The Deep X-Ray Radio Blazar Survey. III. Radio Number Counts, Evolutionary Properties, and Luminosity Function of Blazars

Our knowledge of the blazar surface densities and luminosity functions, which are fundamental parameters, relies still on samples at relatively high flux limits. As a result, our understanding of this rare class of active galactic nuclei is mostly based on relatively bright and intrinsically luminous sources. We present the radio number counts, evolutionary properties, and luminosity functions of the faintest blazar sample with basically complete (~95%) identifications. Based on the Deep X-Ray Radio Blazar Survey (DXRBS), it includes 129 flat-spectrum radio quasars (FSRQs) and 24 BL Lac objects down to a 5 GHz flux and power ~50 mJy and ~1024 W Hz-1, respectively, an order of magnitude improvement as compared to previously published (radio-selected) blazar samples. DXRBS FSRQs are seen to evolve strongly, up to redshift ≈1.5, above which high-power sources show a decline in their comoving space density. DXRBS BL Lac objects, on the other hand, do not evolve. High-energy and low-energy peaked BL Lac objects (HBLs and LBLs, respectively) share the same lack of cosmological evolution, which is at variance with some previous results. The observed luminosity functions are in good agreement with the predictions of unified schemes, with FSRQs getting close to their expected minimum power. Despite the fact that the large majority of our blazars are FSRQs, BL Lac objects are intrinsically ~50 times more numerous. Finally, the relative numbers of HBLs and LBLs in the radio and X-ray bands are different from those predicted by the so-called blazar sequence and support a scenario in which HBLs represent a small minority (≈10%) of all BL Lac objects.

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