  We use high resolution angle-resolved photoemission to study the electronic
structure of the iron based high-temperature superconductors
Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ as a function of Ru concentration. We find that
substitution of Ru for Fe is isoelectronic, i. e., it does not change the value
of the chemical potential. More interestingly, there are no measured,
significant changes in the shape of the Fermi surface or in the Fermi velocity
over a wide range of substitution levels ($0<x<0.55$). Given that the
suppression of the antiferromagnetic and structural phase has been associated
with the emergence of the superconducting state, Ru substitution must achieve
this via a mechanism that does not involve changes of the Fermi surface. We
speculate that this mechanism relies on magnetic dilution which leads to the
reduction of the effective Stoner enhancement.
