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

¹ Osservatorio Astronomico di Padova, INAF, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: renato.falomo@oapd.inaf.it
² Osservatorio Astronomico di Trieste, INAF, via Tielopo, Trieste, Italy
³ Universita' dell'Insubria (Como), Italy
⁴ Istituto di Radioastronomia, INAF  c/o CNR, via Gobetti 101 40129 Bologna, Italy
⁵ Dipartimento di Astronomia - Bologna University - via Ranzani 1 40127 Bologna, Italy
⁶ Osservatorio Astronomico di Bologna, INAF, via Ranzani 1, 40127 Bologna, Italy
⁷ Osservatorio Astronomico di Brera, INAF, via Bianchi 46, Merate, Italy
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

Received: 16 March 2009
Accepted: 17 April 2009

Abstract

Context. BL Lac objects are low-power active nuclei exhibiting a variety of peculiar properties caused by the presence of a relativistic jet and orientation effects.

Aims. We present adaptive optics near-IR images at high spatial resolution of the nearby BL Lac object PKS 0521-365, which is known to display a prominent jet both at radio and optical frequencies.

Methods. The observations were obtained in Ks-band using the ESO multi-conjugated adaptive optics demonstrator at the Very Large Telescope. This allowed us to obtain images with 0.1 arcsec effective resolution. We performed a detailed analysis of the jet and its related features from the near-IR images, and combined them with images previously obtained with HST in the R band and by a re-analysis of VLA radio maps.

Results. 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 is dominated by synchrotron radiation, while the nucleus also exhibits a significant inverse Compton component. We discovered the near-IR counterpart of the radio hotspot and found that the near-IR flux is consistent with being a synchrotron emission from radio to X-ray wavelengths. The bright red object (red-tip), detached but well aligned with the jet, is well resolved in the near-IR and has a linear light profile. Since it has no radio counterpart, we propose that it is a background galaxy not associated with the jet.

Conclusions. The new adaptive optics near-IR images and previous observations at other frequencies allow us to study the complex environment around the remarkable BL Lac object PKS 0521-365. These data exemplify the capabilities of multi conjugate adaptive optics observations of extragalactic extended sources.