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

TL;DR

This study targets a massive, dynamically unrelaxed galaxy group in the COSMOS field at $z=0.349$, identified by two tailed radio galaxies including a wide-angle tail (WAT). It combines 3 GHz VLA radio data, HST-ACS imaging, COSMOS2020 CLASSIC photometric redshifts, COSMOS2015 photometry, spectroscopic redshifts, and Chandra/XMM-Newton X-ray observations to characterize the tailed radio galaxies, their hosts, and the intragroup medium (IGM). The X-ray analysis yields a hot, irregular IGM with $T_X = 2.4 \\pm 0.6$ keV and $n_e = (8.2 \\pm 0.3) imes 10^{-4} \, \mathrm{cm}^{-3}$, while the optical analysis reveals an irregular, red-dominated overdensity with weak substructure. The BGG shows a velocity offset relative to the IGM of $v_{BGG/IGM} \gtrsim 540$ km s$^{-1}$, implying substantial bulk motion of the IGM and supporting an early-stage galaxy group merger scenario in which WATs act as effective tracers of dynamical youth. Overall, the results indicate a dynamically young system in the early assembly phase, where tailed radio morphologies reflect the interplay between galaxies and their surrounding hot plasma. Future deep, high-resolution multiwavelength observations and hydrodynamical modeling will further constrain the physical mechanisms linking tailed radio galaxies to group assembly processes.

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 ($\log_{10}(M_*/M_{\odot})=11.88\pm0.03$ and $\log_{10}(M_*/M_{\odot})=11.49\pm0.06$), 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 ($T_X=2.4\pm0.6\hspace{0.1cm}\mathrm{keV}$) and an electron density of $(8.2\pm0.3)\times 10^{-4}\hspace{0.1cm}\mathrm{cm^{-3}}$. 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 ($v_{\mathrm{BGG/IGM}} \gtrsim 540$ 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.

Figures (14)

• Figure 1: Multiwavelength view of our massive galaxy group in the COSMOS field. The background is an RGB composite created from Subaru i-, r-, and g-band observations capak2008, revealing the optical galaxy distribution. The magenta overlay indicates extended X-ray emission from the intragroup medium, derived from wavelet-filtered 0.5–2 keV data from Chandra and XMM–Newton Gozaliasl, tracing diffuse hot gas on scales of 16–256 arcseconds. The blue structures highlight the radio emission from the radio galaxies 10913 (WAT, the brightest group galaxy) and 44 (tailed, the second brightest group galaxy), tracing radio jets launched from their central engines. The emission is based on the 3 GHz map from the VLA-COSMOS 3 GHz Large Project 3ghz_smol_a. Together, the image captures the interaction between galaxies, the radio AGNs, and the surrounding hot plasma in a galaxy group at a redshift of $z = 0.349$.
• Figure 2: Cumulative distribution functions of the source (galaxy) density scaled with the mean value of source density (for a given sample) for 100 simulated background samples (red curves). Vertical black lines represent the mean density thresholds at $P=0.7$, $P=0.8$, and $P=0.9$ based on 100 simulations.
• Figure 3: Results of our Voronoi tesselation analysis: Colored cells are left after filtering according to the pre-calculated source density thresholds (0.7, 0.8, and 0.9 in different colors; see Section \ref{['sec:voronoi']} for details). Black dots are galaxies with the local source density higher than $80\%$ of the density values from the total sample, and together with the hosts of radio galaxies 10913 and 44 (manually added, see the text for details) form a sample of probable group members. Host galaxies of the 7 radio sources are shown as open gray circles. 10913, 44, and 4092 correspond to the first, second, and third brightest galaxies of the group, respectively, in the SC $i^{+}$ band (white stars).
• Figure 4: Panel A: CMD for 76 probable group members. Triangles represent the 23 most probable members with high-confidence spectroscopic redshifts in a narrow range [0.335, 0.365] centered on the group's redshift (see panel B, shown as an inset of panel A). Galaxies are separated in blue and red according to color-determination scheme explained in Section \ref{['sec:defining_the_red_sequence']}. Panel C: 76 probable group galaxies' bimodal distribution in color compared to the one of the large COSMOS-0.35 sample. Dot-dashed vertical lines indicate the inflection points of the two distributions.
• Figure 5: CSMD for 76 probable group members. Stellar masses are calculated as explained in Section \ref{['sec:csmd_analysis']}. The first and the second most massive galaxies correspond to the hosts of radio galaxies 10913 (WAT) and 44 (tailed), respectively. Markers and colors follow the scheme described in the caption of Fig. \ref{['fig:CMD_B-r']}.