Gaseous Dynamical Friction: A Challenge to Modern Hydrodynamical Schemes

Abstract

The process of momentum and energy transfer from a massive body moving through a background medium, known as dynamical friction (DF), is key to our understanding of many astrophysical systems. We present a series of high-resolution simulations of gaseous DF using Lagrangian hydrodynamics solvers, in the state-of-the-art multi-physics code, GIZMO. The numerical setup is chosen to allow direct comparison to analytic predictions for DF in the range of Mach 0.2 ≤ M ≤ 3. We investigate, in detail, the DF drag force, the radial structure of the wake, and their time evolution. The subsonic forces are shown to be well resolved, except at Mach numbers close to M = 1. The supersonic cases, close to M = 1, significantly undershoot the predicted force. We find that for scenarios with 0.7 < M < 2, between 10% − 50% of the expected DF force is missing. The origin of this deficit is mostly related to the wake structure close to the perturber, where the density profile of the Mach cone face shows significant smoothing, which does not improve with time. The spherical expanding perturbation of the medium is captured well by the hydro-scheme, but it is the sharp density structure, at the transition from Mach cone to average density, that introduces the mismatch. However, we find a general improvement of the force deficit with time, though significant differences remain, in agreement with other studies. This is due to (1) the structure of the far field wake (𝑟 ≥ 4𝑟 𝑠 , with 𝑟 𝑠 as the gravitational smoothing scale) being reproduced well, and (2) the fraction of total drag from the far field wake increasing with time. Dark matter sub-haloes, in typical cosmological simulations, occupy parameters similar to those tested here, suggesting that the DF which these sub-haloes experience is significantly underestimated, and hence their merger rate. Dynamical friction is a relevant benchmark and should be included as one of the standard hydro test for astrophysical simulations, since it is critical to our understanding of many systems.

The impact of galaxy selection on the splashback boundaries of galaxy clusters

“…However, since more massive present-day subhaloes tend to come from more massive accreting subhaloes, we still expect any impact that dynamical friction has to increase with subhalo mass. An additional consideration is that dynamical friction is believed to be significantly underestimated in relatively low-resolution cosmological simulations like TNG-300 (van den Bosch & Ogiya 2018;Morton, Khochfar & Oñorbe 2021).…”

The importance of black hole repositioning for galaxy formation simulations

“…For SMBHs moving through a gaseous background, on the other hand, the hydrodynamic disturbance generated by their passage generates a gaseous wake, which similarly leads to a deceleration (e.g. Ostriker 1999;Beckmann et al 2018;Morton et al 2021), at least in the absence of feedback (as pointed out by e.g. Gruzinov et al 2020 andToyouchi et al 2020).…”