Multi-frequency characterisation of remnant radio galaxies in the Lockman Hole field

Abstract

Context. Remnant radio galaxies represent an important phase in the life-cycle of radio active galactic nuclei. It is suggested that in this phase, the jets have switched off and the extended emission is fading rapidly. This phase is not well-studied due to the lack of statistical samples observed at both low and high frequencies. Aims. In this work, we study a sample of 23 candidate remnant radio galaxies previously selected using the Low Frequency Array at 150 MHz in the Lockman Hole field. We examine their morphologies and study their spectral properties to confirm their remnant nature and revise the morphological and spectral criteria used to define the initial sample. Methods. We present new observations with the Karl G. Jansky Very Large Array at 6000 MHz at both high and low resolution. These observations allowed us to observe the presence or absence of cores and study the spectral curvature and steepness of the spectra of the total emission expected at these high frequencies for the remnant candidates. Results. We confirm 13 out of 23 candidates as remnant radio sources. This corresponds to 7% of the full sample of active, restarted, and remnant candidates from the Lockman Hole field. Surprisingly, only a minority of remnants reside in a cluster (23%). The remnant radio galaxies show a range of properties and morphologies. The majority do not show detection of the core at 6000 MHz and their extended emission often shows ultra-steep spectra (USS). However, there are also remnants with USS total emission and a detection of the core at 6000 MHz, possibly indicating a variety of evolutionary stages in the remnant phase. We confirm the importance of the combination of morphological and spectral criteria and this needs to be taken into consideration when selecting a sample of remnant radio sources.

The galaxy group NGC 507: Newly detected AGN remnant plasma transported by sloshing

“…However, we note that the core prominence of the source at 1400 MHz, defined as CP = S core /S tot , is equal to 0.02. This is more than a factor ten below the value expected for a typical radio galaxy of similar power according to de Ruiter et al (1990) and more consistent with recent results on remnant radio galaxies (Mahatma et al 2018;Jurlin et al 2021). This suggests, once again, that the radio AGN is experiencing a low-power phase, and that it is not able to sustain the activity of large-scale jets.…”

“…Jets from active galactic nuclei (AGN) are a recurrent phenomenon in the lifetime of a galaxy. This is clearly demonstrated by observations of jetted AGN exhibiting multiple pairs of radio lobes (the so-called 'restarted radio galaxies', Saikia & Jamrozy 2009;Schoenmakers et al 2000;Morganti 2017;Jurlin et al 2021), as well as by the presence of multiple generations of X-ray cavities in the intracluster and intragroup medium (ICM and IGrM) surrounding the central AGN of galaxy clusters Fits files of the radio maps are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: //cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/661/A92 and groups (e.g., Randall et al 2011;Vantyghem et al 2014;Biava et al 2021a).…”

The LOFAR view of giant, early-type galaxies: Radio emission from active nuclei and star formation

“…12, the red squares are those located in the LINERs region, the gray ones are those for which no optical spectral classification is possible, the gray downward pointing triangles are upper limits. Jurlin et al (2020Jurlin et al ( , 2021 study the properties of 158 extended radio sources in the Lockman Hole and by combining several indicators (e.g., core prominence, low-surface brightness of the extended emission, and steep spectrum of the central region) find a fraction between 13% and 15% of candidate restarted radio galaxies and 7% of remnant radio galaxies. Quici et al (2021) explored 104 radio sources in the GAMA 23 field with data covering the frequency range 0.1−9 GHz; by adopting a morphological criterion based on the absence of a radio core, they found ten candidate remnant sources, three of them also have a steep spectrum and a diffuse radio morphology.…”

The LOFAR view of giant, early-type galaxies: radio emission from active nuclei and star formation

“…Remnant and restarted sources has been the subject of several studies, spurred in particular by the advent of the deep low frequency images provided by LOFAR. Brienza et al (2017) and Jurlin et al (2020Jurlin et al ( , 2021 study the properties of 158 extended radio sources in the Lockman Hole and by combining several indicators (e.g., core prominence, low-surface brightness of the extended emission, and steep spectrum of the central region) find a fraction between 13% and 15% of candidate restarted radio galaxies and 7% of remnant radio galaxies. Quici et al (2021) explored 104 radio sources in the GAMA 23 field with data covering the frequency range 0.1-9 GHz; by adopting a morphological criterion based on the absence of a radio core, they found ten candidate remnant sources, three of them also have a steep spectrum and a diffuse radio morphology.…”