Skip to main content
SpringerLink
Log in

An optical view of BL Lacertae objects

  • Review Article
  • Published:
The Astronomy and Astrophysics Review Aims and scope

Abstract

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.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Fig. 13
Fig. 14
Fig. 15
Fig. 16
Fig. 17
Fig. 18
Fig. 19
Fig. 20
Fig. 21

Similar content being viewed by others

Notes

  1. This term was coined by E. Spiegel during the Conference on BL Lac Objects in Pittsburgh, in April 1978, from a contraction of “BL Lac” and “quasar”.

  2. \(\varGamma \) is defined as \((1 - \beta ^2)^{-0.5}\), where \(\beta = v/c\), with \(v\) representing the plasma velocity.

  3. In general, the luminosity enhancement is a factor \(\delta ^p\), where \( 3 {\mathop {<}\limits _{\sim }}p {\mathop {<}\limits _{\sim }}4\), depending on the geometry of the emitting region and on spectral index.

  4. Fanaroff and Riley (1974) distinguished radiogalaxies into two types: FR I and FR II according to whether their luminosities at 178 MHz were lower or higher than \(2 \times 10^{32}\) erg s\(^{-1}\) Hz\(^{-1}\)sr\(^{-1}\). Their radio morphologies are also different, being core-dominated or lobe-dominated, respectively. The comparison of radio power and morphology and host galaxy luminosity of BLLs and radiogalaxies suggests that the dominant parent population consists of FR I radiogalaxies.

  5. The overlap of several spatial emission components may also make the radio spectrum deviate from a Rayleigh–Jeans shape, so that it appears thin, although the individual components may be self-absorbed.

  6. see list at http://tevcat.uchicago.edu.

  7. GASP/WEBT, http://www.oato.inaf.it/blazars/webt/.

  8. The only other known sources showing similar polarization are optical counterparts of GRBs, at least during the first seconds to minutes after explosion, and indeed have in common with blazars the non-thermal origin of their continuum.

  9. As an aside, the detection of a broad H\(\alpha \) emission line at the redshift of the host galaxy of BL Lacertae proved that the BLL prototype is not a high-redshift micro-lensed quasar (Ostriker and Vietri 1985, 1990).

References

Download references

Acknowledgments

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.

Author information

Authors and Affiliations

  1. INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padua, Italy

    Renato Falomo

  2. INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Via P. Gobetti 101, 40129, Bologna, Italy

    Elena Pian

  3. Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126, Pisa, Italy

    Elena Pian

  4. Sezione di Pisa, INFN, Pisa, Italy

    Elena Pian

  5. Università dell’Insubria, Via Valleggio 11, 22100, Como, Italy

    Aldo Treves

  6. Sezione di Milano Bicsocca, INFN, Milan, Italy

    Aldo Treves

  7. ICRA, Milan, Italy

    Aldo Treves

Authors
  1. Renato Falomo
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Elena Pian
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Aldo Treves
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Renato Falomo.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Falomo, R., Pian, E. & Treves, A. An optical view of BL Lacertae objects. Astron Astrophys Rev 22, 73 (2014). https://doi.org/10.1007/s00159-014-0073-z

Download citation

  • Received:

  • Published:

  • DOI: https://doi.org/10.1007/s00159-014-0073-z

Keywords

Search

Navigation