THE PROPERTIES OF X-RAY-SELECTED ACTIVE GALACTIC NUCLEI .3. THE RADIO-QUIET VERSUS RADIO-LOUD SAMPLES

Using a sample of 406 active galactic nuclei (AGNs), extracted from th e Einstein Observatory Extended Medium Sensitivity Survey (EMSS), for which complete radio information exists, we present and discuss some p roperties of the radio-loud and radio-quiet objects, and compare them with those of radio-observed, optically selected AGNs. The radio to op tical spectral index of the EMSS AGN sample shows a bimodal distributi on with a minimum at alpha(ro) approximately 0.35. A bimodal distribut ion is observed also in samples of optically selected AGNs, making thi s characteristic a general property of the total AGN population. This feature allows us to split, almost unambiguously, the EMSS AGN sample into radio-loud (RL: alpha(ro) > 0.35: 43 objects) and radio-quiet (RQ : alpha(ro) < 0.35: 363 objects) objects. An abrupt change in the radi o properties of the X-ray selected AGN population is observed at M(B) approximately -24: in the optically bright subsample (30 +/- 4)% of th e objects are RL, while in the optically faint subsample (3 +/- 1)% of the objects are RL. This difference is statistically confirmed at a h igh confidence level by the comparison of the alpha(ro) distributions of the two subsamples. These results have been compared with those obt ained from a compilation of optically selected AGNs. The G(> alpha(ro) ) distributions for optically and X-ray selected optically faint (M(B) > -24) AGN are not statistically different. On the contrary, the same distributions for optically bright AGNs are statistically different a t a high confidence level. As expected from previous results, the frac tion of RL AGNs among optically bright X-ray selected AGNs is approxim ately 2.5 times larger than among the optically selected ones. We have determined, for the first time, the X-ray luminosity functions (XLF) at different redshifts, the cosmological evolution and the de-evolved XLF of radio-quiet and radio-loud objects separately. There is no evid ence that the evolutionary properties of radio-loud objects differ sig nificantly from those of radio-quiet ones: the best-fit evolution para meter of the RL AGNs is higher than, but consistent within errors with , that of the RQ AGNs. On the contrary, the shape of the de-evolved XL F of the two classes appears to be different and a flattening of the X LF of the RL subsample is visible for L(X)(z = 0) less than or similar 3 x 10(44) ergs s-1. Similar results were obtained, in the optical do main, from an analysis of the radio properties of the PG sample, the o nly sizeable complete sample of optically selected AGNs with full radi o information. This measured difference adds to the already significan t number of reasons for postulating the two distinct classes of AGNs: RL and RQ. Using the de-evolved XLF and the best-fit evolution paramet ers for the two classes of AGN, we have computed the expected fraction of RL objects as a function of the X-ray flux: this fraction ranges f rom approximately 9% for f(X) approximately 5 x 10(-12) ergs cm-2 s-1, to approximately 13% for f(X) approximately 2 x 10(-13) ergs cm-2 s-1 , and then decreases to approximately 2.5% for f(X) approximately 2 x 10(-15) ergs cm-2 s-1. Recent deep radio observations of a deep ROSAT field seem to be in agreement with our calculations.