Abstract
We present high-resolution imaging and low-resolution spectroscopy of the BL Lac object PKS 0537-441 (z = 0.893) and its environment. The observations were designed to clarify, whether the properties of PKS 0537-441 are affected by gravitational microlensing, or whether PKS 0537-441 and its environment act as a lensing system itself. Our observations show that neither case seems to be likely. We did not find a galaxy along the line-of-sight to the BL Lac as claimed previously, our spectroscopy shows that none of the four closest companion galaxies is at high redshift. Two of the four nearby companion galaxies to PKS 0537-441 are within 200 km/s 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. We also detected extended [O II] emission in the off-nuclear spectrum of PKS 05371-441, which is most likely due to photoionization from the active nucleus. Alternatively, the extended [O II] emission is due to jet-cloud interaction with the counterjet of PKS 0537-441. Our clustering analysis indicates that PKS 0537-441 is 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 (Delta z < 0.002). We found serendipitously even more galaxies at somewhat higher redshifts (z = 0.9-1). Thus, PKS0537-441 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.
Highlights
BL Lac objects are a relatively rare and extreme subclass of the Active Galactic Nuclei (AGN)
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
We found 32 galaxies (4 SED1, 5 SED2, 12 SED3, 4 SED4 and 7 SED5), 2 AGN (PKS 0537−441 and A1, which will be discussed ) and 2 stars
Summary
BL Lac objects are a relatively rare and extreme subclass of the Active Galactic Nuclei (AGN). Romero et al (1995) derived from their radio variability measurements brightness temperatures in excess of 1021 K, which is 9 orders of magnitude above the inverse Compton limit Based on their claimed detection of a galaxy along the line-of-sight to PKS 0537−441 Stickel et al (1988, see below) discussed, whether the BL Lac could be microlensed by small masses in this foreground galaxy. In a reanalysis of the same HST-image mentioned above, Lewis & Ibata (2000) found indications for a galactic component after the subtraction of a scaled PSF (which was recently shown to be due to scattered light by Pian et al 2002) They detected 3 companion galaxies close to PKS 0537−441 (2 of which had already been found by Falomo et al 1992) and discussed an alternative scenario in which PKS 0537−441 is not subject to lensing effects, but is rather a member of a group of galaxies which lense a distant background galaxy.
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