THE VLA SURVEY OF THE CHANDRA DEEP FIELD-SOUTH. III. X-RAY SPECTRAL PROPERTIES OF RADIO SOURCES

We discuss the X-ray properties of the radio sources detected in a deep 1.4 and 5 GHz VLA Radio survey of the Extended Chandra Deep Field-South (E-CDFS). Among the 266 radio sources detected, we find 89 sources (1/3 of the total) with X-ray counterparts in the catalog of the 1 Ms exposure of the central 0.08 deg(2) or in the catalog of the 250 ks exposure of the 0.3 deg(2) E-CDFS field. For 76 (85%) of these sources, we have spectroscopic or photometric redshifts, and therefore we are able to derive their intrinsic properties from X-ray spectral analysis, namely intrinsic absorption and total X-ray luminosities. We find that the population of submillijansky radio sources with X-ray counterparts is composed of a mix of roughly 1/3 star-forming galaxies and 2/3 active galactic nuclei (AGNs). The distribution of intrinsic absorption among X-ray-detected radio sources is different from that of the X-ray-selected sample. Namely, the fraction of low-absorption sources is at least 2 times larger than that of X-ray selected sources in the CDFS. This is mostly due to the larger fraction of star-forming galaxies present among the X-ray-detected radio sources. If we investigate the distribution of intrinsic absorption among sources with L-X > 10(42) erg s(-1) in the hard 2-10 keV band (therefore in the AGN luminosity regime), we find agreement between the X-ray population with and without radio emission. In general, radio-detected X-ray AGNs are not more heavily obscured than the non-radio-detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGNs. For the radio sources without cataloged X-ray counterparts, we measure their average photometric properties in the X-ray bands with stacking techniques. We detect emission with very high confidence level in the soft band and marginally in the hard band. Given their redshift distribution, the average X-ray luminosity of these sources is consistent with being powered by star formation. We note that on average, the spectral shape of our radio sources is soft with HR similar to -0.5 and constant in different bins of radio flux. This result shows that the statistics do not indicate a significant trend in the average X-ray spectral properties, but it is consistent with the radio source population being dominated by star-forming galaxies below 100 mu Jy, as shown by our morphological and multiwavelength analysis presented in Mainieri et al. and Padovani et al..