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7. Cosmology II: Inhomogeneities in the Universe
(and clusters), thus contributing to the density, but also
these baryons have modified the density profile of dark
matter halos in the course of cosmic evolution. Baryons
are dissipative, they can cool, form a disk, and accrete
inwards. The change in the resulting density distribution
of baryons by dissipative processes cause a change of the
gravitational potential over time, to which dark matter
also reacts. The dark matter profile in real galaxies is
thus modified compared to pure dark matter simulations.
Despite these difficulties, it has been found that the
X-ray data of many clusters are compatible with an
NFW profile. Analyses based on the weak lensing effect
also show that an NFW mass profile provides a very
good description for shear data. In Fig. 7.16 it is shown
that the radial profile of the galaxy density in clusters
on average follows an NFW profile, where the mean
concentration index is c ≈ 3, i.e., smaller than expected
for the mass profile of clusters. One interpretation of this
result is that the galaxy distribution in clusters is less
strongly concentrated than the density of dark matter.
7.5.5 The Substructure Problem
As we will discuss in detail in the next chapter, the
CDM model of cosmology has proven to be enormously
successful in describing and predicting cosmological
observations. Because this model has achieved this success
and is therefore considered the standard model,
results that apparently do not fit into the standard model
are of particular interest. The rotation curves of LSB
galaxies mentioned above are one such result. Either
one finds a good reason for this apparent discrepancy
between observation and the predictions of the
CDM model or, otherwise, results of this kind indicate
the necessity to introduce extensions to the CDM
model. In the former case, the model would have overcome
another hurdle in demonstrating its validity and
would be confirmed even further, whereas in the latter
case, new insights would be gained into the physics of
cosmology.
Sub-Halos of Galaxies and Clusters of Galaxies.
Besides the rotation curves of LSB galaxies, there is
another observation that does not seem to fit into the
picture of the CDM model at first sight. Numerical
simulations of structure formation show that a halo of
mass M contains numerous halos of much lower mass,
Fig. 7.16. The galaxy distribution averaged over 93 nearby
clusters of galaxies, as a function of the projected distance
to the cluster center. Galaxies have been selected in the NIR,
and cluster masses, and thus r 200 , have been determined from
X-ray data. Plotted is the projected number density of cluster
galaxies, averaged over the various clusters, versus the scaled
radius r/r 200 . In the top panel the galaxy sample is split into
luminous and less luminous galaxies, while in the bottom
panel the cluster sample is split according to the cluster mass.
The solid curves show a fit of the projected NFW profile,
which turns out to be an excellent description in all cases. The
concentration index is, with c ≈ 3, roughly the same in all
cases, and smaller than expected for the mass profile of clusters
so-called sub-halos. For instance, a halo with the mass
of a galaxy cluster contains hundreds or even thousands
of halos with masses that are orders of magnitude lower.