and Cosmology

6.4 Scaling Relations for Clusters of Galaxies
the formation and evolution of the gas component in
clusters of galaxies.
Despite this discrepancy between the simple models
and the observations, Fig. 6.28 shows a clear correlation
between mass and luminosity, which can thus empirically
be used after having been calibrated. Although
the temperature is the preferred measure for a cluster’s
mass, one will in many cases resort to the relation between
mass and X-ray luminosity because determining
the luminosity (in a fixed energy range) is considerably
simpler than measuring the temperature, for which
significantly longer exposure times are required.
259
Fig. 6.28. For the galaxy clusters in the extended HIFLUGCS
sample, the X-ray luminosity in the energy range of the
ROSAT satellite is plotted versus the mass of the cluster.
The solid points show the clusters of the HIFLUGCS sample
proper. For the full sample and for the main HIFLUGCS
sample, a best-fit power law is indicated by the solid line and
dashed line, respectively
ature of the intergalactic gas is a better mass indicator
than the X-ray luminosity or the velocity dispersion of
the cluster galaxies.
However, determining the slope of the relation from
the data approximately yields L