST GALAXIES

We present full results of a wide-energy, population study of X-ray emission from a sample of 51 candidate starburst galaxies selected from the IRAS Bright Galaxy Sample. Super posed low and high energy X-ray emission from these galaxies in the Einstein IPC and HEAO-1 A2 and A4 energy bands, which span 0.5 to 160 keV, is detected at the 99.99% confidence level, after allowing for confusion noise in the HEAO-1 data. Above 13 keV the confidence level is only 85%. A power-law fit to the mean spectral luminosity yields a (photon) index of 1.47 +/- 0.26. We consider and assess likely environments and mechanisms for X-ray emission in starburst galaxies. These include thermal emission from massive binaries, supernova remnants, and galactic halos, and nonthermal emission resulting from Compton scattering of relativistic electrons by the far IR and the cosmic microwave background radiation fields. The contribution of the population of sources represented by this sample to the 3-50 keV residual cosmic X-ray background is estimated to be at the 3 - -4% level assuming no evolution. This contribution is significantly higher if the population evolved moderately.