XGAP with uGMRT

I. Old active galactic nuclei plasma in merging galaxy groups

¹ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, S. P. Pune University, Ganeshkhind, Pune, 411007, India
² Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, P.O. Box 94 Makhanda 6140, South Africa
³ South African Radio Astronomy Observatory, Black River Park North, 2 Fir St, Cape Town, 7925, South Africa
⁴ Istituto Nazionale di Astrofisica (INAF) – Istituto di Radioastronomia (IRA), via Gobetti 101, 40129 Bologna, Italy
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, CH-1290 Versoix, Switzerland
⁷ INAF – IASF Milano, via A. Corti 12, 20133 Milano, Italy
⁸ Department of Physics, University of Helsinki, P. O. Box 64, FI00014 Helsinki, Finland
⁹ Department of Physics and Astronomy, University of Alabama in Huntsville, 301 Sparkman Drive, Huntsville, AL 35899, USA
¹⁰ Department of Computer Science, Aalto University, PO Box 15400 Espoo FI-00076, Finland
¹¹ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 November 2025
Accepted: 4 February 2026

Abstract

Context. Galaxy groups are significantly affected by outflows from central active galactic nuclei (AGN) due to their shallower gravitational potential compared to galaxy clusters. The group-binding energy is comparable to the energy output from AGN, making it an important factor in mutual evolution.

Aims. To understand the central AGN evolution, we performed a multi-wavelength analysis of three dynamically active groups–SDSSTG8102, SDSSTG16393, and SDSSTG28674–which are part of the XMM-Newton Group AGN Project X-GAP sample, a statistically complete sample of 49 galaxy groups.

Methods. We combined proprietary uGMRT 400 MHz observations with 144 MHz LOFAR and XMM-Newton observations to study the radio sources associated with the respective brightest group galaxies (BGGs).

Results. The BGGs in SDSSTG8102 and SDSSTG16393 have extended radio emission with asymmetric distortions in their morphologies. SDSSTG28674 has a compact flat-spectrum radio source associated with the BGG and an extended lobe on one side. The source is connected to the extended lobe by a faint bridge, which was detected with LOFAR. The integrated spectral indices of the three BGGs are −0.96 ± 0.09 (SDSSTG8102), −1.35 ± 0.09 (SDSSTG16393), and −1.6 ± 0.02 (SDSSTG28674). X-ray images revealed elongated morphologies in all three groups, with SDSSTG28674 showing evidence of a binary merger, while thermodynamical maps highlighted temperature variations.

Conclusions. In SDSSTG8102, lobes are bent and displaced by intragroup medium (IGrM) flows, while SDSSTG16393 hosts steep-spectrum relic-like plasma coinciding with X-ray emission. SDSSTG28674, with its ultra-steep spectrum lobe and disturbed morphology, likely traces merger-driven activity, as would be consistent with a remnant or revived radio phoenix. The spectral diversity across the systems reflects different stages of AGN fading governed by duty cycle, source age, and confinement by the hot IGrM. The presence of bright group-scale X-ray halos (∼300 kpc) and radio emission > 50 kpc, combined with disturbed morphologies, underscores the central role of IGrM confinement and merger-driven gas motions in sustaining extended diffuse structures. Multi-band radio follow-up of the entire X-GAP sample will allow for further insights.