An optical view of BL Lacertae objects

  • Renato FalomoEmail author
  • Elena Pian
  • Aldo Treves
Review Article


BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. This implies the existence of a parent population of sources with a misaligned jet that have been identified with low-power radiogalaxies. The spectrum of BL Lacs, dominated by non-thermal emission over the whole electromagnetic range, together with bright compact radio cores, high luminosities, rapid and large amplitude flux variability at all frequencies and strong polarization makes these sources an optimal laboratory for high energy astrophysics. A most distinctive characteristic of the class is the weakness or absence of spectral lines, that historically hindered the identification of their nature and ever thereafter proved to be a hurdle in the determination of their distance. In this paper, we review the main observational facts that contribute to the present basic interpretation of this class of active galaxies. We overview the history of the BL Lac objects research field and their population as it emerged from multi-wavelength surveys. The properties of the flux variability and polarization, compared with those at radio, X-ray and gamma-ray frequencies, are summarized together with the present knowledge of the host galaxies, their environments, and central black hole masses. We focus this review on the optical observations, which played a crucial role in the early phase of BL Lacs studies, and in spite of extensive radio, X-ray, and recently gamma-ray observations, could represent the future major contribution to the unveiling of the origin of these sources. In particular, they could provide a firm conclusion on the long debated issue of the cosmic evolution of this class of active galactic nuclei and on the connection between formation of supermassive black holes and relativistic jets.


Galaxies: active Galaxies: nuclei Galaxies: jets  BL Lacertae objects: general Quasars: supermassive black holes Radiation mechanisms: non-thermal 



We are grateful to Catherine Boisson, Alessandro Caccianiga, Stefano Covino, Luigi Foschini, Gabriele Ghisellini, Oliver King, Alan Marscher, Claudia Raiteri, Kenji Toma, Massimo Villata, Andreas Zech for inputs in the preparation of this work. In particular, we thank Gabriele Giovannini and Massimo Persic for useful suggestions, comments and discussions on this review. We are also very grateful to Angela Sandrinelli and Marco Landoni for a careful reading of the manuscript and for fruitful comments. We acknowledge support from ASI/INAF Grant I/088/06/0 and PRIN MIUR 2010/2011.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.INAF-Osservatorio Astronomico di PadovaPaduaItaly
  2. 2.INAF-Istituto di Astrofisica Spaziale e Fisica CosmicaBolognaItaly
  3. 3.Scuola Normale SuperiorePisaItaly
  4. 4.Sezione di PisaINFNPisaItaly
  5. 5.Università dell’InsubriaComoItaly
  6. 6.Sezione di Milano BicsoccaINFNMilanItaly
  7. 7.ICRAMilanItaly

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