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Session 86 - Measurement of Cosmological Parameters.
Oral session, Wednesday, January 17
1st Floor, La Villita Assembly Building
Powerful extended radio sources are observed to relatively large redshift, and thus provide a potentially valuable cosmological tool. This requires that we understand the physics of the sources and their interaction with the ambient medium quite well.
Classical double radio sources are thought to be powered by a highly collimated outflow from a central active galactic nucleus (AGN). The collimate outflow terminates in the radio hot spot and lobe, which drives a strong shock front into the ambient gas; Cygnus A provides a low-redshift example. Detailed multifrequency radio maps may be used to deduce the shock properties, which allow us to understand the way that the source size depends on the properties of the AGN, such as the beam power, and the properties of the ambient medium, such as the ambient gas density.
Powerful classical doubles could be used for cosmology if the redshift evolution of the lobe-lobe sizes were understood and could be accounted for. We have developed a model that can account for many of the properties of powerful extended radio sources, including the redshift evolution of the lobe-lobe source size. A comparison of model predictions and observations is very strongly dependent on cosmological parameters, especially when high redshift sources are included.
We have applied the model to 14 3CR radio galaxies with redshifts between zero and two. The model predictions and the observations track each other quite well in all cosmological models. However, a better fit is obtained for a low density universe with Ømega \sim 0.1 than for a flat matter dominated universe with Ømega = 1, where Ømega is the mean mass density relative to the critical value. A low value of Ømega \sim 0.1 with space curvature or with a cosmological constant fit the data equally well. These results will be presented and the implications discussed. Note that this is a strictly geometrical test, and hence is independent of models of structure formation and evolution.
