The Cambridge-Cambridge ROSAT Serendipity Survey - III. VLA observations and the evolution of radio-quiet and radio-loud objects

Abstract

We present the results of the VLA radio observations at 1.475 GHz (20 cm) of the active galactic nuclei (AGN) in the Cambridge-Cambridge ROSAT Serendipity Survey (CRSS), a sample of 123 faint X-ray sources with fx(0.5−2.0keV)≥2×10−14ergs−1cm−2 fx(0.5−2.0keV)≥2×10−14ergs−1cm−2 . Of the 80 AGN in the sample, seven show radio emission at the 5 σ level and only two ( 2.5+4.0–1.7 2.5–1.7+4.0 per cent) qualify as radio-loud (RL) objects (αro ≥ 0.35). This result, compared with 13 per cent RL in the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) sample of AGN [flux limit fx(0.3−3.5keV)∼2×10−13ergs−1cm−2 fx(0.3−3.5keV)∼2×10−13ergs−1cm−2 ], confirms that the fraction of X-ray-selected RLAGN drops rapidly as the X-ray flux limit is lowered. Combining the CRSS AGN sample with that extracted from the EMSS we study the X-ray luminosity function (XLF) and evolutionary properties for radio-quiet (RQ) and RLAGN separately. We find that the RQ and RLAGN populations show the same cosmological evolution within the errors. In fact, when the luminosity evolution is parametrized with a power law of the form L∗x(z)=L∗x(0)(1+z)k Lx∗(z)=Lx∗(0)(1+z)k , we find k = 2.43 ± 0.26 and 2.71 ± 0.10 for RL and RQ AGN populations respectively. In addition, the shapes of the de-evolved XLFs of the two classes appear to be different at both the low-luminosity (Lx < 1044 erg s–1) and high-luminosity ends. These results are robust for different cosmological models (using q0 = 0.0 and 0.5) and for different values of the threshold αro used to distinguish between RQ and RL objects. Finally, we find that the differences in the shapes of the XLFs of RQ and RLAGN can be explained by introducing an X-ray beaming model to separate the observed X-ray luminosity of radio quasars into relativistically beamed and isotropic contributions.