  We investigate the steady state properties of an active fluid modeled as an
assembly of soft repulsive spheres subjected to Gaussian colored noise. Such a
noise captures one of the salient aspects of active particles, namely the
persistence of their motion and determines a variety of novel features with
respect to familiar passive fluids. We show that within the so-called
multidimensional unified colored noise approximation, recently introduced in
the field of active matter, the model can be treated by methods similar to
those employed in the study of standard molecular fluids. The system shows a
tendency of the particles to aggregate even in the presence of purely repulsive
forces because the combined action of colored noise and interactions enhances
the the effective friction between nearby particles. We also discuss whether an
effective two-body potential approach, which would allow to employ methods
similar to those of density functional theory, is appropriate. The limits of
such an approximation are discussed.
