  We introduce the public version of the cosmological magnetohydrodynamical
moving-mesh simulation code Arepo. This version contains a finite-volume
magnetohydrodynamics algorithm on an unstructured, dynamic Voronoi tessellation
coupled to a tree-particle-mesh algorithm for the Poisson equation either on a
Newtonian or cosmologically expanding spacetime. Time-integration is performed
adopting local timestep constraints for each cell individually, solving the
fluxes only across active interfaces, and calculating gravitational forces only
between active particles, using an operator-splitting approach. This allows
simulations with high dynamic range to be performed efficiently. Arepo is a
massively distributed-memory parallel code, using the Message Passing Interface
(MPI) communication standard and employing a dynamical work-load and memory
balancing scheme to allow optimal use of multi-node parallel computers. The
employed parallelization algorithms of Arepo are deterministic and produce
binary-identical results when re-run on the same machine and with the same
number of MPI ranks. A simple primordial cooling and star formation model is
included as an example of sub-resolution models commonly used in simulations of
galaxy formation. Arepo also contains a suite of computationally inexpensive
test problems, ranging from idealized tests for automated code verification to
scaled-down versions of cosmological galaxy formation simulations, and is
extensively documented in order to assist adoption of the code by new
scientific users.
