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Previous models of galactic disk heating in interactions invoke restrictiveassumptions not necessarily valid in modern LCDM contexts: that satellites andorbits are rigid and circular, with slow decay over many orbital times fromdynamical friction. This leads to a linear scaling of disk heating withsatellite mass: disk heights and velocity dispersions scale ~M_sat/M_disk. Inturn, observed disk thicknesses present strong constraints on merger histories:the implication for the Milky Way is that <5% of its mass could come frommergers since z~2, in conflict with cosmological predictions. Morerealistically, satellites merge on nearly radial orbits, and once near thedisk, resonant interactions efficiently remove angular momentum while tidaleffects strip mass, leading to rapid merger/destruction in a couple offree-fall plunges. Under these conditions the proper heating efficiency isnon-linear in mass ratio, ~(M_sat/M_disk)^2. We derive the scaling of diskscale heights and velocity dispersions as a function of mass ratio and disk gascontent in this regime, and show this accurately describes the results ofsimulations with proper 'live' halos and disks. Under realistic circumstances,disk heating in minor mergers is suppressed by an order of magnitude relativeto expectations of previous models. We show that the Milky Way disk could haveabsorbed ~5-10 1:10 mass-ratio mergers since z=2, in agreement withcosmological models. These distinctions lead to dramatic differences in whichmass ratios are most important for disk heating and in the isophotal shapes ofdisk+bulge systems.

The Radical Consequences of Realistic Satellite Orbits for the Heating and Implied Merger Histories of Galactic Disks