High-resolution imaging from Mauna Kea: the triple quasar in 0.3-arc s seeing

The `triple' quasar, PG1115 +08 (which actually contains at least four objects: A₁, A₂, B and C) was the second gravitational lens object to be found¹. But because of its small angular scale (<2.5 arc s separation between components) and lack of detectable radio emission, which precludes the use of high-resolution interferometric radio techniques, not much is known about this object. In particular, the lensing object(s) have not been identified. Here we report imaging observations made on a night of exceptional seeing, as well as spectroscopy of two galaxies near the quasar. We find that there is a galaxy centred approximately midway between the two A components. The properties of this galaxy are consistent with it being the brightest member of a small group at a group redshift of 0.305. Although detailed modelling is required, it is likely that the galaxy and its group are the lenses.