Cluster components

Clusters of galaxies represent the largest quasi–virialized systems in the Universe. They typically contain 102–103 galaxies in a region of about 1–3 Mpc, with a total mass of_∼1014_M

¯ forrich groups to∼1015M¯ forrich clusters. First observations

showed that these structures are associated with deep gravitational potential wells containing galaxies with a typical velocity dispersion along the line–of–sight ofσv ∼

103_{km s}−1_{. The crossing time for a cluster of size R can be defined as:} tcr = R σv = 1 µ R 1 Mpc ¶ ³ σ_v 103_{km s}−1 ´−1 Gyr

1.6 Observational results

Therefore, in a Hubble time (_≈10h−1_{Gyr), the system has enough time in its internal} region (.1h−1Mpc) to dynamically relax. This condition cannot be attained in the surrounding environment. Assuming virial equilibrium, the typical cluster mass is:

M _' Rσ 2 v G ' µ R 1h−1_Mpc ¶_³ σv 103_{km s}−1 ´2 1015h−1M_¯

A number of different cluster properties have been traditionally used in order to con- struct a morphological classification of these systems. The Bautz & Morgan (1970) system, for example, is based on the degree to which the cluster is dominated by the brightest galaxies within it. In a similar manner, but with a finer classification, Rood & Sastry (1971) introduce a classification system that is based on the nature and distribution of the ten brightest cluster galaxies. Another commonly used classi- fication system was first introduced by Oemler (1974), who simply classified clusters on the basis of their morphological content. Specifically, he defined a cluster asspiral rich if the spirals represent the dominant population, asspiral poor if the fraction of lenticulars is higher than the corresponding fraction of spirals, and as acD cluster if it is dominated by a central cD galaxy with ellipticals representing the majority of the population.

Somewhat surprisingly, these different classifications appear to be highly corre- lated, with the result that clusters can be represented very crudely using a one– dimensional sequence fromregular toirregular. Regular cluster are highly symmetric in shape with a high concentration of galaxies towards the centre. Moreover subclus- tering, is absent or weak in regular clusters. In contrast, irregular clusters have little symmetry or central concentration and usually show significant subclustering.

It is important to note that there is no one–to–one correspondence between the regularity of a cluster and its richness, related to the number of galaxies associated with that cluster. Regular clusters are usually rich, while irregular clusters may be either rich or sparse.

In Fig. 1.5, I show our two closest clusters: the Coma cluster (on the top), a rich cluster with mass _∼ 1015_M

¯ containing thousands of galaxies, and Virgo (on the

bottom), a relatively sparse and irregular cluster with mass_∼1014M_¯.

The galaxy population in clusters exhibits a remarkablemorphology–density rela- tion (Dressler 1980; Whitmore et al. 1993), with an increasing space density of early type galaxies towards the cluster centre. Which physical processes are responsible for establishing the observed morphological mix is still a matter of debate and it is possible that both global cluster properties and the local density environment may play a role.

Galaxies in clusters also exhibit a luminosity segregation (with more luminous galaxies being more centrally concentrated). This is however limited to only the very bright galaxies (Stein 1997; Biviano et al. 2002). Cluster galaxies also exhibit a

Figure 1.5: The Coma cluster (top panel) and the Virgo cluster (bottom panel).

kinematical segregation (Adami et al. 1998), with late type galaxies having a higher velocity dispersion than early type galaxies.

Many observational results suggest that the stellar population of galaxies in cluster cores are generally old, with most of the stars formed at redshifts larger than 2. These conclusions usually rely on the argument that recent episodes of star formation would produce an excessive scatter in observed colour–magnitude relation or the fundamen-

1.6 Observational results

Figure 1.6: ROSAT image of the Coma cluster.

tal plane of elliptical galaxies, which are both very tight (van Dokkum & Franx 1996; Kodama et al. 1998). I will come back to this topic in Chapter 6, where I will study the colour magnitude relation in clusters at redshift_∼0.8.

Early X–ray observations (Meekins et al. 1971; Gursky et al. 1971) showed that clusters of galaxies contain a hot plasma, with a typical temperature of 2–14 keV and a central density of _∼ 10−3 electrons cm−3. The hot plasma detected through the luminous X-ray emission, is produced by thermal bremsstrahlung radiation and has a net luminosity of _∼ 1043−45 _{erg sec}−1_{. Fig. 1.6 shows the X–ray map of the} Coma cluster taken with ROSAT.

The total mass of the hot gas in a cluster is generally larger than the total mass in stars, suggesting that at least part of this gas has a cosmological origin. On the other hand, the detection of line emission from iron suggests that a substantial portion of this gas must have been ejected from galaxies into the intra–cluster medium,

although the relative importance of different physical mechanisms that can provide viable explanations for this transfer of mass (and metals), is still a matter of debate (see Chapter 4).