  Recent radio VLBI observations of the relativistic jet in M87 radio galaxy
have shown a triple-ridge structure that consists of the conventional
limb-brightened feature and a central narrow ridge. Motivated by these
observations, we examine a steady axisymmetric force-free model of a jet driven
by the central black hole (BH) with its electromagnetic structure being
consistent with general relativistic magnetohydrodynamic simulations, and find
that it can produce triple-ridge images even if we assume a simple Gaussian
distribution of emitting electrons at the base of the jet. We show that the
fluid velocity field associated with the electromagnetic field produces the
central ridge component due to the relativistic beaming effect, while the
limb-brightened feature arises due to strong magnetic field around the jet edge
which also induces the electrons to be dense there. We also show that the
computed image strongly depends on the electromagnetic field structure, viewing
angle, and parameters related to the electrons' spatial distribution at the jet
base. This study will help constraining the non-thermal electron injection
mechanism of BH jets and be complementary to theoretical analyses of the
upcoming data of Event Horizon Telescope.
