Numerical Study of Cosmic Ray Confinement through Dust Resonant Drag
  Instabilities
  We investigate the possibility of cosmic ray (CR) confinement by charged dust
grains through resonant drag instabilities (RDIs). We perform
magnetohydrodynamic particle-in-cell simulations of magnetized gas mixed with
charged dust and cosmic rays, with the gyro-radii of dust and GeV CRs on
$\sim\mathrm{AU}$ scales fully resolved. As a first study, we focus on one type
of RDI wherein charged grains drift super-Alfv{\'e}nically, with Lorentz forces
strongly dominating over drag forces. Dust grains are unstable to the RDIs and
form concentrated columns and sheets, whose scale grows until saturating at the
simulation box size. Initially perfectly-streaming CRs are strongly scattered
by RDI-excited Alfv{\'e}n waves, with the growth rate of the CR perpendicular
velocity components equaling the growth rate of magnetic field perturbations.
These rates are well-predicted by analytic linear theory. CRs finally become
isotropized and drift at least at $\sim v_\mathrm{A}$ by unidirectional
Alfv\'{e}n waves excited by the RDIs, with a uniform distribution of the pitch
angle cosine $\mu$ and a flat profile of the CR pitch angle diffusion
coefficient $D_{\mu\mu}$ around $\mu = 0$, without the "$90$ degree pitch angle
problem." With CR feedback on the gas included, $D_{\mu\mu}$ decreases by a
factor of a few, indicating a lower CR scattering rate, because the
backreaction on the RDI from the CR pressure adds extra wave damping, leading
to lower quasi-steady-state scattering rates. Our study demonstrates that the
dust-induced CR confinement can be very important under certain conditions,
e.g., the dusty circumgalactic medium around quasars or superluminous galaxies.
