BL Lac object

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A BL Lac object, BL Lacertae object is a type of active galaxy with an active galactic nucleus (AGN) and is named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large amplitude flux variability and significant optical polarization. Because of these properties, the prototype of the class (BL Lac) was originally believed to be a variable star. When compared to the more luminous active nuclei (quasars) with strong emission lines, BL Lac objects have spectra dominated by a featureless non-thermal continuum.

The optical spectrum of the BL Lac object PG 1553+11.

In the unified scheme of radio-loud active galactic nuclei, the observed nuclear phenomenology of BL Lacs is interpreted as due to the effects of the relativistic jet that is pointing towards the observer.

The BL Lac object H 0323+022 (z=0.147) imaged at ESO NTT (R filter). The host galaxy and close companions are visible.

BL Lacs are believed to be intrinsically identical to low power radio galaxies but with the jet closely aligned to the line of sight of the observer. These active nuclei are hosted in massive spheroidal galaxies. From the point of AGN classification, BL Lacs are a blazar subtype.