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

I. Radio observations

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

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

Received: 14 April 2025
Accepted: 20 October 2025

Abstract

Context. The general population of radio galaxies includes several classes of peculiar objects. These include giant radio galaxies (GRGs) with linear sizes greater than 0.7 Mpc and double-double radio galaxies (DDRGs) that show signs of recurrent jet-formation activity. Observations and models developed in recent decades suggest that recurrent activity, among other factors, may stimulate GRG growth. The connection between the two classes of objects is still poorly understood, however, and few of the known giant DDRGs are studied thoroughly so far.

Aims. We studied the giant DDRGs J1021+1216, J1528+0544, and J2345–0449. The collection of radio maps together with the basic parameters of the sources, such as independent measurements of the flux density in the inner and outer lobes, measurements of the linear sizes, and the axial ratio of the lobes will facilitate further in-depth modeling of the objects in the second part of our study.

Methods. We performed multifrequency radio observations of the selected sources with the Karl G. Jansky Very Large Array and the upgraded Giant Metrewave Radio Telescope. The analysis of the radio-lobe morphology and the structure of the spectral index maps provided preliminary information about the environment and activity of the sources.

Results. Outer doubles with asymmetric backflows and minor misalignments were observed in J1021+1216, J1528+0544, and J2345–0449. Low surface brightness wings occur close to the core in J2345–0449. In all three sources, the shorter lobe is the brighter one, which suggests large-scale inhomogeneities in the ambient medium. A trace of emission with low values of the spectral index appears in the outer double maps of J1528+0544. They might indicate the orientation of the jet in the initial activity phase. The asymmetries of the inner lobes in J1021+1216 and J2345–0449 can be explained by a relativistic Doppler effect, while the innder double structure of J1528+0544 requires contamination of the relic radio cocoon.

Conclusions. Our examples of asymmetric backflows and wings in the outer doubles of the sources show inhomogeneities in the external medium around the host galaxies on the size of the radio lobe. This suggests a complex evolution of the radio cocoons. The initial jets appear to clear the ambient medium for the subsequent activity phases. A spectral aging analysis and dynamical modeling are required to confirm the results. We provided this in the second part of this study.