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Register a new userThe Deep X-Ray Radio Blazar Survey (DXRBS). III. Radio Number Counts, Evolutionary Properties, and Luminosity Function of Blazars
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P. Padovani
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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 (FSRQ) and 24 BL Lacs down to a 5 GHz flux and power ~ 50 mJy and ~ 10^{24} W/Hz, respectively, an order of magnitude improvement as compared to previously published (radio-selected) blazar samples. DXRBS FSRQ 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 Lacs, on the other hand, do not evolve. High-energy (HBL) and low-energy (LBL) peaked BL Lacs 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 FSRQ getting close to their expected minimum power. Despite the fact that the large majority of our blazars are FSRQ, BL Lacs are intrinsically ~ 50 times more numerous. Finally, the relative numbers of HBL and LBL in the radio and X-ray bands are different from those predicted by the so-called blazar sequence and support a scenario in which HBL represent a small minority (~ 10%) of all BL Lacs.
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We have undertaken a survey for blazars by correlating the ROSAT WGACAT database with publicly available radio catalogs, restricting our candidate list to serendipitous flat-spectrum sources (alpha_r <= 0.7). We discuss here our survey methods, identification procedure and first results. Our survey is found to be ~ 95% efficient at finding blazars, a figure which is comparable to or greater than that achieved by other radio and X-ray survey techniques. DXRBS provides a much more uniform coverage of the parameter space occupied by blazars than any previous survey. Particularly important is the identification of a large population of flat-spectrum radio quasars with ratios of X-ray to radio luminosity >~ 10^-6 (alpha_rx <~ 0.78) and of many low-luminosity flat-spectrum radio quasars. Moreover, DXRBS fills in the region of parameter space between X-ray selected and radio-selected samples of BL Lacs.
We have undertaken a survey of archived, pointed ROSAT PSPC data for blazars by correlating the ROSAT WGACAT database with several publicly available radio catalogs, restricting our candidate list to serendipitous flat radio spectrum sources (alpha_r <= 0.70). Here we discuss our survey methods, identification procedure and first results. Our survey is found to be ~ 95% efficient at finding flat-spectrum radio-loud quasars (FSRQs, 59 of our first 85 IDs) and BL Lacertae objects (22 of our first 85 IDs), a figure which is comparable to or greater than that achieved by other radio and X-ray survey techniques. The identifications presented here show that all previous samples of blazars (even when taken together) did not representatively survey the blazar population, missing critical regions of (L_X,L_R) parameter space within which large fractions of the blazar population lie. Particularly important is the identification of a large population of FSRQs (>~ 25% of DXRBS FSRQs) with ratios of X-ray to radio luminosity >~ 10^-6 (alpha_rx <~ 0.78). In addition, due to our greater sensitivity, DXRBS has already more than doubled the number of FSRQs in complete samples with 5 GHz (radio) luminosities between 10^31.5 and 10^33.5 erg/s/Hz and fills in the region of parameter space between X-ray selected and radio-selected samples of BL Lacs. DXRBS is the very first sample to contain statistically significant numbers of blazars at low luminosities, approaching what should be the lower end of the FSRQ luminosity function.
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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 1Ms exposure of the central 0.08 deg^2 (Giacconi et al. 2002; Alexander et al. 2003) or in the catalog of the 250 ks exposure of the 0.3 deg^2 E-CDFS field (Lehmer et al. 2005). 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 AGN. 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 two 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 AGN 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 AGN.
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