Dark matter mass loss in galaxy flybys: dependence on impact parameter

Abstract

Galaxy flybys, interactions where two independent halos inter-penetrate but detach at a later time and do not merge, occur frequently at lower redshifts. These interactions can significantly impact the evolution of individual galaxies from mass loss and shape transformation to the emergence of tidal features and the formation of morphological disc structures. The main focus of this paper is on the dark matter mass loss of the secondary, intruder galaxy, with the goal of determining a functional relationship between impact parameter and dark matter mass loss. Series of N-body simulations of typical galaxy flybys (10:1 mass ratio) with differing impact parameters show that dark matter halo leftover mass of intruder galaxy follows logarithmic growth law with impact parameter, regardless of the way total halo mass is estimated. Lost mass then, clearly, follows exponential decay law. Stellar component stretches faster as impact parameter decreases, following exponential decay law with impact parameter. Functional dependence on impact parameter in all cases seems universal, but fitting parameters are likely sensitive to interaction parameters and initial conditions (e.g. mass ratio of interacting galaxies, the initial relative velocity of intruder galaxy, interaction duration). While typical flybys, investigated here, could not be the sole culprit behind the formation of ultra-diffuse or dark matter deficient galaxies, their effects should not be disregarded as they can at least contribute substantially. Rare, atypical and stronger flybys are worth exploring further.