Speaker
Description
Understanding how often, and for how long, radio-loud AGN switch their jets on and off remains a central problem in extragalactic astrophysics. Double-double radio galaxies (DDRGs), whose two distinct pairs of lobes preserve the imprint of successive jet episodes, are rare laboratories for tackling this issue, particularly when the systems reach giant ( >1 Mpc) dimensions, giving exceptionally clean separation between activity cycles. Until now, however, the lack of deep, well-matched imaging across the full radio band has limited quantitative age-dating of the plasma in such sources.
We present the first systematic spectral-ageing study of 14 giant DDRGs drawn from the LOFAR Two-Metre Sky Survey (LoTSS). Each source is now covered by six high-fidelity datasets spanning 54 MHz to 11 GHz: LOFAR LBA (LoLSS, 54 MHz), LOFAR HBA (LoTSS, 144 MHz), uGMRT Band 4 (550-750 MHz), and JVLA L, S, C, and X bands. In this presentation, I will showcase the new six-band images, which recover the full diffuse emission for nearly every target and unveil previously unseen features in greater detail. I will discuss how the morphological variety across the 14-source sample, ranging from edge-brightened inner doubles to hotspot-free relic lobes, encapsulates a continuum of restarted phases, and how the derived age distributions refine our picture of jet intermittency, particle acceleration and feedback on megaparsec scales. The LBA–HBA combination, along with uGMRT Band 4, secures precise injection indices. When coupled with the uGMRT–VLA data, we achieve robust curvature constraints, essential for diagnosing radiative losses, adiabatic expansion, and in-situ re-acceleration.
Such morphological diversity is echoed in the spectral properties. Low-frequency index maps from the LBA-HBA pair show injection spectra as flat as α ≈ 0.6 in several fresh hotspots, while outer relic lobes without hotspots steepen beyond α ≈ 1.2 between 54 MHz and 1.4 GHz, signalling severe radiative ageing. Fitting JP, KP and Tribble models with BRATS yields spectral ages ranging from a few Myr for the newly ignited inner jets to as much as ~80 Myr for the oldest outer cocoons. Combining these epochs gives duty-cycle estimates of 40% to 60%, implying that even the most powerful radio galaxies may spend comparable periods in active and quiescent states.
We place the giant DDRGs in their broader cosmic context by analysing both their surrounding environment and the megaparsec environments that host them. By comparing sources embedded in dense groups with those in more isolated regions, we explore how environmental pressure and fuel supply regulate jet shutdown, quiescence and re-ignition, shedding new light on the conditions that favour recurrent radio activity.
This study represents a major step forward in decoding AGN duty cycles, jet dynamics and particle acceleration mechanisms, refining our understanding of AGN feedback in large-scale structure formation.
Overall, these results demonstrate how LOFAR’s low-frequency leverage, coupled with uGMRT and JVLA coverage, can pin down both morphology and energetics of restarted AGN on megaparsec scales. They lay the groundwork for much larger duty-cycle studies with forthcoming LoTSS-DR3 and LOFAR 2.0 surveys, and ultimately for assessing the long-term impact of intermittent jets on galaxy and large-scale-structure evolution.
