Systematic derivation of coarse-grained fluctuating hydrodynamic equations for many Brownian particles under nonequilibrium conditions.

We study the statistical properties of many Brownian particles under the influence of both a spatially homogeneous driving force and a periodic potential with period l in a two-dimensional space. In particular, we focus on two asymptotic cases lint<<l and lint>>l , where lint represents the interaction length between two particles. We derive fluctuating hydrodynamic equations describing the evolution of a coarse-grained density field defined on scales much larger than l for both cases. Using the obtained equations, we calculate the equal-time correlation functions of the density field to the lowest order of the interaction strength. We find that the system exhibits long-range correlation of the type r-d (d=2) for the case lint>>l , while no such behavior is observed for the case lint<<l .