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The dark matter halos in $\Lambda$CDM cosmological simulations are triaxialand highly flattened. In many cases, these triaxial equilibria are alsotumbling slowly, typically about their short axes, with periods of order aHubble time. Halos may therefore exert a slowly changing external torque onspiral galaxies that can affect their dynamical evolution in interesting ways.We examine the effect of the external torques exerted by a tumbling quadrupolartidal field on the evolution of spiral galaxies using N-body simulations withrealistic, disk galaxy models. We measure the amplitude of the externalquadrupole moments of dark halos in cosmological simulations and use these toforce disk galaxy models in a series of N-body experiments for a range ofpattern speeds. We find that the torques are strong enough to induce long livedtransient warps in disks similar to those observed in real spirals and alsoinduce the bar instability at later times in some galaxy models that areotherwise stable for long periods of time in isolation. We also observe forcedspiral structure near the edge of the disk where normally self-gravity is tooweak to be responsible for such structure. This overlooked influence of darkhalos may well be responsible for many of the peculiar aspects of disk galaxydynamics.

Warps and Bars from the External Tidal Torques of Tumbling Dark Halos