Search

We aim to obtain a complete sample of redshift radio quasi-stellar objects (QSOs) from the Faint Images of the Radio Sky at Twenty cm survey (FIRST) sources having star-like counterparts in the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) photometric survey . Our starting sample of 8665 FIRST–DR5 pairs includes 4250 objects with spectra in DR5, 52 of these being QSOs. We found that simple supervised neural networks, trained on the sources with DR5 spectra, and using optical photometry and radio data, are very effective for identifying high- z QSOs in a sample without spectra. For the sources with DR5 spectra the technique yields a completeness (fraction of actual high- z QSOs classified as such by the neural network) of 96 per cent, and an efficiency (fraction of objects selected by the neural network as high- z QSOs that actually are high- z QSOs) of 62 per cent. Applying the trained networks to the 4415 sources without DR5 spectra we found QSO candidates. We obtained spectra of 27 of them, and 17 are confirmed as high- z QSOs. Spectra of 13 additional candidates from the literature and from SDSS Data Release 6 (DR6) revealed seven more QSOs, giving an overall efficiency of 60 per cent (24/40). None of the non-candidates with spectra from NASA/IPAC Extragalactic Database (NED) or DR6 is a QSO, consistently with a high completeness. The initial sample of high- z QSOs is increased from 52 to 76 sources, i.e. by a factor of 1.46. From the new identifications and candidates we estimate an incompleteness of SDSS for the spectroscopic classification of FIRST QSOs of 15 per cent for . [ABSTRACT FROM AUTHOR]

We have used the Australia Telescope Compact Array (ATCA) at 95 GHz to carry out continuum observations of 130 extragalactic radio sources selected from the Australia Telescope 20 GHz (AT20G) survey. We use a triple-correlation method to measure simultaneous 20- and 95-GHz flux densities for these objects, and over 90 per cent of our target sources are detected at 95 GHz. We demonstrate that the ATCA can robustly measure 95-GHz flux densities with an accuracy of ∼10 per cent in a few minutes for sources stronger than about 50 mJy. We measure the distribution of radio spectral indices in a flux-limited sample of extragalactic sources, and show that the median 20–95 GHz spectral index does not vary significantly with flux density for . This finding allows us to estimate the extragalactic radio source counts at 95 GHz by combining our observed 20–95 GHz spectral-index distribution with the accurate 20-GHz source counts measured in the AT20G survey. Our derived 95-GHz source counts at flux densities above 80 mJy are significantly lower than those found by several previous studies. The main reason is that most radio sources with flat or rising spectra in the frequency range 5–20 GHz show a spectral turnover between 20 and 95 GHz. As a result, there are fewer 95-GHz sources (by almost a factor of 2 at 0.1 Jy) than would be predicted on the basis of extrapolation from the source populations seen in lower-frequency surveys. We also derive the predicted confusion noise in cosmic microwave background surveys at 95 GHz and find a value 20–30 per cent lower than previous estimates. The 95-GHz source population at the flux levels probed by this study is dominated by quasi-stellar objects with a median redshift . We find a correlation between optical magnitude and 95-GHz flux density which suggests that many of the brightest 95-GHz sources may be relativistically beamed, with both the optical and millimetre continuum significantly brightened by Doppler boosting. [ABSTRACT FROM AUTHOR]

We report on the search for distant radio-loud quasars in the Cosmic Lens All Sky Survey (CLASS) of flat spectrum radio sources with S[sub 5GHz]>30 mJy. Unresolved optical counterparts were selected from APM scans of POSS-I plates, with e<19.0 and red o-e>2.0 colours, in an effective area of ∼6400 deg[sup 2]. Four sources were found to be quasars with z>4, of which one was previously known. This sample bridges the gap between the strong radio surveys with S[sub 5GHz]>200 mJy and the samples of radio-weak quasars that can be generated via radio observations of optically selected quasars. In addition, four new quasars at z>3 have been found. The selection criteria result in a success-rate of ∼1:7 for radio-loud quasars at z>4, which is a significant improvement over previous studies. This search yields a surface density of 1 per 1600 deg[sup 2], which is about a factor of ∼15 lower than that found in a similar search for radio-quiet quasars at z>4. The study presented here is strongly biased against quasars beyond z>4.5, since the e-passband of the POSS-I only samples the spectra shortward of 1200 Å at these redshifts. [ABSTRACT FROM AUTHOR]