 INTERLUDE 25-1 BL Lac Objects |
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| In 1929 an object thought to be a variable star was discovered in the constellation Lacerta. Astronomers gave it the two-letter code BL, so it became known as BL Lacertae, or BL Lac for short. Not until the 1970s, when it became clear that BL Lac was a strong radio source, did anyone question its classification as a star. As more objects like BL Lac were found, astronomers began to realize that they had stumbled across a whole new class of extragalactic sources. BL Lac objects seem starlike in a telescope and vary greatly in luminosity over time. They are also powerful compact radio sources, sometimes colloquially referred to as "blazars."
Astronomers originally placed BL Lac objects into a special class because they displayed no spectral lines. Their redshifts could not be determined, and their distances were unknown. However, recent discoveries of extremely faint spectral lines strongly suggest that BL Lac objects are extremely distant, making them nearly as luminous as quasars (discussed later in the text). Further careful observations have shown that blazars reside at the centers of relatively normal elliptical galaxies. We consider these galaxies active because of the tremendous strength of the blazar radio emission. The weak spectral lines may be the key needed to unravel the nature of the BL Lac objects. Evidently, the thermal emission from any stars |
they contain, which would provide spectral lines, is swamped by their nonthermal synchrotron radiation (see Section 25.4), which does not contain spectral features. Other types of active galaxies show a mixture of thermal and nonthermal radiation, so it is more difficult to study the nonthermal radiation alone. BL Lac objects, then, may offer astronomers a chance to study the "bare machine" (which generates the nonthermal radiation) powering these active galaxies.
In some ways the BL Lac objects seem to represent a "link" or transition phase between radio galaxies and quasars. The luminosities of the compact radio sources inside BL Lac objects span the whole range from the relatively weak cores observed in radio galaxies to the stronger ones found in quasars(see Section 25.5). Perhaps there exists an evolutionary sequence of some sort, in which the ancient quasars use up their fuel, turning into the weaker yet more erratic BL Lac objects, which subsequently become the rather inactive cores of radio galaxies. An alternative possibility is that BL Lacs—and perhaps also the weaker core—halo radio sources—are just lobe radio galaxies, but radio galaxies in which the jet happens to be pointing nearly straight at us. We would then be seeing the galaxy through the nearer radio lobe. Only further observations and better statistics will tell us for sure which (if either) of these two possibilities is correct. |