  It is widely believed that the global baryon content and mass-to-light ratio
of groups and clusters of galaxies are a fair representative of the matter mix
of the universe and therefore, can be used to reliably determine the cosmic
mass density parameter Omega_M. However, this fundamental assumption is
challenged by growing evidence from optical and X-ray observations that the
average gas mass fraction and mass-to-light ratio increase mildly with scale
from poor groups to rich clusters. Although a number of time-consuming
hydrodynamical simulations combined with semi-analytic approaches have been
carried out, which permit a sophisticated treatment of some complicated
processes in the formation and evolution of cosmic structures, the essential
physics behind the phenomenon still remains a subject of intense debate. In
this Letter, using a simple analytic model, we show that radiative cooling of
the hot intragroup/intracluster gas may allow one to reproduce the observed
scale-dependence of the global stellar and gas mass fractions and mass-to-light
ratio of groups and clusters, provided that about half of the cooled gas is
converted into stars. Together with the recent success in the recovery of the
entropy excess and the steepening of the X-ray luminosity-temperature relations
detected in groups and clusters, radiative cooling provides a simple, unified
scheme for the evolution of hot gas and the formation of stars in the largest
virialized systems of the universe.
