Quasars in 7C

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1999MNRAS.307..293Riley+

Quasars from the 7C Survey - I: sample selection and radio maps

Julia M.Riley 1, Steve Rawlings 2, Richard G.McMahon 3, Katherine M.Blundell 2, Philip Miller 1,4, Mark Lacy 2 and Elizabeth M. Waldram 1

1 Mullard Radio Astronomy Observatory, Astrophysics Group, Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE.

2 Astrophysics, Department of Physics, Keble Road, Oxford, OX1 3RH.

3 Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA.

4 Present address: Venerable English College, Via di Monserrato, 45, 00186 Roma, Italy. \\

Abstract

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151 MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10h28m, Dec +41deg and covers ~0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting `7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S_151 > 100 mJy; POSS-I E-plate magnitude E ~R < 20; and POSS-I colour (O - E) < 1.8; the effective area of the survey drops significantly below S_151 ~200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al. (1998b), that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.