A multiwavelength study of an early galaxy group merger in COSMOS revealed by two tailed radio galaxies at z = 0.35

¹ Department of Physics, University of Zagreb, Bijenička cesta 32, Zagreb, Croatia
² Department of Computer Science, Aalto University, PO Box 15400 00076 Espoo, Finland
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
⁴ Department of Physics, University of Helsinki, P.O. Box 64 FI-00014 Helsinki, Finland

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

Received: 4 July 2025
Accepted: 23 September 2025

Abstract

We report the discovery of two tailed radio galaxies in the COSMOS field, associated with a massive, dynamically unrelaxed galaxy group detected in X-rays at z = 0.349. One of them is a wide-angle tail (WAT) galaxy, supporting the role of WATs as tracers of dynamically young groups and clusters. Our multiwavelength analysis combines VLA radio data, HST-ACS imaging, COSMOS2020 photometric redshifts, COSMOS2015 photometry, the newest compilation of spectroscopic redshifts in COSMOS, and X-ray observations from Chandra and XMM-Newton. We used these data to study the tailed radio galaxies, their host galaxies, and the group environment. Both radio galaxies are hosted by massive (log₁₀(M_*/M_⊙) = 11.88 ± 0.03 and log₁₀(M_*/M_⊙) = 11.49 ± 0.06), red, elliptical galaxies with extended stellar halos, as revealed by a color, magnitude, and stellar mass analysis combined with GALFIT modeling and surface-brightness profiles. One corresponds to the brightest group galaxy (BGG), while the other is the second-brightest. A diffuse intragroup medium (IGM) is characterized by its irregular shape and the analysis of the X-ray spectra of the group core reveals high temperature (T_X = 2.4 ± 0.6 keV) and an electron density of (8.2 ± 0.3)×10⁻⁴ cm⁻³. A galaxy overdensity associated with the group was detected via Voronoi tessellation, using COSMOS2020 CLASSIC photometric redshifts, displaying an irregular morphology, along with evidence of substructure. Assuming the jet bending results from interaction with the IGM, we find a high relative velocity between the BGG and the IGM (v_BGG/IGM ≳ 540 km/s), primarily due to bulk gas motion. Our findings indicate a dynamically young system in the early stages of assembly via group-group merging.