Quintessence, Cosmology, and Fanaroff-Riley Type IIB Radio Galaxies

Abstract

Fanaroff-Riley type IIb (FR IIb) radio galaxies provide a modified standard yardstick that allows constraints to be placed on global cosmological parameters. A sample of 20 FR IIb radio galaxies with redshifts between 0 and 2 are compared with the parent population of 70 radio galaxies to determine the coordinate distance to each source. The coordinate-distance determinations are used to constrain the current mean mass-energy density of quintessence $Ω_q$, the equation of state of the quintessence w, and the current mean mass-energy density of nonrelativistic matter $Ω_m$; zero space curvature is assumed. Radio galaxies alone indicate that the universe is currently accelerating in its expansion (with 84% confidence); most of the allowed parameter space falls within the accelerating universe region on the $Ω_m$-w plane. This provides verification of the acceleration of the universe indicated by high-redshift supernovae and suggests that neither method is plagued by systematic errors. It is found that $Ω_m$ must be less than about 0.5 and the equation of state w of the quintessence must lie between -0.25 and -2.5 at about 90% confidence. Fits of the radio galaxy data constrain the model parameter $β$, which describes a relation between the beam power of the active galactic nucleus (AGN) and the total energy expelled through large-scale jets. It is shown that the empirically determined model parameter is consistent with models in which the outflow results from the electromagnetic extraction of rotational energy from the central compact object. A specific relation between the strength of the magnetic field near the AGN and the spin angular momentum per unit mass of the central compact object is predicted.