Tracing the localization of 4f electrons: Angle-resolved photoemission on YbCo2Si2, the stable trivalent counterpart of the heavy-fermion YbRh2Si2

${\mathrm{YbCo}}_{2}{\mathrm{Si}}_{2}$ is considered to serve as a stable-valent, isoelectronic reference for the extensively studied heavy-fermion system ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$ which is situated very close to an antiferromagnetic quantum critical point (QCP). The investigation of the Fermi surface (FS) topology of ${\mathrm{YbCo}}_{2}{\mathrm{Si}}_{2}$ and its comparison to ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$ could help to unravel the strongly disputed nature of this quantum phase transition, whether it originates from a ``local'' or ``itinerant'' QCP. Here we study the electronic structure and FS of ${\mathrm{YbCo}}_{2}{\mathrm{Si}}_{2}$ by means of angle-resolved photoelectron spectroscopy (ARPES) and compare it to ab initio band structure calculations and FS modeling. Our approach allows the electronic structure at the surface and in the bulk to be disentangled. Identifying the bulk contribution, we demonstrate that ${\mathrm{YbCo}}_{2}{\mathrm{Si}}_{2}$ exhibits a ``small'' FS, confirming the formation of a ``large'' FS in ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$. This favors an itinerant QCP, instead of the widely discussed local scenario. Our study demonstrates that ARPES is a reliable tool for the study of bulk electronic states in intermetallic Kondo lattice systems despite the complexity induced by their three-dimensional character and the presence of pronounced surface states.