Gaseous Dynamical Friction: A Challenge to Modern Hydrodynamical Schemes
  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 under-shoot 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 differencesremain, in agreement
with other studies. This is due to (1) the structure of the far field wake
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 tests for astrophysical simulations.
