Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf

2.2. The thermal component 15
Figure 2.1: Panel (a): Chandra observation of the massive (regular) cluster of galaxies A 1689
(blue) superimposed on the optical image (yellow, Hubble Space Telescope). Panel (b): fitting of
the surface brightness profile with a doubleβ model described in the inset. Taken from Xue & Wu
(2002).
gas temperatures increasing with the size and the mass of the environment. Typical temperatures
for clusters, groups and individual galaxies are given in Table 2.1.
2.2.2 Morphology of the X-ray emitting gas
Imaging of the X-ray emission from massive ellipticals, groups and clusters of galaxies has shown
that the morphologies of the emitting gas are quite heterogeneous, but that they can be related to
the dynamical state of the systems. Much of work has been carried out for galaxy clusters, but the
conclusions can be extended to sparser environments such as groups of galaxies and field galaxies,
which can be regarded as scaled-down version of rich clusters.
Forman & Jones (1982) first proposed a classification into “regular” and “irregular” X-ray cluster
morphologies, with a connection to the evolutionary state.
Essentially, regular clusters are those which show approximately round, centrally condensed
X-ray brightness distributions, decreasing smoothly outwards. Their temperatures and X-ray
luminosities are usually high and they often host a central dominant galaxy. Galaxy clusters
belonging to this class are thought to be evolved systems which have undergone dynamical
relaxation.
When the X-ray emission can be approximated as regular, its surface brightness profile is well
parametrized by the function:
] −3β+1/2
S (r)=S (0)
[1+ r2
(2.2)
15
r 2 c