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Abstract: The gravitational lens system CLASS B2108+213 has two lensed images separatedby 4.56 arcsec. Such a wide image separation suggests that the lens is either amassive galaxy, or is composed of a group of galaxies. To investigate thestructure of the lensing potential we have carried out new high resolutionimaging of the two lensed images at 1.7 GHz with the VLBA and at 5 GHz withglobal VLBI. Compact and extended emission is detected from the two lensedimages, which provides additional constraints to the lensing mass model. Wefind that the data are consistent with either a single lensing galaxy, or a twogalaxy lens model that takes account of a nearby companion to the main lensinggalaxy within the Einstein radius of the system. However, for an ensemble ofglobal power-law mass models, those with density profiles steeper thanisothermal are a better fit. The best-fitting profile for a single sphericalmass model has a slope of $\gamma=$~2.45$ {-0.18}^{+0.19}$. The system also hasa third radio component which is coincident with the main lensing galaxy. Thiscomponent is detected at milli-arcsecond scales for the first time by the 1.7GHz VLBA and 5 GHz global VLBI imaging. However, the third radio component isfound not to be consistent with a core lensed image because the radio spectrumdiffers from the two lensed images, and its flux-density is too high whencompared to what is expected from simple mass models with a variable power-lawdensity profile and-or a reasonable core radius. Furthermore, 1.4 GHz imagingof the system with the MERLIN finds extended lobe emission on either side ofthe main lensing galaxy. Therefore, the radio emission from the third radiocomponent is almost certainly from an AGN within the main lensing galaxy, whichis classified as an FR I type radio source.

Autor: A. More, J. P. McKean, T. W. B. Muxlow, R. W. Porcas, C. D. Fassnacht, L. V. E. Koopmans


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