A limit to the X-ray luminosity of nearby normal galaxies

Abstract

The origin of the extragalactic diffuse X-ray background is not well understood. It is isotropic to the extent that a galactic origin can be excluded. Indeed, even a halo of radius as large as 12 kpc could provide no more than 3% of the flux¹. The spectrum between 3 and 50 keV fits a 40-keV thermal bremsstrahlung form which is suggestive of emission from a hot intergalactic medium². This or some other truly diffuse mechanism may be responsible. If, however, discrete sources play a major part, we require either strong evolution of an already recognised X-ray emitting class, such as quasars, or as we consider here, a hitherto elusive population of low luminosity emitter.