Evidence of a double-double morphology in B0818+214

TL;DR

This paper presents evidence of a double-double radio source in B0818+214, revealing a compact inner double structure within a smaller-than-typical size, suggesting intermittent activity and different accretion mechanisms.

Contribution

It provides the first detailed observation of a double-double morphology in a radio source smaller than 0.7 Mpc, challenging existing size-based criteria for such structures.

Findings

Inner lobes are within 5.7 kpc, immersed in host galaxy's ISM.

Inner double likely in coasting phase due to host galaxy inactivity.

Different accretion mechanisms may trigger outer and inner doubles.

Abstract

The so-called double-double structure in radio sources is the most conspicuous signature of their restarted activity. Observations indicate that in the majority of double-double radio sources (DDRS), the span of the radio lobes is larger than 0.7 Mpc. This lower limit is also suggested by theory. However, it seemed likely that the apparent core of B0818+214, a radio galaxy with an overall linear size of its radio structure below that limit, could harbour a compact double well aligned with the outer lobes so that the whole object would fulfil the criteria of a DDRS. Here, we present evidence that the central component of B0818+214, when magnified through the EVN+MERLIN 18-cm observations, shows two FR II-like lobes. As the separation of the inner lobes is not greater than 5.7 kpc, they are immersed in the ISM of the host galaxy. This circumstance is the likely reason why the inner double has become visible, despite the predictions of the theory according to which B0818+214 as a whole is too small for a new double to develop inside the cocoon inflated during the previous active phase. Moreover, we speculate that its host galaxy is not active at the moment and so the inner double may be in the coasting phase often observed in other medium-sized symmetric objects with intermittent activity. It could be, therefore, that two different mechanisms of accretion disk instabilities, ionisation and radiation-pressure driven, may be independently responsible for triggering active phases, manifesting as the outer and the inner doubles, respectively.