Multifrequency study of three giant radio galaxies with recurrent jet-formation activity: J1021+1216, J1528+0544, and J2345–0449

II. Spectral aging analysis and dynamical modeling

Astronomical Observatory, Jagiellonian University, Orla 171, PL-30-244 Krakow, Poland

^★ Corresponding author kamil.wolnik@doctoral.uj.edu.pl

Received: 14 April 2025
Accepted: 26 September 2025

Abstract

Context. In the first paper of this series, we presented radio observations of three giant double-double radio galaxies: J1021+1216, J1528+0544, and J2345–0449. We reported the asymmetries and minor misalignments identified in the outer and inner doubles of all three sources, in addition to an uncommon trace of emission with a relatively flat spectrum in the spectral index map of J1528+0544. Furthermore, we discovered core extensions in the J1021+1216 and J1528+0544 high-resolution maps, suggesting that the two sources are triple-double radio galaxies. In this paper, we continue our investigation of the three sources in search of the causes behind these observed peculiarities.

Aims. Our goal is to carry out a detailed study of a selection sample of giant double-double radio galaxies. By determining the properties of these sources and their environments, we obtained a comprehensive image of the processes influencing their evolution, which we could then use to make comparisons with the model results on radio-galaxy evolution from the literature.

Methods. In this work, we used the radio maps prepared and presented in the first paper of this study to perform a spectral aging analysis with the Broadband Radio Astronomy ToolS software and dynamical modeling with the DYNAGE software. From this modeling, we recovered a range of parameters describing the conditions in and around the observed sources, including the duration of the active and quiescent phases, jet power, and external medium density.

Results. Based on our radiative and dynamical models, we report long durations for the active phases in the outer doubles of J1021+1216 and J2345–0449. We report ages of t_rad, J10 = 43 ± 4 Myr and t_dyn, J10 = 250 Myr for J1021+1216, and t_rad, J23 = 42 ± 4 Myr and t_dyn, J23 = 176 Myr for J2345–0449. The inner double of J1021+1216 was found to be expanding at a speed ∼0.5c inside a relic cocoon with a density of log(ρ₀ [kg m⁻³]) = −25.7. In J1528+0544, all the parameters that could influence the evolution of the outer lobes are not out of the ordinary. However, we found a radiatively young structure in the outer lobes, which we interpreted as a trace of a restarted jet belonging to an “intermediate” phase of activity.

Conclusions. We conclude that there is no single universal factor stimulating the growth of the GRGs. In J1021+1216 and J2345–0449 outer doubles, with projected sizes ∼1.85 Mpc and ∼1.7 Mpc, respectively, the main factor stimulating their growth is the exceptionally long duration of their active phases. In J1021+1216 inner double, with a projected size of ∼1 Mpc, the main factor is its fast expansion inside a low-density medium. The outer double J1528+0544, with a projected size ∼715 kpc, represents the case of a giant radio galaxy, where growth was stimulated by the recurrent activity of the galactic nucleus. Furthermore, we report the discovery of two radio galaxies with three separate phases of activity visible at once: J1021+1216 and J1528+0544.