2025MNRAS.542...62J

The luminous spirals NGC5713 and NGC5719 form an interacting galaxy pair 94 kpc apart and are connected by a straight, elongated neutral hydrogen structure extending over 200 kpc. Their 14 velocity-confirmed satellite galaxies and the two hosts separate into two distinct subgroups in their line-of-sight velocities and on-sky distribution revealing a prominent coherent kinematic structure with a velocity amplitude of $67\pm 12$ km s$^{-1}$ . We test four scenarios to explain the observed velocity field: isotropic motions in a dark matter halo, a plane of satellites seen nearly face-on, and a kinematically mixed satellite system with a co-rotating edge-on plane and an isotropic component, and a merger of two small galaxy groups. Taking the geometry and dynamical state of the NGC5713/19 pair into account together with their positions and motions in the Boötes Strip the most consistent picture is the infall of two satellite systems that follow their host galaxies along a cosmic filament. We believe this is the first clear example of an equal-mass $L^{*}$ disc galaxy merger where a kinematically coherent satellite system is in the process of formation. These observations reinforce the importance of major mergers as a channel for producing co-rotating satellite systems.