High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars

Abstract

We discuss our ATCA 18.5 and 22 GHz flux density measurements of Southern extragalactic sources in the complete 5 GHz sample of Kühr et al. (1981, A&AS, 45, 367). The high frequency (5-18.5 GHz) spectral indices of steep-spectrum sources for which we have 18.5 GHz data (66% of the complete sample) are systematically steeper than the low frequency (2.7-5 GHz) ones, with median α2.75 = 0.76, median α 518.5 = 1.18 (Sv ∝ v-α), and median steepening Δα = 0.32, and there is evidence of an anti-correlation of Δα518.5 with luminosity. The completeness of 18.5 GHz data is much higher (89%) for flat-spectrum sources (mostly quasars), which also exhibit a spectral steepening: median α2.75 = -0.14, median α518.5 = 0.16 (Sv ∝ v-α), and median Δα = 0.19. Taking advantage of the almost complete redshift information on flat-spectrum quasars, we have estimated their 5 GHz luminosity function in several redshift bins. The results confirm that their radio luminosity density peaks at zpeak≃ 2.5 but do not provide evidence for deviations from pure luminosity evolution as hinted at by other data sets. A comparison of our 22 GHz flux densities with WMAP K-band data for flat-spectrum sources suggests that WMAP flux densities may be low by a median factor of ≃ 1.2. The extrapolations of 5 GHz counts and luminosity functions of flat-spectrum radio quasars using the observed distribution of the 5-18.5 GHz spectral indices match those derived directly from WMAP data, indicating that the high frequency WMAP survey does not detect any large population of FSRQs with anomalous spectra. © ESO 2005.