Disk tilts in Milky Way-like galaxies and their link to mergers

Abstract

We investigated disk reorientation and its relationship to mergers in a sample of 93 Milky Way-mass galaxies drawn from the TNG50 simulations. From this sample, we identified 117 individual disk-tilt events with clearly defined start and end times, and we divided them into two classes based on whether the disk evolves smoothly or undergoes rapid disruption and regrowth during the tilt duration. For each tilt event, we linked the most massive merger that occurs in close temporal association with it. We focus on those tilt events occurring during phases of smooth disk evolution, which typically show tilt rates below 10 degrees per Gyr, persist for 2-9 Gyr, and are generally connected to minor mergers. Roughly 40% of these events reach tilt angles exceeding 30 degrees, most often triggered by satellites whose masses are comparable to or less massive than the galaxy's total stellar mass, merging on tangential orbits with orbital angular momentum that is retrograde or nearly perpendicular to the disk spin. The satellite consistently drives the disk tilt toward alignment with the satellite's orbital spin; satellites that came in on retrograde orbits relative to the disk spin can thus induce large tilt angles and can produce an almost orthogonal configuration between the stellar disk and the satellite's orbital plane as an intermediate phase. Owing to the comparatively slow tilt rates, the stellar disk can keep stable and continue to grow throughout the reorientation process. Consequently, galaxies that experience strong disk tilts can still maintain stellar disks that are as thin as those in systems without significant reorientation.