Speaker
Description
Peaked-spectrum (PS) radio sources - Gigahertz-Peaked Spectrum (GPS) and Megahertz-Peaked Spectrum (MPS) - are known to trace the youngest stages of jet evolution. Milliarcsecond VLBI images often reveal compact double morphologies embedded in kpc-scale cocoons, suggesting that many PS objects could later grow into classical FR II or FR I radio galaxies. A key open question is whether this growth is typical or PS activity instead consists of short bursts separated by long quiescent periods remains an open question that can be answered only by detecting relic, low–surface-brightness plasma around the compact cores. Over the last three decades only a handful of PS AGN have been shown to sit inside much older, faint radio cocoons, leaving the duty–cycle of young jets essentially unconstrained. Previous wide-area radio surveys lacked the surface-brightness sensitivity and resolution to recognise these “compact-core + relic/extended emission” systems in statistically useful numbers. LoTSS-DR2/3 changes the landscape with its excellent sensitivity at 144 MHz across the northern sky, exactly the regime where hundred-kiloparsec lobes survive tens of Myr after the jets switch off.
Taking advantage of the high sensitivity and resolution of LoTSS-DR2/3, we examine the duty–cycle of young radio galaxies. The sensitivity and resolution of LoTSS are ideal for unveiling the faint, steep-spectrum cocoons that trace previous jet episodes. We focus on the large LoTSS-area catalogues GPS & MPS sources. The LoTSS images reveal faint, steep-spectrum cocoons that record past jet episodes: so far I have identified 60 PS objects with diffuse emission extending ≥ 50 kpc beyond their unresolved cores. Roughly half reside inside giant radio galaxies that already span megaparsec scales. Coexisting peaked cores and relic/extended emission provide unambiguous evidence of restarted activity; spectral fitting shows that the extended emissions are tens of Myr old, whereas the new cores are younger than a Myr, implying duty cycles dominated by long dormant phases even at low radio powers. In this presentation we will detail the search strategy, analysis pipeline, the current results, and the future work now underway, including optical/IR SED modelling that will probe any synchrony between jet re-ignition and host-galaxy star formation.
By measuring how frequently PS sources carry relics or extended emission beyond core, we can constrain the life-cycle of compact AGN and test whether most eventually mature into large-scale doubles or terminate after brief spurts. These first results demonstrate LOFAR’s unique ability to connect AGN activation to its afterglow, refine models of radio-mode feedback, and deliver the first large, homogeneously selected sample of restarted peaked-spectrum galaxies.
This pilot census also acts as a path-finder: the methods and framework developed here are directly scalable to deeper LOFAR surveys with LOFAR 2.0 and, ultimately, SKA-Low & SKA-Mid surveys, where still fainter afterglows will be detectable in the tens of thousands—opening a new window on the full temporal ecology of radio-loud AGN.
