  We study the electronic properties of InAs/EuS/Al heterostructures as
explored in a recent experiment [S. Vaitiekenas \emph{et al.}, Nat. Phys.
(2020)], combining both spectroscopic results and microscopic device
simulations. In particular, we use angle-resolved photoemission spectroscopy to
investigate the band bending at the InAs/EuS interface. The resulting band
offset value serves as an essential input to subsequent microscopic device
simulations, allowing us to map the electronic wave function distribution. We
conclude that the magnetic proximity effects at the Al/EuS as well as the
InAs/EuS interfaces are both essential to achieve topological superconductivity
at zero applied magnetic field. Mapping the topological phase diagram as a
function of gate voltages and proximity-induced exchange couplings, we show
that the ferromagnetic hybrid nanowire with overlapping Al and EuS layers can
become a topological superconductor within realistic parameter regimes, and
that the topological phase can be optimized by external gating. Our work
highlights the need for a combined experimental and theoretical effort for
faithful device simulation.
