ar X iv : a st ro - p h / 02 10 54 6 v 1 2 4 O ct 2 00 2 On the time evolution of giant radio galaxies

A time evolution of giant lobe-dominated radio galaxies (with projected linear size D > 1 Mpc for H0=50 km sMpc and q0=0.5) is analysed on the basis of dynamical evolution of the entire FRII-type population. Two basic physical parameters, namely the jet power Q0 and central density of the galaxy nucleus ρ0 are derived for a sample of giants with synchrotron ages reliably determined, and compared with the relevant parameters in a comparison sample of normalsize sources consisting of 3C, B2, and other sources. Having apparent luminosity (power) P and linear size D of each sample source, Q0 and ρ0 are fitted using the dynamical model of Kaiser et al. (1997) but modified by allowing an evolution of the cocoon axial ratio with time as suggested by Blundell et al. (1999). In a result, we found that: (i) there is not a unique factor governing the source size; giants are old sources with high enough jet power evolved in a relatively low-density environment. The size is dependent, in order of decreasing partial correlation coefficients, on age; then on Q0; next on ρ0, (ii) a number of the sample sources, having similar Q0 and ρ0 but different ages and axial ratios (called a ‘clan’), fit very well the evolutionary luminosity–size (P–D) and energy density–total energy (ueq–Etot) tracks derived from the model for a ‘fiducial’ source with Q0 and ρ0 equal to the means of relevant values obtained for the ‘clan’ members. Therefore, these sources can be considered as ‘the same’ source observed at different epochs of its lifetime, and hence very useful for an obEmail: [email protected] servational constraint of evolutionary models of the source dynamics, (iii) in some cases a slow acceleration of the average expansion speed of the cocoon along the jet axis seems to be present, and (iv) apparent increase of the lowest internal pressure value, observed within the largest sources’ cocoon, with redshift is obscured by the intrinsic dependence of their age on redshift, which hinders making definite conclusions about a cosmological evolution of intergalactic medium (IGM) pressure.