  We study the scaling relations between the baryonic content and total mass of
groups of galaxies, as these systems provide a unique way to examine the role
of non-gravitational processes in structure formation. Using Planck and ROSAT
data, we conduct detailed comparisons of the stacked thermal Sunyaev-Zel'dovich
(tSZ) and X-ray scaling relations of galaxy groups found in the Galaxy And Mass
Assembly (GAMA) survey and the BAHAMAS hydrodynamical simulation. We use weak
gravitational lensing data from the Kilo Degree Survey (KiDS) to determine the
average halo mass of the studied systems. We analyse the simulation in the same
way, using realistic weak lensing, X-ray, and tSZ synthetic observations.
Furthermore, to keep selection biases under control, we employ exactly the same
galaxy selection and group identification procedures to the observations and
simulation. Applying this comparison, we find that the simulations reproduce
the richness, size, and stellar mass functions of GAMA groups, as well as the
stacked weak lensing and tSZ signals in bins of group stellar mass. However,
the simulations predict X-ray luminosities that are higher than observed for
this optically-selected group sample. As the same simulations were previously
shown to match the luminosities of X-ray-selected groups, this suggests that
X-ray-selected systems may form a biased subset. Finally, we demonstrate that
our observational processing of the X-ray and tSZ signals is free of
significant biases. We find that our optical group selection procedure has,
however, some room for improvement.
