Finding Flybys in the Illustris Simulation
The lifespan of a galaxy is punctuated by both mergers and flybys, yet most methods to track the galaxy assembly history within a cosmological simulation are designed to identify only mergers. Here, we present the full halo interaction network for several runs of the Illustris simulation suite, available as a value-added catalog for the community. We describe the method to pinpoint both mergers and flybys, outline the features of the catalog, and identify differences and characteristics exhibited by the different interaction types. We find there are two distinct epochs during the age of the universe, the merger epoch (z=3 to z=1.5), where mergers are the dominant type of interaction, followed by the flyby epoch (z=1.5 to the present), where flybys occur at similar rates as mergers. These time periods are evident across hydrodynamic runs of varying resolution, as well as the dark matter only run. We find that massive halos near 1013 Msun have the highest rates of flybys per halo, with up to 1000 flybys per Gyr near z=0. Mass ratios of flybys tend to be less disparate than those of mergers. Flybys also happen at higher relative speeds than mergers, and have their closest approach between 1.5-2R200,mean of the host halo. The merging population of halos is captured at closer distances to the host and, unlike flybys, preferentially have a majority of their relative velocity in the direction of the host. This paints a picture of flybys as frequent, grazing encounters experienced by low-mass halos on the outskirts of their larger companion. Lastly, the prevalence of flybys in simulations does not appear to be affected by the incorporation of baryons. We conclude that the presence of flybys cannot be ignored in the broader picture of galaxy evolution especially when studying galaxies near the present time.