Sinking Satellites and Tilting Disk Galaxies

Abstract

The infall of a satellite galaxy onto a galactic disk generally brings in angular momentum that is not aligned with the axis of the disk. The main dynamical issues addressed are what fraction of the orbital angular momentum of the satellite and the associated energy is added to the disk, as opposed to being left in the halo, and whether the absorbed fraction is added coherently or thermalized in the disk. By employing fully self-consistent disk+halo+satellite N-body simulations, we study the particular case of the satellite and main halo having similar density profiles, with internal velocities having the ``cosmological'' scaling $σ\propto M^{1/3}$. We find that most of the orbital angular momentum of the infalling satellite is left in the tidally stripped satellite remnants, with only $2\%$, $6\%$ and $9\%$ of the orbital angular momentum being transferred to disks and halos for $10\%$, $20\%$ and $30\%$ disk-mass satellites respectively. Because the disks are tilted by the infall of $10\%$, $20\%$ and $30\%$ disk-mass satellites by angles of $(2.9\pm0.3)^\circ$, $(6.3\pm0.1)^\circ$ and $(9.7\pm0.2)^\circ$ respectively, the kinetic energy associated with the vertical motion in the initial coordinate frame of the three disks is respectively increased by $(6\pm3)\%$, $(26\pm3)\%$ and $(51\pm5)\%$ whereas the corresponding disk thermal energy associated with the vertical random motion in the tilted coordinate frame is only increased by $(4\pm3)\%$, $(6\pm2)\%$ and $(10\pm2)\%$, respectively. Under our initial conditions, a satellite having up to 20\% of the disk mass would produce little observable thickening whereas a 30\% disk-mass satellite produces little observable thickening inside the half-mass radius of the disk but great damage beyond the half-mass radius.