LoTSS DR1: Double-double radio galaxies in the HETDEX field

Abstract

Context. Double-double radio galaxies (DDRGs) represent a short but unique phase in the life-cycle of some of the most powerful radio-loud active galactic nuclei (RLAGN). These galaxies display large-scale remnant radio plasma in the intergalactic medium left behind by a past episode of active galactic nuclei (AGN) activity, and meanwhile, the radio jets have restarted in a new episode. The knowledge of what causes the jets to switch off and restart is crucial to our understanding of galaxy evolution, while it is important to know if DDRGs form a host galaxy dichotomy relative to RLAGN. Aims. The sensitivity and field of view of LOFAR enables the observation of DDRGs on a population basis rather than single-source observations. Using statistical comparisons with a control sample of RLAGN, we may obtain insights into the nature of DDRGs in the context of their host galaxies, where physical differences in their hosts compared to RLAGN as a population may allow us to infer the conditions that drive restarting jets. Methods. We utilised the LOFAR Two-Metre Sky Survey (LoTSS) DR1, using a visual identification method to compile a sample of morphologically selected candidate DDRGs, showing two pairs of radio lobes. To confirm the restarted nature in each of the candidate sources, we obtained follow-up observations with the Karl. G. Jansky Very Large Array (VLA) at higher resolution to observe the inner lobes or restarted jets, the confirmation of which created a robust sample of 33 DDRGs. We created a comparison sample of 777 RLAGN, matching the luminosity distribution of the DDRG sample, and compared the optical and infrared magnitudes and colours of their host galaxies. Results. We find that there is no statistically significant difference in the brightness of the host galaxies between double-doubles and single-cycle RLAGN. The DDRG and RLAGN samples also have similar distributions in WISE mid-infrared colours, indicating similar ages of stellar populations and dust levels in the hosts of DDRGs. We conclude that DDRGs and 'normal' RLAGN are hosted by galaxies of the same type, and that DDRG activity is simply a normal part of the life cycle of RLAGN. Restarted jets, particularly for the class of low-excitation radio galaxies, rather than being a product of a particular event in the life of a host galaxy, must instead be caused by smaller scale changes, such as in the accretion system surrounding the black hole.

Radio spectral properties and jet duty cycle in the restarted radio galaxy 3C388

“…This possibility has previously been discussed from an empirical stand point (e.g. Harwood et al 2017Harwood et al , 2019Mahatma et al 2019) and is also supported through modelling considerations and numerical simulations (Rudnick 2002;Turner et al 2017), which show that the mixing of different aged electrons strongly affects the spectrum at each point of the radio source leading to poor spectral age estimates. This mixing may have an observational origin, when the resolution of the observation is not high enough to distinguish different particle populations or in case of projection effects (see Harwood et al 2019), and also an intrinsic origin, when there is an actual mixing of particles that are accelerated by different events (i.e.…”

Revisiting the Fanaroff–Riley dichotomy and radio-galaxy morphology with the LOFAR Two-Metre Sky Survey (LoTSS)

“…It will be possible to adapt LoMorph for use with data from other instruments, but it is important to emphasise that the same sources can present very different appearances depending on the frequency, sensitivity and angular scales to which a survey is sensitive. For example, FIRST data generally only show the inner, newer structures of most double-double sources identified in LoTSS (Mahatma et al 2019;Williams et al 2019), and due to the higher frequency and comparatively poorer sensitivity of FIRST to extended structure, sometimes isolated components belonging to the same source may not be correctly identified as such. It is also crucial to note that deep surveys such as LoTSS contain a large number of ordinary galaxies with star-formation associated radio emission, which we have been able to pre-filter by using the sample selection of Hardcastle (2018a).…”

“…However, sensitive low frequency observations are at the same time revealing a more complex extended source population, including candidate hybrid radio galaxies, restarting and remnant radio galaxies (e.g. Kapińska et al 2017;Brienza et al 2016Brienza et al , 2017Mahatma et al 2018Mahatma et al , 2019. Simple classification schemes may therefore risk obscuring important physical distinctions.…”

RAiSE X: searching for radio galaxies in X-ray surveys

“…Sabater et al (2019) found that up to 100 per cent of all large galaxies host an active nucleus, noting that their accretion rate is modulated such that half the energy output is released in outbursts of increased activity lasting less than two percent of the time. Studies of individual objects and large sky surveys have also found populations of remnant (Brienza et al 2017;Mahatma et al 2018) and restarted (Mahatma et al 2019) radio galaxies, setting a lower bound for the dutycycle. Turner (2018) used a radio source dynamical model to provide an upper limit on the duty cycle of δ < 0.15 in the remnant B2 0924+30.…”