On Intergalactic Gas as a Possible Absorber of Extragalactic Radio Noise

Abstract

IT has recently been pointed out by Smith¹ that most of the background radiation at low frequencies (< 10 Mc/s) is probably of extragalactic origin. The intensity of this radiation increases with decreasing frequency until about 3 Mc/s where it turns over sharply^1–3. This turnover is probably due to absorption by ionized hydrogen, the required emission measure being about 4 cm⁻⁶ pc (for an electron temperature of 10⁴ °K). It was suggested by Hoyle and Ellis⁴ that the ionized hydrogen lies in a slab parallel to the galactic plane, with an electron concentration n_e ∼ (0.15 cm⁻³ and a thickness ∼200 pc. This model has been ruled out by the rocket and satellite observations of Walsh et al.³ and Smith¹, which show that even when integrated over a hemisphere the radiation has a very sharp turnover in its spectrum, corresponding to the emission measure being nearly isotropic around us. These authors suggest⁵ that we are surrounded by a more or less spherical H II region with, say, n_e ∼ 1 cm⁻³ and radius ∼4 pc (parameters which would not lead to unreasonably large Faraday rotation effects).