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 the mass loss and shape transformation to the emergence of tidal features and 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 the 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 the dark matter halo leftover mass of the intruder galaxy follows a logarithmic growth law with impact parameter, regard- less of the way the total halo mass is estimated. The lost mass then, clearly, follows the exponential decay law. The stellar component stretches faster as the impact parameter decreases, following the exponential decay law with impact parameter. Functional dependence on impact parameter in all cases seems universal, but the fitting parameters are likely sensitive to the interaction parameters and initial conditions (e.g. the mass ratio of interacting galaxies, initial relative velocity of the 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, they can still contribute significantly. Rare, atypical and stronger flybys are worth further exploring.

The role of impact parameter in typical close galaxy flybys

“…Similarly, the intruder's dark matter halo also gains angular momentum after the pericentre anti-correlated with the impact parameter. However, it is followed by the immediate and steep angular momentum loss, which coincides with the period of intense tidal stripping of the dark matter halo (Mitrašinović, 2022). During this period, two angular momentum vectors are almost aligned in most simulations (B30-B55).…”

“…We used these simulations to examine the intruder's dark matter mass loss (Mitrašinović, 2022). In this paper, our focus is on the primary galaxy and its structural evolution.…”

The isolated dark matter-poor galaxy that ran away

“…Since the galaxy is almost constantly tidally disturbed during this intense phase, as a measure of its center, we used the position of a particle with the lowest potential energy, instead of a calculated center of mass. This provides a more accurate estimate, as the difference between the two can become significant due to tidal structures (e.g., Mitrašinović 2022). Aside from being tidally disturbed, during this period, the galaxy occasionally and briefly becomes a satellite of more massive cluster members as it interacts with them.…”