Tilted Dark Halos Are Common and Long-lived, and Can Warp Galactic Disks

In the Lambda cold dark matter paradigm, the dark halo governs the gravitational potential within which a galaxy can form and evolve. In this Letter we show that the present-day inner ( r < 50 kpc) dark halo can be significantly misaligned with the stellar disk. To this end, we use the TNG50 run from the cosmological magnetohydrodynamic IllustrisTNG simulation suite. Such “tilted” dark halos can arise from a variety of processes including major mergers, massive flybys, or interactions with satellite companions. Furthermore, we show that tilted dark halos (1) are well traced by tilted stellar halos, (2) can maintain their tilt for >5 Gyr in isolated evolution, and (3) can generate warps in the outer disks that are stable over many Gyr. A tilted dark halo holds clues to important events in the formation history of a galaxy, and could help explain the abundance of warped disks in galaxy observations, including the Milky Way.