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G. Fossati (UCSD/CASS), C.M. Urry (STScI)
More than 95% of all catalogued blazars have been found in either shallow radio or shallow X-ray surveys. Because of the range of blazar spectral energy distributions (SED) the two selection methods yield different types, the ``red'' objects (with the peak of the synchrotron emission at IR-optical wavelengths) in radio samples, and the ``blue'' (whose synchrotron emission peaks at UV-X-ray wavelengths) in X-ray samples. The differences in the SEDs do reflect different physical states but only as the extrema of an underlying continuous population.
The relative space densities of the different types, not to mention their absolute space densities or their evolution in cosmic time still remain indeterminate. The ratio of the extreme ``red'' and ``blue'' types, at least for BL~Lac objects, could be 10:1 either way! The blazar demographics are this uncertain essentially because the flux limits of current complete samples are high, so only the tip of the population is sampled. The interpretation of observed phenomenology depends on the complicated sensitivity of diverse surveys to a range of spectral types. Results from quantitative simulations suggest that intrinsically continuous distributions can be made to look bimodal simply because of these selection effects.
This means we do not know which kind of jets nature preferentially makes: those with and high B and \gamma\rm e (``blue'' blazars) or low B and \gamma\rm e (``red'' blazars). We also do not know whether they evolve differently and/or if ``red'' blazars dominate at high redshift and evolve into ``blue'' blazars at low redshift, and what is the relationship between the ``non-thermal'' and ``thermal'' power/components. The implications for understanding jet formation are obvious.
Larger and deeper samples, combined with further simulations, will help resolve the present uncertainty over the space density of blazars of all types.
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