2024A&A...692A.175G

Context. The study of substructure is an important step in determining how galaxy clusters form.
Aims. We aim to gain new insights into the controversial dynamical status of MACS J0329-0211 (MACS0329), a massive cluster at z = 0.4503 ± 0.0003, through a new analysis using a large sample of member galaxies as kinematic tracers.
Methods. Our analysis is based on extensive spectroscopic data for more than 1700 galaxies obtained with the VIMOS and MUSE spectrographs at the Very Large Telescope (VLT) in combination with B and R_C Suprime-Cam photometry from the Subaru archive. According to our member selection procedure, we defined a sample of 430 MACS0329 galaxies within 6 Mpc, corresponding to approximately three times the virial radius.
Results. We estimated the global velocity dispersion, σ_V841_–36⁺²⁶ km s^–1, and present the velocity dispersion profile. We estimated a mass of M₂₀₀ = (9.2 ± 1.5)×10¹⁴ M_⊙ using 227 galaxies within R₂₀₀ = (1.71 ± 0.07) Mpc, for which σ_V,200841_–48⁺⁴⁰ km s^–1. The spatial distribution of the red galaxies traces a SE-NW elongated structure without signs of a velocity gradient. This structure likely originates from the main phase of cluster assembly. The distribution of the blue galaxies is less concentrated and more rounded, and it shows signs of substructure, all characteristics indicating a recent infall of groups from the field. We detected two loose clumps of blue galaxies in the south and southwest at a distance of ∼R₂₀₀ from the cluster center. The strong spatial segregation among galaxy populations is not accompanied by a kinematical difference. Thanks to our extensive catalog of spectroscopic redshift, we were able to study galaxy systems that are intervening along the line of sight. We identified two foreground galaxy systems, GrG1 at z ∼ 0.31 and GrG2 at z ∼ 0.38, and one background system, GrG3 at z ∼ 0.47. We point out that the second brightest galaxy projected onto the MACS0329 core is in fact the dominant galaxy of the foreground group GrG2. MACS0329, GrG3, and two other systems detected using DESI DR9 photometric redshifts are close to each other, suggesting the presence of a large-scale structure.
Conclusions. MACS0329 is close to a state of dynamical equilibrium despite being surrounded by a very rich environment. We emphasize that the use of an extensive spectroscopic redshift survey is essential to avoiding misinterpretation of structures projected along the line of sight.