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During the early 1960s, some radio sources were shown to coincide in position with objects that looked like stars. These became known as quasars (quasi-stellar radio source). It was later discovered that only about one in ten of these objects is a strong radio emitter, the radio-quiet type is named quasi-stellar object (QSO). The term quasar is still widely used to describe both kinds of objects. Figure 05-05a shows the quasar 3C273 (3C denotes the third Cambridge Catalogue of radio sources) discovered in 1962. The radio, optical, and X-ray images are displayed in the top from left to right. The lower picture is a drawing of a quasar. These objects have high redshift, some of which translate into distance well in excess of 10 billion light-years. In order to appear as bright as they do, quasars must be extremely luminous at more than ten thousands times the luminosity of the normal galaxies. Quasars radiate strongly over a wide range of wavelenghts, and although emission lines are present in their spectra, the overall spectrum is consistent with synchrotron emission. Their powerful energy sources are compact and variable, with some quasars |
Figure 05-05a Quasar 3C273 |
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varying substantially in brightness over periods as short as a few days. Some has a jet (e.g, 3C273), or pair of jets emerging from their centers similar to the radio galaxies. There are many more high redshift quasars than low redshift ones. No known quasar has a redshift less than 0.06, and quasar numbers seem to be highest at redshifts of around 2-3. It follows that quasar activity must have been more prevalent among galaxies billions of years ago, when the universe was younger than it is now.
There is a class of objects called BL Lacertae objects or blazars (Figure 05-05b). They are star-like radio sources, similar in appearance to quasars, but with no obvious emission lines in their featureless spectra. They may be quasars seen almost end-on with the jet pointing to the line of sight. Astronomers divide blazars roughly into two groups: lower-energy, relatively nearby BL Lacertae objects and higher-energy, distant soruces. More than 1000 blazars have been catgaloged. |
Figure 05-05b Blazar [view large image] |
It is possible that redshift of the blazars may be masked by the approaching jet, which shifts the light to shorter wavelength. |
