Double-Double radio galaxies:The restarted class of AGN
Pratik Dabhade (Instituto de Astrofísica de Canarias)
30 August 2023
One of the most striking evidence of recurrent activity in AGN is the existence of double-double radio galaxies (DDRGs). These consist of two types of radio lobes, usually along a similar jet axis. DDRGs tend to have FR-II morphology, with the outer pairs of lobes, which are from an earlier cycle of activity, being more diffuse and extended, while the more recent pairs of lobes display hotspots. If the timescale of the restarting is large or if the large-scale environment is quite inhomogeneous, then we also find the offset in the two pairs of lobes with respect to each other. DDRGs allow us to constrain the duty cycle of the AGN activity, where the source morphology and the information from the radio spectra can help us reconstruct the history of the episodic activity. The existence of these sources poses several interesting questions related to the triggering and fueling mechanism of the central black hole, the time-cycle of AGN activity, the evolution of radiogalaxies, and feedback processes from AGN. In order to understand the growth and evolution of radio galaxies, studies of remnant and restarted radio galaxies (e.g., DDRGs) are of vital importance, where low-frequency telescopes like LOFAR are playing a crucial role in identifying relatively elusive populations. Surveys with LOFAR provide excellent sensitivity to diffuse and spectrally steep plasma, which is prevalent in both remnant and restarted radio sources. As per the last compilation of DDRGs in 2017, only about 70 such sources were known in the literature, and that number has not drastically increased as expected. This is mainly due to a lack of dedicated systematic searches through the LOFAR surveys. In order to understand the conditions under which these AGNs undergo recurrent activity, we need to build a statistically large sample of DDRGs and then study them using multi-frequency radio data. In this talk, Pratik will discuss an overview of the literature on this topic, and some recent results of their group will be presented, covering methods employed in searches and the multi-frequency analysis of their new large sample.