Central engines of active galactic nuclei: Properties of collimated outflows and applications for cosmology

Powerful extended radio galaxies in the 3CR sample are observed out to redshifts of about 2. For redshifts greater than 0.3, the average lobe-lobe size of these sources decreases monotonically with redshift for all reasonable cosmological parameter choices. This suggests that the characteristic time for which an active galactic nucleus (AGN) produces highly collimated outflows that power radio emission is shorter for high-redshift sources than it is for low-redshift sources. The analysis presented here supports this conclusion. The relation between the active lifetime and the beam power of powerful extended radio galaxies is investigated here. It is found that the data are accurately described by a model in which the active life-time of the source, t_*, is written as a power law in the energy extraction rate, L_j. The exponent of the power law is estimated to be β ≃ 2.1 ± 0.6, where β is defined by t_* ∝ L_j^-β/3. Note that the value of β for an Eddington-limited system of zero is excluded by this analysis. The fact that β is constrained to lie within a certain range may be used to constrain models of large-scale jet production and cosmological parameters. The comparison of the redshift evolution of characteristic source sizes with the average lobe-lobe size for powerful extended 3CR radio galaxies can be used to constrain cosmological parameters if three empirically estimated quantities can be accurately determined for a subset of the sources. This method of using radio sources as a modified standard yardstick is very similar to the use of supernovae as a modified standard candle. As discussed here, one observable quantity, the lobe propagation velocity, is beset by potential biases that are not completely understood. The analysis presented here shows that these biases do not significantly affect the results on β but must be studied in more detail before cosmological parameters can be estimated precisely. Allowing for the potential biases mentioned above, best fits of the data yield a low value of Ω₀ which is about 2 σ away from a flat, matter-dominated universe.