Density Profile of a Cool Core of Galaxy Clusters
  The density profile of a cool core of intracluster gas is investigated for a
cluster of galaxies that is initially in the virial equilibrium state, and then
undergoes radiative cooling. The initial gas profile is derived under the
assumption that the gas is hydrostatic within the dark-matter potential
presented by the NFW or King model, and has a polytropic profile. The
contribution of masses of gas and galaxies to the potential in the calculation
is ignored compared to the dark matter. The temperature and density profiles of
gas in its quasi-hydrostatic cooling phase, which is expected to last for ~Gyr,
is then calculated for different initial gas profiles. It is found that in the
quasi-hydrostatic cooling phase, while the temperature decreases to about 1/3,
the density increases by a factor of 4-6 at the cluster center in comparison
with their initial polytropic values, though the profiles over the core depend
on the dark-matter potential. Hence, the core radius in the quasi-hydrostatic
cooling gas appears to be smaller than that in the initial polytropic gas. We
compared the density profile of the cool core with observations to find that,
while the initial density is around the upper bounds of large-core (>100 kpc)
clusters, most likely relaxed, but the cooling is not yet significant, the
central density under quasi-hydrostatic cooling falls between the mid- and
high-values of small (<100 kpc)- or cool-core clusters. It is also found for
the quasi-hydrostatic cooling gas that the entropy profile roughly agrees with
the best-fit model to the ACCEPT sample with a low central entropy; also, the
pressure gradient in the inner core is close to that of the REXCESS sample.
X-ray surface brightness calculated for the quasi-hydrostatic cooling gas is
well represented by the conventional double beta-model, giving a physical basis
for applying the double beta-model to cool-core clusters.
