The gravitational lens system CLASS B2108+213 has two lensed images separated by 4.56 arcsec .
Such a wide image separation suggests that the lens is either a massive galaxy , or is composed of a group of galaxies .
To investigate the structure of the lensing potential we have carried out new high resolution imaging of the two lensed images at 1.7 GHz with the Very Long Baseline Array ( VLBA ) and at 5 GHz with global Very Long Baseline Interferometry ( VLBI ) .
Compact and extended emission is detected from the two lensed images , which provides additional constraints to the lensing mass model .
We find that the data are consistent with either a single lensing galaxy , or a two galaxy lens model that takes account of a nearby companion to the main lensing galaxy within the Einstein radius of the system .
However , for an ensemble of global power-law mass models , those with density profiles steeper than isothermal are a better fit .
The best-fitting profile for a single spherical mass model has a slope of \gamma = 2.45 _ { -0.18 } ^ { +0.19 } .
The system also has a third radio component which is coincident with the main lensing galaxy .
This component is detected at milli-arcsecond scales for the first time by the 1.7 GHz VLBA and 5 GHz global VLBI imaging .
However , the third radio component is found not to be consistent with a core lensed image because the radio spectrum differs from the two lensed images , and its flux-density is too high when compared to what is expected from simple mass models with a variable power-law density profile and/or a reasonable core radius .
Furthermore , 1.4 GHz imaging of the system with the Multi-Element Radio Link Interferometric Network ( MERLIN ) finds extended lobe emission on either side of the main lensing galaxy .
Therefore , the radio emission from the third radio component is almost certainly from an AGN within the main lensing galaxy , which is classified as an FR I type radio source .