A group of merging galaxies falling into Abell 2142

¹ LUX, Observatoire de Paris, Sorbonne Université, Université PSL, CNRS F-75014 Paris, France
² Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg Mönchhofstr. 12–14 69120 Heidelberg, Germany
³ Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij Pr. 13 Moscow 119234, Russia
⁴ Collège de France 11 Place Marcelin Berthelot 75005 Paris, France

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

Received: 10 July 2025
Accepted: 30 December 2025

Abstract

Galaxy clusters produce a very hostile environment for galaxies: their gas gets stripped by ram pressure, they undergo galaxy interactions, and their star formation is quenched. Clusters, like Abell 2142, grow not only through galaxy accretion but also through galaxy group infall. Our goal was to study the physical and dynamical state of the most conspicuous infalling group, which is located at a projected distance of 1.3 Mpc from the Abell 2142 centre. The galaxy group G is the leading edge of a spectacular 700 kpc long X-ray tail of hot gas stripped by ram pressure. The infalling galaxies are not quenched yet and are ideal objects for studying the transformation processes due to the cluster environment. We used integral field spectroscopy from MaNGA to derive stellar and gas kinematics, and MegaCam for photometry. Stellar populations (with age and metallicity) were obtained through full-spectrum fitting using NBURSTS. The gas kinematics and excitation were derived from the line emission of Hα, [N II], [O III], and Hβ. The group contains four galaxies, two of which are merging and partly superposed on the line of sight. With a simple parametric model for each velocity field, we succeeded in disentangling the contribution of each galaxy and derived their physical state and kinematics. They are primarily rotating discs, but perturbations and out-of-equilibrium gas manifest as regions of elevated dispersion and as tidal tails and loops of intra-group material. All galaxies show sustained star formation, with a global star formation rate of 42 M_⊙/yr. We conclude that the long X-ray tail must have come from the hot intra-group medium, present before the group infall, and does not correspond to the ram-pressure stripping of the galaxy gas. Ongoing interactions between the group members enhance the star formation activity by inducing mixing of dense gas from their gas-rich galactic discs.