AAS 197, January 2001
Session 125. Jets and Variability in AGN and Radio Galaxies
Oral, Thursday, January 11, 2001, 1:30-3:00pm, San Diego

## [125.01] Investigating the nature of relativistic jets by studying the demographics of Blazars

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 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.

This problematic is tightly connected with the much debated issue of the unified picture(s) of radio--loud AGNs, which indeed revolves around the existence, and the properties of relativistis jets.

We address these issues by comparing simultaneously the properties of the collection of heterogenously selected samples that are available now, with extensive simulations of realizations of a set of plausible unifications scenarions for the whole blazar population. We show that it is already possible to make significant progress even by using only the present samples. The important role of selection effects is discussed. For instance we show that the multiple flux selections typical of available surveys could induce some of the correlations found in color--color diagrams. These latter results should apply to any study of flux limited samples.