Mon. Not. R. Astron. Soc. 307, 293±314 (1999) 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, 1w 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 Accepted 1999 February 9. Received 1999 February 9; in original form 1997 June 19 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 10 h 28 m , Dec.1418 and covers <0.057sr. 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; POSS-I colour O ÿ 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., that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars. Key words: galaxies: active ± quasars: general ± radio continuum: galaxies. 1 INTRODUCTION The advent of fast plate-measuring machines and extensive deep radio surveys with good positional accuracy has made possible the optical identification of large numbers of radio sources. Statistical studies can now be carried out using samples of radio sources associated with particular classes of optical object such as quasars. Surveys at low radio frequency allow the selection of samples of quasars which avoid the strong orientation biases inherent in samples selected at high radio frequencies: this is because the ratio of extended optically thin radio emission to compact optically thick radio emission increases rapidly with decreasing observing frequency, and it is the latter component which is greatly enhanced by Doppler boosting when radio-emitting material moves at a relativistic speed along a direction close to the line of sight. Thus, although low-frequency samples still contain some quasars whose total radio fluxes are strongly influenced by Doppler (i.e., orientation) effects, they are far out-numbered by objects for which the optically thin (steep-spectrum) component dominates ± we will refer to the latter as steep-spectrum radio-loud quasars (SSQs). In Fig. 1 we show how recent work has extended the coverage of the low-frequency (151-MHz) radio luminosity 1 L 151 versus redshift z plane for SSQs. This provides a large range in L 151 at a given z and vice versa ± these are important requirements if the influences of L 151 and z on the properties of SSQs are to be separately determined. However, no large samples selected at 151- MHz flux densities (S 151 ) below ,0.5 Jy have until now been constructed. As Fig. 1 shows, such samples would improve the L 151 ± z coverage still further. This provided the primary motivation for the project described in this paper. The 7C surveys carried out with the Cambridge Low Frequency Synthesis Telescope (CLFST) at 151 MHz provide a perfect basis for this project because of their low limiting flux density, S 151 , 0:1 Jy (e.g. McGilchrist et al. 1990; Lacy et al. 1995). Moreover, these surveys have a positional accuracy of a few arcsec and can be used to identify candidate quasars by comparison with optical plates. We chose to exploit these features by cross-matching the 7C positions with the positions of optical objects from the Palomar Observatory Sky Survey (POSS-I). We used the Automatic Plate- measuring Machine (APM) at Cambridge to scan the glass copies of the original Palomar Observatory Sky Survey (POSS-I) covering a continuous area of the 7C survey; in the search for quasar candidates we sought identifications (IDs) on both the E q 1999 RAS w Present address: Venerable English College, Via di Monserrato, 45, 00186 Roma, Italy. 1 We take H 0  50 km s 21 Mpc 21 ; q 0  0:5 and a cosmological constant of zero. Downloaded from https://academic.oup.com/mnras/article/307/2/293/1103719 by guest on 31 December 2023