and Cosmology

6.6 Evolutionary Effects
Fig. 6.45. The cluster of galaxies
MS1054−03, observed with the HST, is the
most distant cluster in the EMSS X-ray survey
(z = 0.83). The reddish galaxies in the
image on the left form a nearly linear structure.
This cluster is far from being spherical,
as we have also seen from its weak lensing
results (Fig. 6.38) – it is not relaxed.
The smaller images on the right show blowups
of selected cluster fields where mergers
of galaxies become visible: in this cluster,
the merging of galaxies is directly observable.
At least six of the nine merging pairs
found in this cluster have been shown to be
gravitationally bound systems
273
Fig. 6.46. The cluster of galaxies XMMU J2235.2−2557 was
discovered in the field-of-view of an XMM-Newton image for
which a different source was the original target. The image on
the left shows the X-ray contours, superposed on an R-band
image, while the image on the right shows the central section,
here superposed on a K-band image. Galaxies in the field
follow a red cluster sequence if the color is measured in R − z.
The symbols denote galaxies at redshift 1.37 < z < 1.40. The
strong X-ray source to the upper right of the cluster center is
a Seyfert galaxy at lower redshift. As of 2005, this cluster is the
most distant X-ray selected cluster known, with a temperature
of ∼ 6 keV and a velocity dispersion of σ ∼ 750 km/s
shifted towards bluer colors at higher redshifts – there,
the age of the Universe was smaller, and thus the stellar
population was younger. This effect is of particular
importance at high redshifts. The fact that the color–
magnitude diagram of early-type galaxies in clusters
is not flat, in that more luminous galaxies are redder,
follows from the dependence of galaxy colors on the
metallicity of their stellar populations. The higher the