As already mentioned, the redshift of the lensing galaxy zL is mandatory for the determination of any model. Although PG 1115+080 was discovered in 1980, it is only in 1997 that zL was measured (Kundic et al. 1997), by using the Keck telescope. The galaxy is at low redshift, zL = 0.311. With deeper exposures, its velocity dispersion has also been measured, L = 281 ± 25 km s-1 as well as the velocity dispersion of a group of galaxies found along the line of sight (Fig. 6), grp = 326 km s-1 (Tonry 1998). These measurements provide us with an independent estimate of the total mass of the lens and intervening group. They are crucial when trying to break the mass sheet degeneracy.
Figure 6. Infrared image of PG 1115+080, taken with the HST and the NICMOS camera (Impey et al. 1998). The field of view is 35" on a side and shows several "companions", projected on the plane of the sky. The galaxies labeled "G" are part of a group to which also belongs the main lensing galaxy PG 1115+080. These galaxies have to be taken into account when modeling the potential well responsible for the image configuration.