Sinking Satellites and Tilting Disk Galaxies

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Abstract
We perform fully self-consistent N-body simulations of a system of disk, halo, and satellite to investigate three different dynamical responses of a disk to an infalling satellite: tilting, warping, and thickening. Our model is characterized by two cosmologically significant improvements. First, the satellites start at a distance more than 3 times the radius of the optical disk, which ensures a realistic interaction among the satellite, the disk, and the halo in the course of the satellite infall. Second, we allow evolution of the structure and velocity ellipsoid of the disk due to internal heating. We study the commonly arising case of low-density satellites in contrast to that of compact satellites considered in previous work. We find that disks are mainly tilted rather than heated by infalling satellites. Satellites of 10%, 20%, and 30% of the disk mass tilt the disk by angles of 29 ± 03, 63 ± 01, and 106 ± 02. However, only 3.4%, 6.9%, and 11.1% of the orbital angular momentum is transferred to the parent galaxy. The kinetic energy associated with vertical motion in the initial coordinate frame of the disk is respectively increased by (6 ± 3)%, (26 ± 3)%, and (51 ± 5)%, whereas the corresponding thermal energy associated with the vertical random motion in the tilted coordinate frame is only increased by (4 ± 3)%, (6 ± 2)%, and (10 ± 2)%, respectively. The satellites cause warps that are substantially damped over 30 disk rotations. Given our choice of initial conditions, satellites are mainly accreted to the parent halos. Satellites having up to 20% of the disk mass produce no observable thickening, whereas a satellite with 30% of the disk mass produces little observable thickening inside the half-mass radius but great damage beyond this radius. Hence, high cosmological accretion and thin disks can coexist if most infalling satellites have densities comparable to that of the parent galaxy.

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