[Paper Review] Dying Radio Sources in Clusters

Murgia, M., Parma, P.+7 more|arXiv (Cornell University)|Nov 2, 2010

Galaxies: Formation, Evolution, Phenomena139 citations

TL;DR

This study investigates five nearby 'dying' radio galaxies with extremely steep radio spectra, using VLA and MERLIN data to model synchrotron radiative losses. The results confirm that these sources are in a fading phase with extended lobes persisting long after central engine activity ceased, and all are projected at cluster centers—suggesting cluster environments may prolong the dying phase due to reduced lobe expansion or higher occurrence rates of intermittent AGN activity.

ABSTRACT

We present the study of five `dying' nearby radio galaxies belonging to the WENSS minisurvey and to the B2 bright catalogs: WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 1610+29. These sources have been selected on the basis of their extremely steep broad-band radio spectra. The modeling of the integrated spectra and the deep spectral index images obtained with the VLA confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime although their extended lobes have not yet completely faded away. We found that WNB1851+5707 is in reality composed by two distinct dying galaxies, which appear blend together as a single source in the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes are seen in conjunction with a currently active core. A very faint core is detected also in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources of our sample are located (at least in projection) at the center of an X-ray emitting cluster. Our results suggest that the duration of the dying phase for a radio source in cluster can be significantly higher with respect to that of a radio galaxy in the field. The simplest interpretation is that the low-frequency emission from the fading radio lobes last longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. Radio sources in dense environment, like e.g. the center of cooling core clusters, may have a peculiar accretion mode which results in a bursting duty cycle sequence of active and quiescent periods. This result could have important implications for theories of the life cycles of radio sources and AGN feedback in clusters of galaxies but awaits confirmation from future observations of larger samples of objects.

Motivation & Objective

• To identify and characterize nearby 'dying' radio galaxies with extremely steep radio spectra, indicating post-active phases.
• To determine the duration of the dying phase by fitting synchrotron radiative models to radio spectra.
• To investigate whether the presence of fossil radio lobes in galaxy clusters is due to environmental suppression of lobe expansion or higher intrinsic occurrence rates.
• To examine the connection between dying radio sources and X-ray emitting cluster cores, particularly in cooling-core clusters.
• To assess whether low-frequency radio surveys can reveal a hidden population of fossil radio plasma missed in higher-frequency surveys.

Proposed method

• Selected five sources from the WENSS minisurvey and B2 bright catalog based on extremely steep broad-band radio spectra (α > 1.3).
• Fitted synchrotron radiative loss models to integrated radio spectra to estimate the duration of the dying phase.
• Produced deep spectral index images using VLA data to map spectral steepening and identify regions of fossil plasma.
• Used MERLIN imaging to detect faint nuclear components in sources with extended lobes.
• Cross-correlated radio sources with X-ray cluster data from Chandra and ROSAT to assess environmental context.
• Analyzed optical data and redshifts to confirm host galaxy associations and cluster membership.

Experimental results

Research questions

• RQ1 What is the duration of the dying phase for radio galaxies in dense cluster environments compared to isolated field sources?
• RQ2 Why are dying radio galaxies preferentially found in the centers of X-ray emitting clusters?
• RQ3 Can the persistence of low-frequency radio emission in fading lobes be explained by reduced lobe expansion in high-pressure intracluster media?
• RQ4 Do dying sources in clusters show evidence of ongoing or recent low-level nuclear activity, suggesting a bursting AGN duty cycle?
• RQ5 To what extent are fossil radio lobes missed in current surveys due to their very steep spectra and low flux at cm-wavelengths?

Key findings

• All five dying radio galaxies in the sample are located (at least in projection) at the centers of X-ray emitting galaxy clusters, with high significance (p < 0.5% chance of random alignment).
• The VLA and MERLIN observations confirmed that the central engines have ceased activity for a significant fraction of the sources' lifetimes, yet their extended lobes remain detectable due to synchrotron radiative losses.
• WNB1851+5707 was resolved into two distinct dying galaxies, both with fossil lobes and faint cores, indicating a blended detection in lower-resolution WENSS data.
• WNB1829+6911 and B2 0120+33 show evidence of currently active, low-luminosity cores (flat-spectrum features), suggesting intermittent AGN activity.
• A very faint core was detected in the MERLIN image of WNB1851+5707a, indicating ongoing low-level nuclear activity despite the dominance of fossil lobes.
• The results suggest that the dying phase in cluster environments may last significantly longer than in the field, likely due to suppressed lobe expansion by dense intracluster medium or higher incidence of bursty AGN duty cycles.