PKS 0537-441: Extended [O II] emission and a binary QSO?

We present high-resolution imaging and low-resolution spectroscopy of the BL Lac object PKS 0537-441 (z = 0.893) and its environment carried out with the ESO-NTT and VLT telescopes. The observations were designed to clarify, whether the properties of PKS 0537-441 are affected by gravitational microlensing due to the claimed detection of a galaxy along the line-of-sight to the BL Lac, or whether PKS 0537-441 and its environment act as a lensing system itself, as suggested by the detection of several closeby companion galaxies with similar morphologies close to PKS 0537-441. Our observations show that neither case seems to be likely. Within our images we did not find a galaxy along the line-of-sight to the BL Lac as claimed previously. In addition, our spectroscopy shows that none of the four closest companion galaxies (including one new detection by us) is at high redshift. Instead, two of the four nearby companion galaxies to PKS 0537-441 are within 200 km s - 1 of the systemic velocity of the BL Lac (z = 0.892 and 0.895, respectively). The third companion galaxy is at higher redshift (z = 0.947). The fourth companion galaxy shows evidence of Mg II absorption redwards of its systemic velocity and is perhaps a mini low-ionization BAL QSO at z = 0.885. If the latter can be confirmed, PKS 0537-441 is the first BL Lacertae object being a member of a binary Quasar. While we do not find (micro)lensing effects important for this source or its environment, our observations revealed a highly interesting feature. We detected extended [O II] emission in the off-nuclear spectrum of PKS 0537-441, which is most likely due to photoionization from the active nucleus, although we can not rule out the possibility that the extended emission is due to jet-cloud interaction with the counterjet of PKS 0537-441. According to our analysis of the photometric data, PKS 0537-441 seems to be located in a cluster environment as rich as Abell type 0-1. This is supported by the detection of four galaxies in the field with similar redshifts as the BL Lac (Az < 0.002). However, we serendipitously found even more galaxies at somewhat higher redshifts (z = 0.9-1). Thus, PKS 0537-441 might be located in front of a galaxy cluster at somewhat higher redshift or even be part of a large-scale structure with an extension towards the BL Lac.