SIMBAD references

We present the results of an unbiased radio search for gravitational lensing events with image separations between 15 and 60arcsec, which would be associated with clusters of galaxies with masses >10^13–14M_☉. A parent population of 1023 extended radio sources stronger than 35mJy with stellar optical identifications was selected using the FIRST radio catalogue at 1.4GHz and the APM optical catalogue. The FIRST catalogue was then searched for companions to the parent sources stronger than 7mJy and with separation in the range 15 to 60arcsec. Higher-resolution observations of the resulting 38 lens candidates were made with the VLA at 1.4 and 5GHz, and with MERLIN at 5GHz in order to test the lens hypothesis in each case. None of our targets was found to be a gravitational lens system. These results provide the best current constraint on the lensing rate for this angular scale, but improved calculations of lensing rates from realistic simulations of the clustering of matter on the relevant scales are required before cosmologically significant constraints can be derived from this null result. We now have an efficient, tested observational strategy with which it will be possible to make an order-of-magnitude larger unbiased search in the near future.