Tilted Dark Halos are Common, Long-Lived, and can Warp Galactic Disks

Abstract: In the $\Lambda$-CDM 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 ($r<50\text{ kpc}$) dark halo can be significantly misaligned with the stellar disk. To this end, we use the TNG50 run from the cosmological magneto-hydrodynamic IllustrisTNG simulation suite. Such "tilted" dark halos can arise from a variety of processes including major mergers, massive fly-bys, 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.