An estimate of the counts of faint radio sources at 5 GHz

Abstract

A recent search for 5-GHz emission from nearby E and S0 galaxies produced a frequency distribution of flux densities dominated by background fluctuations. Analysis of this distribution provides an estimate of the number–flux density relation for extragalactic radio sources at levels corresponding to ∼ 106/sr. The results indicate a very slow convergence for the 5-GHz count in comparison with N(S) relations at lower frequencies. This suggests that a flat-spectrum population dominates the 5-GHz count at levels below ∼ 20 mJy, and calculations have been carried out to determine whether the recognized populations of objects with appropriate spectra, i.e. ‘flat-spectrum’ quasars, or ‘active’ galaxies, could be responsible. If it is the former, the cosmological evolution of the objects must be very strong and there can be no redshift cut-off for z < 10; if the latter, this low-luminosity population also must undergo strong evolution, in contradistinction to low-luminosity sources with steep spectra. Finally, if a previously unrecognized population is responsible, the sources must resemble the ‘active’ galaxies in having flat spectra and relatively low luminosities, and considerable evolution is again required.