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BL Lac objects are low--power active nuclei exhibiting a variety of peculiarproperties that are caused by the presence of a relativistic jet andorientation effects.  We present here adaptive optics near-IR images at high spatial resolution ofthe nearby BL Lac object PKS 0521-365, which is known to display a prominentjet both at radio and optical frequencies. The observations were obtained inKs--band using the ESO multi-conjugated adaptive optics demonstrator at theVery Large Telescope. This allowed us to obtain images with 0.1 arcseceffective resolution. We performed a detailed analysis of the jet and itsrelated features from the near-IR images, and combined them with imagespreviously obtained with HST in the R band and by a re-analysis of VLA radiomaps. We find a remarkable similarity in the structure of the jet at radio,near-IR, and optical wavelengths. The broad--band emission of the jet knots isdominated by synchrotron radiation, while the nucleus also exhibits asignificant inverse Compton component. We discovered the near-IR counterpart ofthe radio hotspot and found that the near-IR flux is consistent with being asynchrotron emission from radio to X-ray. The bright red object (red-tip),detached but well aligned with the jet, is well resolved in the near-IR and hasa linear light profile. Since it has no radio counterpart, we propose that itis a background galaxy not associated with the jet. The new adaptive opticsnear-IR images and previous observations at other frequencies allow us to studythe complex environment around the remarkable BL Lac object PKS 0521-365. Thesedata exemplify the capabilities of multi conjugate adaptive optics observationsof extragalactic extended sources.

The jet of the BL Lac object PKS 0521 -365 in the near-IR : MAD adaptive optics observations