Clusters of Galaxies

Cluster of Galaxies Characteristics
The Virgo, Coma, Perseus, and Phoenix Clusters
Cluster of Galaxies Classification
Cluster of Galaxies Formation
Gravitational Lens
Dark Matter

Cluster of Galaxies Characteristics

Long before the era of extragalactic astronomy it was recognized that the distribution of "fuzzy objects" in the sky is not random. Even the small sample of the 35 Messier objects now recognized as galaxies exhibits this nonrandomness; nearly half of these

		objects are in the vicinity of the Virgo Cluster. By the 1960s enough data have been collected to classify the clustering of the galaxies. It can be grouped into two categories - the regular and the irregular. Their properties are shown in Table 04-01 below. Figure 04-01a shows a typical cluster of galaxies at z = 1.10 with contours for X-ray emission. Figure 04-01b shows another cluster about 1 billion light year further away. The X-ray emission shown in purple reveal the hot intracluster gas. It is estimated that the composition of a cluster is 10% galaxies, 20% intracluster medium (gas), and 70% dark matter.
Figure 04-01a Cluster of Galaxies 1 [large image]	Figure 04-01b Cluster of Galaxies 2 [large image]

Table 04-01 Characteristics of Regular and Irregular Clusters

Property	Regular Clusters	Irregular Clusters
Symmetry	Marked spherical symmetry	Little or no symmetry
Concentration	High concentration of members toward cluster center	No marked concentration to a unique cluster center; often two or more nuclei of concentration are present
Collisions	Numerous collisions and close encounters	Collisions and close encounters are relatively rare
Types of galaxies	All or nearly all galaxies in the first 3 or 4 magnitude intervals are elliptical and/or S0 galaxies	All types of galaxies are usually present except in the poor groups, which may not contain giant ellipticals. Late-type spirals and/or irregular galaxies present
Number of galaxies	Order of 10³ or more	Order of 10 to 10³
Diameter (Mpc)	Order of 1 - 10	Order of 1 - 10
Subclustering	Probably absent or unimportant	Often present. Double and multiple systems of galaxies common
Radial velocities dispersion	Order of 10³ km/sec	Order of 10² - 10³ km/sec
Mass (from Virial Theorem)	Order of 10¹⁵ M_sun	Order of 10¹² - 10¹⁴ M_sun
Other characteristics	Cluster often centered about one or two giant elliptical galaxies
Examples	Coma cluster (A1656); Corona Borealis cluster (A2065)	Virgo cluster, Hercules cluster (A2151)

Only in the last two decades, astronomers are able to detect the X-ray component of the cluster of galaxies. It is now known that the cluster is usually dominated by a supermassive black hole with mass that ranges from a few million to hundreds of millions of solar mass (Figure 04-01c). The black hole blows out huge amounts of high-speed material that can drive the evolution of the entire cluster. This process can dictate events on much smaller scales, such as the growth of galaxies, and the temperature variation of the gas. The evolution of the central galaxy runs in cycle as shown in Figure 04-01d, and explained briefly below.

		1. Starting from a system of high temperature gas and a quiet supermassive black hole, the gas cools down and flows inward (called cooling flow) as it emits X-rays, which carry off a lot of energy. 2. Some of the gas in the cool flow condenses into stars that become part of the central galaxy, and some sinks all the way down to feed the supermassive black hole. In so doing, it creates an accretion disk and activates high-power jets. 3. The supermassive black hole in the center of
Figure 04-01c Cluster Structure [view large image]	Figure 04-01d Cluster Evolution [view large image]	galaxy is expected to spin up over time as they accrete gas. By the time the black hole has swallowed enough gas to double its mass, its outer

boundary (the event horizon) should be rotating at nearly the speed of light. The rapid whirling creates a pair of jet in opposite direction. The jets carry off about 1/4 the inflow material, and have two major components: a matter-dominated outflow that moves at 1/3 the speed of light, forming the outer sheath of the funnel, and an inner region along the axis of the funnel that contains a rarefied gas of extremely high energy particles. It is the inner region that carries much of the energy over long distance and creates the bubbles observed by radio and X-ray astronomy. Note that all the examples below (such as the Virgo, Coma, and Perseus clusters) feature either a jet or bubbles.
4. The jet deposits its energy into the gas in the surrounding space via a low pitch sound wave (~ 57 octaves below middle C) producing a web of ripple-like filamentary structures.
5. The heating of the gas greatly diminishes the cooling flow, if not shutting it off altogether.
6. By cutting or shutting down the cooling flow, the supermassive black hole chokes off its own supply of gas and gradually goes dormant. Then the jets fade away, leaving the cluster gas without a heat source. Millions of years later the hot gas in the central region of the cluster finally cools sufficiently to initiate a new cycle of growth for the galaxy and its supermassive black.