Oral Presentation

Living with Neighbors: Interaction fraction and spin-orbit alignment of dark matter halos

Author(s): Sung-Ho An (Yonsei University), Juhan Kim (KIAS), Jun-Sung Moon, and Suk-Jin Yoon (Yonsei University)

Presenter: Sung-Ho An (Yonsei University)

We investigate how neighbors affect galaxy formation and evolution using a set of cosmological simulations. We exploit dark matter halo pairs with a mass range of 10^10.8–10^13.0 h^-1Msun and with a distance smaller than the sum of virial radii of two constituent halos. An et al. (2019) shows the statistical analysis of the halo pairs by measuring the interaction fraction. Dividing into mergers and flybys based on the total energy of the pair system, we find that the merger fraction does not depend on the halo mass and large-scale density, whereas the flyby fraction increases with the decreasing halo mass and with the increasing density. Over cosmic time, the flybys substantially outnumber mergers toward z=0 by a factor of five, implying the flyby’s contribution to galactic evolution is stronger than ever at the present epoch. An et al. (2021, submitted) analyzes the angular alignment between the spin direction of a halo and the orbital angular momentum of its interacting neighbor (i.e., spin-orbit alignment or SOA). The SOA is statistically significant in that 53% of the neighbors are on the prograde orbit, and the fraction increases for the pairs with close merging neighbors. We for the first time find evidence that the merging neighbors moving along the local filament induce the spin-flip phenomenon. We suggest that the SOA is created by the tidal effects of both interactions with neighbors and the local cosmic flow along the filament.