A multifrequency study of giant radio sources III. Dynamical age vs. spectral age of the lobes of selected sources

TL;DR

This study compares dynamical and spectral ages of giant radio galaxy lobes using the DYNAGE algorithm, revealing age ratios between 1 and 5 and discussing factors affecting age estimates and model assumptions.

Contribution

It introduces a self-consistent approach to estimate and compare dynamical and spectral ages of radio lobes, considering deviations from equipartition and magnetic field variations.

Findings

Dynamical to spectral age ratio ranges from ~1 to ~5.

Departure from equipartition can equalize lobe ages.

DYNAGE better accounts for low-frequency radiative effects.

Abstract

The dynamical ages of the opposite lobes of selected giant radio sources are estimated using the DYNAGE algorithm of Machalski et al., and compared with their spectral ages estimated and studied by Jamrozy et al. in Paper II. As expected, the DYNAGE fits give slightly different dynamical ages and other model's parameters for the opposite lobes modelled independently each other, e.g. the age ratios are found between ~1.1 to ~1.4. Demanding similar values of the jet power and the radio core density for the same source, we look for a self-consistent solution for the opposite lobes, which results in different density profiles along them found by the fit. We also show that a departure from the equipartition conditions assumed in the model, justified by X-ray observations of the lobes of some nearby radio galaxies, and a relevant variation of the magnetic-field strengths may provide an equalisation of the lobes' ages. A comparison of the dynamical and spectral ages shows that a ratio of the dynamical age to the spectral age of the lobes of investigated giant radio galaxies is between ~1 and ~5, i.e. is similar to that found for smaller radio galaxies (e.g. Parma et al. 1999). Supplementing possible causes for this effect already discussed in the literature, like uncertainty of assumed parameters of the model, an influence of a possible departure from the energy equipartition assumption, etc. Arguments are given to suggest that DYNAGE can better take account of radiative effects at lower frequencies than the spectral-ageing analysis.The DYNAGE algorithm is especially effective for sources at high redshifts, for which an intrinsic spectral curvature is shifted to low frequencies.