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

Abstract

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. 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. We have combined LOFAR observations at 150 MHz with public surveys at higher frequencies to perform a complete selection and have used, for the first time, a combination of spectral criteria (e.g. the classical ultra-steep spectral index and high spectral curvature) as well as morphological criteria (e.g. low radio core prominence and relaxed shapes). Mock catalogues of radio galaxies are created based on existing spectral and dynamical evolution models combined with observed source properties. 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 subset of the entire population ($\sim$50%) when frequencies higher than 1400 MHz are not included in the selection process, and that they are biased towards old ages.