Search and modelling of remnant radio galaxies in the LOFAR Lockman Hole field

Abstract

Context. The phase of radio galaxy evolution after the jets have switched off, often referred to as the remnant phase, is poorly understood and very few sources in this phase are known. Aims. In this work we present an extensive search for remnant radio galaxies in the Lockman Hole, a well-studied extragalactic field. We create mock catalogues of low-power radio galaxies based on Monte Carlo simulations to derive first-order predictions of the fraction of remnants in radio flux limited samples for comparison with our Lockman-Hole sample. Methods. Our search for remnant radio galaxies is based on LOFAR observations at 150 MHz combined with public survey data at higher frequencies. To enhance the selection process, and obtain a more complete picture of the remnant population, we use spectral criteria such as ultra-steep spectral index and high spectral curvature, and morphological criteria such as low radio core prominence and relaxed shapes to identify candidate remnant radio galaxies. Mock catalogues of radio galaxies are created based on existing spectral and dynamical evolution models combined with observed source properties. Results. We have identified 23 candidate remnant radio galaxies which cover a variety of morphologies and spectral characteristics. We suggest that these different properties are related to different stages of the remnant evolution. We find that ultra-steep spectrum remnants represent only a fraction of our remnant sample suggesting a very rapid luminosity evolution of the radio plasma. Results from mock catalogues demonstrate the importance of dynamical evolution in the remnant phase of low-power radio galaxies to obtain fractions of remnant sources consistent with our observations. Moreover, these results confirm that ultra-steep spectrum remnants represent only a small subset of the entire population (∼50%) when frequencies higher than 1400 MHz are not included in the selection process, and that they are biased towards old ages.

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

“…Brienza et al: Radio spectral properties and jet duty cycle in the restarted radio galaxy 3C388 This is becoming increasingly evident through recent observational campaigns of remnant radio galaxies detecting small fractions, up to 10%, of these sources (see e.g. Brienza et al 2017;Godfrey et al 2017;Mahatma et al 2018), and also through radio galaxy modelling and simulations, which predict visibility timescales for the remnant plasma of the order of a few tens of Myr (e.g. Brienza et al 2017;Godfrey et al 2017;Hardcastle 2018;English et al 2019).…”

Investigating the spectral age problem with powerful radio galaxies

“…at least for Fanaroff-Riley type II (FR-II; Fanaroff & Riley 1974) sources, constrain the lobe magnetic field strengths to be substantially lower than those implied by equipartition (Croston et al 2005;Ineson et al 2017). Furthermore, low-sensitivity, narrow-bandwidth observations from the previous generation of radio telescopes may not accurately trace the oldest radiating particles since last injection -a feature of the rapidly fading nature of synchrotron radiation that also prevents detections of switchedoff radio galaxies even in sensitive low frequency surveys (Brienza et al 2016(Brienza et al , 2017Mahatma et al 2018).…”

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

“…In order to test this with a large sample, sensitivity is crucial. Studies of remnants with the LOw Frequency ARray (LOFAR; van Haarlem et al 2013) in the Herschel-ATLAS field (Mahatma et al 2018), for sources > 80 mJy at 150 MHz, and in the similar LOFAR study by Brienza et al (2017), found upper-limit remnant fractions of ∼ 10 per cent; these are potentially a larger percentage of remnants than the percentage found in 3CRR, although this is clearly a non-systematic comparison.…”