Radio Properties of z > 4 Optically - Selected Quasars

Abstract

We report on two programs to address differential evolution between the radio-loud and radio-quiet quasar populations at high (z > 4) redshift. Both programs entail studying the radio properties of optically-selected quasars. First, we have observed 32 optically-selected, high-redshift (z > 4) quasars with the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and well-understood sample. We detect four quasars above our 3σ limit of ≈ 0.15 mJy, which is sufficiently sensitive to detect all radio-loud quasars at the probed redshift range. Second, we have correlated 134 z > 4 quasars, comprising all such sources that we are aware of as of mid-1999, with FIRST and NVSS. These two recent 1.4 GHz VLA sky surveys reach 3σ limits of approximately 0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z > 4 quasars, of which six were not previously known to be radio-loud. The depth of these surveys does not reach the radio-loud/radio-quiet demarcation luminosity density (L1.4 GHz = 10 h 50 ergs s −1 Hz) at the redshift range considered; this correlation therefore only provides a lower limit to the radio-loud fraction of quasars at high-redshift. The two programs together identify eight new radio-loud quasars at z > 4, a significant increase over the seven currently in the published literature. We find no evidence for radioloud fraction depending on optical luminosity for −25 > MB > −28 at z ≃ 2, or for −26 > MB > −28 at z > 4. Our results also show no evolution in the radio-loud fraction between z ≃ 2 and z > 4 (−26 > MB > −28). Subject headings: Cosmology: observations — galaxies: active — galaxies: evolution — galaxies:formation — quasars: general Department of Astronomy, University of California, Berkeley, CA 94720; dan@slothrop.berkeley.edu Palomar Observatory, Caltech, Pasadena, CA 91125; george@deimos.caltech.edu NRAO, P.O. Box 0, Socorro, NM 87801-0387; rperley@nrao.edu Observatorio Nacional, Rio de Janeiro, Brazil; reinaldo@voyager.on.br Department of Astrophysics, University of Oxford, Keble Road, Oxford, UK OX13RH; jvw@astro.ox.ac.uk

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Cite this paper

@inproceedings{Stern2000RadioPO, title={Radio Properties of z > 4 Optically - Selected Quasars}, author={Daniel Stern and S. George Djorgovski and Richard Perley and Reinaldo R. de Carvalho}, year={2000} }