  We consider a rigid assembly of two active Brownian particles, forming an
active colloidal dimer, in a gradient of activity. We show analytically that
depending on the relative orientation of the two particles the active dimer
accumulates in regions of either high or low activity, corresponding to,
respectively, chemotaxis and antichemotaxis. Certain active dimers show both
chemotactic and antichemotactic behavior, depending on the strength of the
activity. Our coarse-grained Fokker-Planck approach yields an effective
potential, which we use to construct a nonequilibrium phase diagram that
classifies the dimers according to their tactic behavior. Moreover, we show
that for certain dimers a higher persistence of the motion is achieved similar
to the effect of a steering wheel in macroscopic devices. This work could be
useful for designing autonomous active colloidal structures which adjust their
motion depending on the local activity gradients.
