A new radio double lens from CLASS: B1127+385

Abstract

We present the discovery of a new gravitational lens system with two compact radio images separated by 0.701 ± 0.001 arcsec. The lens system was discovered in the Cosmic Lens All Sky Survey (CLASS) as a flat-spectrum radio source. Both radio components show structure in a Very Long Baseline Array (VLBA) 8.4-GHz radio image. No further extended structure is seen in Very Large Array (VLA), Multi-Element Radio Linked Interferometer Network (MERLIN) or VLBA images. Hubble Space Telescope (HST) WFPC2 images in F555W and F814W show two extended objects close to the radio components, which we identify as two lens galaxies. Their colours and mass-to-light ratios seem to favour two late-type spiral galaxies at relatively high redshifts (z_d ≥ 0.5). Faint emission is also detected at positions corresponding to the radio images. A two-lens mass model can explain the observed VLBA structure. The best-fitting model has a reduced χ^2 of 1.1. The relative positions of the VLBA subcomponents are reproduced within 0.08 mas, and the flux density ratios within 20 per cent. We also reproduce the position angle and separation of the two VLBA subcomponents in A and B within the observational errors, which we consider strong evidence for the validity of the lens model. Moreover, we find a surface density axis ratio of 0.74^(+0.10)_(0.12) for the primary lens (G1), consistent with the surface brightness axis ratio of 0.69 ± 0.15. Also, the surface density position angle of (4.9^(+28.2)_(-22.4)° of G1 compares well with the (−6±13) position angle of the surface brightness distribution. The errors indicate the 99 per cent confidence interval.

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