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Session 12 - Cosmology, Large-Scale Structure and Distance Scales.
Display session, Monday, June 10
Great Hall,
Powerful extended radio galaxies are observed out to high redshift, and have coordinate distances (a_or) that depend on parameters such as Ømega_o, the mass density of the universe relative to the critical value, and the normalized cosmological constant \lambda_o=\Lambda/H_o^2, where H_o is the Hubble parameter. At high redshift, (a_or) depends significantly on Ømega_o and \lambda_o. If the lobe propogation and interactions with the ambient medium were understood, these sources would be a valuable cosmological tool.
Double radio sources are thought to be powered by a highly collimated outflow from an active galactic nucleus (AGN). The 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. Multi-frequency radio maps are used to deduce the shock properties, which are used to calculate the source size which depends on the properties of the AGN, such as the beam power, ambient gas density.
Radio galaxies can be used to constrain cosmological parameters (Ømega_o amp; \lambda_o in this work, but not H_o) if the redshift evolution of the lobe-lobe sizes can be predicted. A model has been developed that accounts for the redshift evolution of the lobe-lobe source size. A comparison of model predictions and observations is very strongly dependent on (a_or) for the model of powerful extended radio galaxies decribed here.
The model is applied to 14 radio galaxies with redshifts between zero and two. The model predictions and the observations track each other quite well for most cosmological models. Fitting the ratio of predicted and observed lengths to a constant independent of redshift favors a low-density universe over a flat, matter-dominated universe. The best fit for a curved universe is Ømega_o=0.10^+.45_-.30, and is insenstive to the data cuts examined. While for a flat universe with a cosmological constant, the best fit is Ømega_o=0.35^+.30_-.25 for all 14 points; excluding Cygnus A, the best fit is Ømega_o=0.10^+.30_-.10. A flat matter-dominated universe is ruled out only at the 2\sigma level. The results and implications will be discussed in detail.
