Probing the nature of the massive black hole binary candidate SDSS J1536+0441

We present an imaging study of the black hole binary candidate SDSS J1536+0441 (z ≈ 0.3893), based on deep, high-resolution V z K images collected at the ESO/Very Large Telescope. The images clearly show an asymmetric elongation, indicating the presence of a companion source at ~1'' (≈5 kpc projected distance) east from the quasar. The host galaxy of the quasar is marginally resolved. We find that the companion source is a luminous galaxy, the light profile of which suggests the presence of an unresolved, faint nucleus (either an obscured active galactic nucleus or a compact stellar bulge). The study of the environment around the quasar indicates the occurrence of a significant overdensity of galaxies with a redshift compatible with z ≈ 0.4. This suggests that it resides in a moderately rich cluster of galaxies