The optical structure and local environment of the BL Lac object 1E 1415.6 + 2557

Abstract

The X-ray-selected BL Lac object 1E 1415.6 + 2557 is of current interest, because its host galaxy morphology and clustering environment appear to be inconsistent with standard beaming models for BL Lac objects. The host galaxy of 1E 1415.6 + 2557 was originally identified as a luminous spiral galaxy, but more recently this galaxy has been classified as an isolated giant elliptical galaxy with an optical jet. We present deep multicolour high-resolution optical images of this object, and confirm that the host galaxy is a giant elliptical galaxy. We find that the optical 'jet' is more likely to be an inclined spiral galaxy seen in projection against the BL Lac object, or a nearby companion galaxy. We also present evidence that 1E 1415.6 + 2557 resides in a compact group or a poor (Abell class 0) cluster of galaxies. These results are consistent with unification models in which BL Lac objects are low-luminosity radio galaxies seen at a small angle to the line of sight of a relativistic jet.