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.