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 deg2 or in the catalog of the 250 ks exposure of the 0.3 deg2 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 > 1042 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 ~ –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 μJy, as shown by our morphological and multiwavelength analysis presented in Mainieri et al. and Padovani et al..