Ionising flux of cosmic background radiation

Abstract

THE intensity of the diffuse cosmic background radiation for photon energies of 13.6 to ∼ 150eV cannot be measured directly because of absorption by the neutral component of the interstellar medium. In the range 150–300 eV, there is considerable galactic emission within a few hundred parsecs of the Sun which places severe limitations on attempts to measure the intensity and spectrum of the metagalactic background radiation at these energies. An indirect means of estimating the diffuse metagalactic flux of ionising radiation was proposed by Sunyaev¹, who studied the interaction of this radiation with neutral hydrogen at the peripheries of galaxies. More locally, the intensity of this radiation within ∼ 1 kpc of the Sun can be probed by studying the ionisation of heavy elements in the interstellar medium². Here we re-examine Sunyaev's proposal in the context of more detailed studies of the interaction of metagalactic ionising radiation with galactic gas. We give a firm upper limit on the intensity of metagalactic ionising background radiation to which the neutral interstellar medium is exposed, and discuss some important consequences of this result. Our principal conclusion is that further than ∼ 30 kpc from the centre of the Galaxy, self-shielding by H II^3,4 is possible only if the flux of ionising radiation is below a certain critical value^5–7.