2 The JVAS gravitational lens survey

 

2.1 The sample

The Jodrell Bank-VLA Astrometric Survey (JVAS) is a survey for flat-spectrum radio sources with a flux density greater than 200mJy at 5GHz. Flat-spectrum radio sources are likely to be compact, thus making it easy to recognise the lensing morphology. In addition, they are likely to be variable, making it possible to determine H₀ by measuring the time delay between the lensed images. (See Biggs et al. 1998, for the description of a time delay measurement in a JVAS gravitational lens system.) JVAS is also a survey for MERLIN phase-reference sources and as such is described in Patnaik et al. (1992), Browne et al. (1998) and Wilkinson et al. (1998). JVAS as a gravitational lens survey, the lens candidate selection, followup process, confirmation criteria and a discussion of the JVAS gravitational lenses is described in detail in King et al. (1999) (see also King & Browne 1996).

In order to have a parent sample which is as large as possible and as cleanly defined as practical, our "JVAS gravitational lens survey sample'' is slightly different than the "JVAS phase-reference calibrator sample''. For the former, the source must be a point source and must have a good starting position (so that the observation was correctly pointed) while its precise spectral index is not important. For the latter, only the spectral index is important, as the source can be slightly resolved or the observation can be less than perfectly pointed. Thus, the JVAS astrometric sample (Patnaik et al. 1992; Browne et al. 1998; Wilkinson et al. 1998) contains 2144 sources. To these must be added 103 sources which were too resolved to be used as phase calibrators and 61 sources which had bad starting positions (thus the observations were too badly pointed to be useful for the astrometric sample), bringing the total to 2308. This formed our gravitational lens sample, since these additional sources were also searched for gravitational lenses (King et al. 1999) (none were found meeting the JVAS selection criteria).

  

Table 1: JVAS lenses used in this analysis. Of the information in the table, for this analysis we use only the source redshift $z_{\mathrm{s}}$and the image separation $\Delta\theta$

2.2 The lenses

We have used the gravitational lens systems in Table 1 in this analysis. The JVAS lens B1938+666 (King et al. 1998) was not included because it is not formally a part of the sample, having a too steep spectral index and having been recognised on the basis of a lensed extended source as opposed to lensed compact components. Also, the JVAS lens B2114+022 (Augusto et al. 1999) was not included because it is not a single-galaxy lens system.