Resurging from the ashes: A spectral study of seven candidate revived radio fossils in nearby low-mass galaxy clusters

TL;DR

This study investigates revived fossil radio sources in the intracluster medium of nearby low-mass galaxy clusters by combining LOFAR LoTSS-DR2 144 MHz imaging with uGMRT 300–500 MHz follow-up to produce high-resolution morphologies and spatially resolved spectral-index maps for seven candidate revived fossils. The authors demonstrate that a morphology-based, low-frequency selection efficiently identifies fossil plasma, with all targets exhibiting regions of very steep spectra and complex filamentary structures that require high-resolution, multi-band data to classify accurately. They find a diverse set of revived fossil manifestations, including radio phoenices, GReET-like tails, remnants, and tailed galaxies undergoing re-energisation, often in disturbed cluster environments; several sources show indications of ongoing re-energisation, while others remain ambiguous remnants. The results highlight the role of fossil electrons as seed cosmic rays in the ICM and the influence of cluster dynamics on revival processes, and they outline a path forward with upcoming data releases (e.g., LoTSS-DR3), machine-learning identification, LOFAR-VLBI, and SKA precursors to systematically map revived fossil activity.

Abstract

Complex energy transfer processes in the intracluster medium (ICM) can revive fossil (with spectral ages $\gg100$ Myr) plasma initially generated by radio galaxies. This leads to the re-ignition of faint radio sources with irregular and filamentary morphologies, and ultra-steep ($α\gtrsim 1.5$) synchrotron spectra, which can be more easily detected at low frequencies ($\sim 100$ MHz). These sources offer the opportunity to investigate the microphysics of the ICM and its interplay with radio galaxies, the origin of seed relativistic electrons, the merging history of the host cluster, and the phenomenology of radio filaments. The study of revived sources has so far been hampered by the requirement of sensitive and high-resolution multi-frequency radio data at low frequencies to characterise their spatial properties and provide a proper classification. We aim to perform the analysis of a sample of candidate revived sources identified among nearby ($z\leq0.35$) and low-mass ($M_{500}\leq5\times 10^{14} M_\odot$) \textit{Planck} clusters in the footprint of LoTSS-DR2. By inspecting LoTSS-DR2 images at 144 MHz, we identified 7 targets with patchy and filamentary morphologies, which have been followed-up with the uGMRT at 400 MHz. By combining LOFAR and uGMRT data, we obtained high-resolution images and spectral index maps, which we used to interpret the nature of the sources. All targets show regions with very steep spectra, confirming the effectiveness of our morphology-based selection in identifying fossil plasma. Based on their morphology, spectral properties, and optical associations, we investigated the origin of the targets. We found a variety of promising revived fossil sources, while also showing that apparently intricate structures can be easily misclassified in the absence of high-resolution and multi-band data.

Figures (16)

• Figure 1: Overview of the candidate revived fossil sources identified by visual inspection among low-$z$ and low-$M_{\rm 500}$ PSZ2 clusters in LoTSS-DR2 based on their irregular and filamentary morphology.
• Figure 2: Images of G071.63+29.78. Top: LOFAR at 144 MHz and uGMRT at 400 MHz radio images (units are ${\rm Jy \; beam^{-1}}$). Resolution and noise are reported on top of each panel. The contour levels are $[\pm3, \;6, \;12,\; 24,\; ...]\times \sigma$. Bottom left: Pan-STARRS optical (composite i, r, g filters) image. Orange contours are from radio images on top panels. Green circles and cyan lines indicate optical sources and regions discussed in the text. Bottom centre: 144-400 MHz spectral index map (the corresponding error map is shown in Fig. \ref{['fig: errspixmap']} in Appendix \ref{['sect: errspixmap']}). The contour levels are $[\pm3, \;6, \;12,\; 24,\; ...]\times \sigma$ from the 144 MHz image. Bottom right: X-ray image of the cluster. Green contours are from the LOFAR image. Red and black crosses indicate the Planck centre and X-ray peak, respectively.
• Figure 3: Images of G088.53+41.18. The description of each panel is the same as for Fig. \ref{['fig: mappefullres1']}.
• Figure 4: Images of G113.29-29.69. The description of each panel is the same as for Fig. \ref{['fig: mappefullres1']}.
• Figure 5: Images of G137.74-27.08. The description of each panel is the same as for Fig. \ref{['fig: mappefullres1']}.