Deep Radio Source Counts and Small Scale Fluctuations of the Microwave Background Radiation

Abstract

Source counts which now extend to surface densities of ∿10⁵ sources/sr make possible a direct evaluation of the radio source contribution to the small-scale fluctuations in the microwave background on scales larger than ∿10′, at wavelengths ≧6 cm. Comprehensive radio spectral data permit a straightforward and largely model-independent extrapolation of the N(S) relation to shorter wavelengths. On the other hand, Peacock and Gull (1981, hereafter PG) have constructed a set of models which incorporate a wealth of additional data, such as local luminosity functions, luminosity/redshift distributions, luminosity-spectral index correlations; they can therefore be exploited to optimize the extrapolations both to higher frequencies and to fainter flux densities. Only one of these models, however, namely No. 4, is consistent with the recent P(D) results (Wall et al. 1982; Ledden et al. 1980) which provide information on the areal density of sources at s∿lmJy; therefore, in the following we shall focus on it. (It is interesting to note, in passing, that P(D) counts do not reflect the faint end of counts at 408 MHz as it would be expected in view of the rapid convergence of the flat spectrum source counts below 500 mJy. In fact, PG model 4 predicts that 5 GHz counts at S<lmJy are dominated by flat spectrum sources.