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
References
Index

Cluster of Galaxies 1 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^2,3 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.

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The Virgo 4, Coma⁵, Perseus⁶, and Phoenix Clusters

		Figure 04-02a shows the Virgo cluster as an example of the irregular cluster. It is the closest cluster of galaxies to the Milky Way. There are about 150 large galaxies of many types in this cluster and at least a thousand known dwarf galaxies. At the core of the Virgo cluster lie the three large elliptical galaxies M84 (center), M86 (upper right) and M87 (with the jet in Figure 04-02b). These galaxies probably formed from the merger of many smaller galaxies and are much more massive than our own galaxy. The cluster contains not only galaxies filled with stars but
Figure 04-02a The Virgo Cluster [view large image]	Figure 04-02b M87 [view large image]	also gas so hot it glows in X-rays. Motions of galaxies in and around clusters indicate that they contain more dark matter than any visible matter. The Virgo cluster is the dominant member within the Virgo supercluster. All the other members move around this center by its gravitational pull.

Figure 04-03 shows the Coma cluster (a member of the Coma supercluster) as an example of the regular cluster. It displays the central region with a dominant elliptical galaxy at the center. The X-ray⁷ emission from the hot and tenuous gas is illustrated in the picture on the right. The gas is thought to be produced by matter ejected from stars in the galaxies over a period of about a billion years and reaches a temperature of 10⁷ ^oC.

Figure 04-03 The Coma Cluster of Galaxies

In Figure 04-04a, each of those fuzzy blob is a galaxy, together making up the Perseus Cluster. The dominant galaxy is NGC1275 - the large galaxy on the image left. It is a prodigious source of x-rays and radio emission as gas and galaxies falling

		into its center. The x-ray hot gas (not the individual galaxies) appears in the left panel of Figure 04-04b, a false color image from the Chandra Observatory. The bright central source flanked by two dark cavities is the cluster's supermassive black hole. At right, the panel shows the x-ray image data specially processed to enhance contrasts and reveals a strikingly regular pattern of pressure waves
Figure 04-04a Perseus Cluster [view large image]	Figure 04-04b Hot Gas in Perseus Cluster [view large image]	rippling through the hot gas. In other words, sound waves, likely generated by bursts of activity from the black hole, are ringing through the Perseus Galaxy Cluster.

It was found by many world-class observatories that the Phoenix cluster at a distance of 5.7 lys from Earth is the most powerful X-ray source and the most massive among any known galactic clusters (Figure 04-04c). The X-ray is produced from gas cooling in the central region. Since the central black hole emits only moderate jets, gas cooling proceeds at a much faster pace (than the other clusters) causing star formation at a rate about 20 times faster than in the Perseus cluster.

Figure 04-04c Phoenix Cluster [view large image]

It is believed that this is only a short-lived phenomena. The black hole will become more active via the feeding of more material, and eventually the outflow of energy will reverse the trend.

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Cluster of Galaxies Classification

Several classification schemes have been developed and correlate reasonably well with one another. One of these is the Rood and Sastry (RS) classification, which is based on the projected distribution of the brightest 10 members. They recognize these types:

cD - single dominant cD (elliptical) galaxy (A2029, A2199)
B - dominant binary, like Coma
L - linear array of galaxies (Perseus)
C - single core of galaxies
F - flattened (IRAS 09104+4109)
I - irregular distribution (Hercules)

These appear to have affinities in a tuning-fork pattern with concentration generally increasing leftward as shown in Figure 04-05a. It conveys a strong impression that the different types are the same kind of system at different stages of development. The evolution sequence seems to be running from right to left. It is highly improbable that clusters of galaxies have been built up by chance encounters of galaxies in the general field. It just takes too long (longer than the age of the universe) to complete the process. Therefore, either that clusters are systems whose member galaxies became gravitationally bound at more or less the same time, or that the clusters represent condensations from pregalaxian material and that subcondensations within them became galaxies.

Figure 04-05a Cluster of Galaxies Classification

Numerous collisions and close encounters are believed to have taken place within clusters of the cD type, with the giant ellipticals having grown by absorbing smaller ones in successive acts of "galactic cannibalism." Figure 04-05b shows a comet-like tail of glowing gas, 200,000 light-years long, streams from galaxy C153 as it plunges through galaxy cluster Abell 2125 at

nearly 8 million kilometers per hour. Itself a member of the giant cluster of galaxies, C153 may once have been a spiral galaxy like the Milky Way. In this series of images, false-color composites of x-ray and optical data, zooms in on this galaxy's fate. A passage through the hot intracluster gas in the central regions of Abell 2125 is seen to be stripping C153 of its own star forming material and distorting its shape. As other galaxies in the cluster suffer a similar fate, the hot gas collecting in the cluster's core should become enriched in heavy elements. The violent spectacle was taking place about 3 billion lys from Earth and is thought to illustrate a common process in the cosmic evolution of large clusters of galaxies.

Figure 04-05b Cluster of Galaxies Evolution [view large image]

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Cluster of Galaxies Formation

One of the cluster formation scenarios suggests that a group of newly formed galaxies (protogalaxy) are expanding away from each other some one billion years or so after the Big Bang. This grouping represented a fluctuation above the average density of its surroundings. Its density would grow and would exert a gravitational pull on itself that is strong enough to counteract the expansion of the universe, and the expansion of the region slowed down. It reached a maximum radius when its density is about five times greater than the background density as shown in Figure 04-06a.

Figure 04-06a Cluster of Galaxies Formation [view large image]

At its radius of maximum expansion, the cloud of galaxies had only gravitational potential energy and no kinetic energy. Then it begins to collapse, it loses potential energy and gains kinetic energy in the form of random motions of the galaxies. Once the kinetic energy equals one-half of the (negative) potential energy, the cloud reached a stage of energy balance and there is a dynamic equilibrium (where the Virial Theorem is applicable). The gravitational field of the cloud no longer changes with time. The kinetic energy of the randomly moving galaxies acts like a pressure that balances the force of gravitational attraction, the result is to produce a stable cluster of galaxies.

Cosmological simulations show that the progenitors of present-day galactic clusters were the largest structures at redshift of about 6. These proto-clusters are characterized by local over-densities of massive galaxies, and extend over tens of mega-parsecs. Owing to the high mass densities and correspondingly high merger rates, extreme phenomena such as star-bursts and quasars occurred frequently in these regions. One such proto-clusters at z = 5.3 have been detected (in 2010) by combined observations in X-ray, optical, infrared, microwave, and radio of the electromagnetic spectrum.

Figure 04-06b Proto-cluster
[view large image]

Diagram (a) in Figure 04-06b shows the proto-cluster LBG among sundry nearby objects such as galaxies and stars. Diagram (b) in the same image is the blow-up of the core region. Analyse of these images and other data reveals some properties of this proto-cluster in remarkable agreement with computer simulations:

The green circle in Diagram (a) spans an area of about 2 Mpc radius. Members of the proto-cluster are marked in white circles. There are 8 of them around the core representing an over-density of 11-fold.
The galaxy with extreme star-burst is shown in Diagram (b) covering an area of about 0.865 Mpc. It is the optical counterpart of the sub-millimetre source.
A quasar (not shown) is detected (in X-ray and visible light) at a distance of 13 Mpc from the core. The associated black hole has a mass of about 10⁸ M_sun implying a stellar mass of about 10¹¹ M_sun for the entire system - among the most luminous and massive objects at this reshift.
The star formation rate of 1500 M_sun/year is estimated from the far-infrared luminosity of this system. It is more than 100 times the rate of an average galaxy at this redshift.
The 3 objects around the star-burst probably represent the progenitor of a massive central cluster galaxy (type cD) in later evolution. These objects are already within the radius of a typical local cD galaxy. However, the observed stellar mass in these galaxies is significantly less than the ~ 10¹¹ M_sun - 10¹² M_sun in a typical local cD galaxy, indicating that the majority of the stars have yet to form.
Actually, the galaxy spectra show absorption features of interstellar gas + young massive stars indicative of a stellar population less than 30 million years old.

Thus the observations show that this proto-cluster of massive galaxies extends over more than 13 Mpc and contains a luminous quasar, star-brust as well as a system rich in molecular gas. These massive galaxies place a lower limit of more than 4x10¹¹M_sun of dark and luminous matter in this region, in consistence with the cosmological simulations for the earliest galaxy clusters.

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Gravitational Lens^8,9,10

		Bending of light is one of the many applications under the theory of general relativity. The Sun deflects star light behind by a small fraction of 1.75" as confirmed by Sir Arthur Eddington's 1919 solar eclipse expeditions. Since the amount of deflection is proportional to the mass of the lensing object, it was suggested back in 1937 that entire galaxies, or even clusters of galaxies could act as giant magnifying lenses in space (Figure 04-07a). Observational difficulty prevented such detection until 1972 when identical quasar images were spotted at both radio and optical wavelength for QSO 0957, which is 5 billion light years behind the intervening giant elliptical galaxy YGKOW G1 (at a distance of 3.7 billion light years from
Figure 04-07a Gravita- tional Lens	Figure 04-07b QSO0957 + 561a, b [view large image]	here, see Figure 04-07b). Gravitational lensing can also be used to estimate the rate of cosmic expansion (via light variation from quasar), and it would yield information about dark matter if the bending cannot be accounted for by the visible (lensing) matter alone.

As shown in Figure 04-07a (illustration at the bottom) when light from a background source, such as a galaxy, passes through the gravitational field of a foreground cluster, it is also deflected by its gravitational field. If the alignment is right, an image of the back-ground object will be produced. The image would be larger and brighter than the object would appear without the "lens". If the source, lens, and observer are lined up along a straight line, a distant point of light will be spread out into a ring called an "Einstein ring", whereas if the alignment is imperfect the background source will be seen as two or more arc-shaped images. An example of this pheno-menon is shown in Figure 04-07c. This is the cluster A2218, which has produced more than 120 arc-shaped images of

Figure 04-07c Einstein Ring, Abell 2218;
the square covers the area in Figure 04-08

galaxies that are members of a remote cluster. Analyses of the lensing effects produced by clusters have confirmed that clusters of galaxies contain from ten to one hundred times as much dark matter as

		luminous matter. The gravitational lensing of Abell 2218 has revealed a galaxy with redshift of 6.6 - 7.1 corresponding to a look back time of about 13x10⁹ years (750 million years after the Big Bang or 5% the age of the universe). It is among the most distant objects ever observed as shown in Figure 04-08 in the form of a pair of faint red arcs (also see ERO - Extremely Red Object). A March, 2004 report indicates a galaxy still further away at a distance of 13.23x10⁹ lys with a size less than 300 lys across (see Figure 04-09).
Figure 04-08 Lensing [view large image]	Figure 04-09 Ancient Galaxy

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Dark Matter

By analyzing the distribution of luminous matter and the properties of the gravitational lensing due to total cluster mass in CL0025+1654, researchers have solved the problem of tracing the dark matter layout. Their resulting map shows the otherwise invisible dark matter in blue, and the positions of the cluster galaxies in yellow. The work, based on extensive Hubble Space Telescope observations, reveals that the cluster's dark matter is not evenly distributed, but follows the clumps of luminous matter closely as shown in Figure 04-10a. The finding bears not so much on whether the dark matter is self-interacting (in addition to gravity) but on the "bottom up" theory in which gravity is believed to have assembled increasingly large structures from small ones as the universe aged and expanded.

In Figure 04-10b the separation of dark matter (blue) from the X-ray emitting gas clouds (red) after the collision of a small and large clusters about 100 million years ago demonstrates that unlike the gas which feels electromagnetism as well as gravity, the dark matter clings to the clusters by gravity only; while the hot intra-cluster gas experiences an additional drag force that slows it down more. However a 2013 study of the collision in the Musket Ball cluster found that the galaxies are separated from its dark matter (Figure 04-10c in blue) by about 19,000 light years. Some dark matter was more in line with the gas (in red). The founding suggests that the dark matter does slow down by some sort of force other than gravity, i.e., it is interacting with itself. The different scenario

			from the Bullet cluster could be related to the timing of the events. While the Musket Ball collision occurred much earlier about 700 million years ago, the Bullet collision happened later so that the galaxies and
Figure 04-10a Dark Matter in CL0025+1654	Figure 04-10b Dark Matter in Bullet Cluster	Figure 04-10c Dark Matter in Musket Ball Cluster	dark matter do not have enough time to divorce.

Research in 2008 indicates that dark matter can produce a weak lensing effect to distort the image of distant galaxies. By noting the degree to which background galaxies appear unusually flat and unusually similar to neighbors, the dark matter distribution producing these weak gravitational lensing distortions can be estimated. Analysis of the shapes of 200,000 distant galaxies imaged does show the presence of a massive network of dark matter. Figure 04-10d is a computer-generated simulation of dark matter distribution. It shows the dark matter (in red) bending the light path from the apparent shape of distant galaxies (in blue) to a more flattened shape.

Figure 04-10d Dark Matter Lensing [view large image]

Figure 04-11 shows the computer simulations of the evolution of cold dark matter (upper sequence from left to right), in which ordinary gases cool, condense and fragment to make galaxies (in corresponding lower sequence). Each panel shows the projected cold dark matter or galaxy distribution in slices of thickness 15 h^-1Mpc, where h denotes the Hubble constant in units of 100 km s^-1Mpc^-1. For the cold dark matter, the color hue from blue to red encodes the local velocity dispersion, and the brightness of each

	pixel is a measurement of the density. In the galaxy evolution panels, each galaxy is weighted by its stellar mass, and the color scale of the images is proportional to the total stellar mass. The cold dark matter evolves from a smooth, nearly uniform distribution into a highly clustered state, quite unlike the galaxies, which are strongly clustered from the beginning.
Figure 04-11 Cold Dark Matter Evolution [view large image]

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References:

Cluster of Galaxies, A Review -- http://nedwww.ipac.caltech.edu/level5/Narlikar2/Nar1_6.html
Messier Objects, Blow-up Images and Description -- http://www.3towers.com/messier.htm
General, Interactive NGC Catalog -- http://www.seds.org/~spider/ngc/ngc.html
The Virgo Cluster -- http://www.seds.org/messier/more/virgo.html
The Coma Cluster -- http://antwrp.gsfc.nasa.gov/apod/ap020203.html
The Perseus Cluster -- http://antwrp.gsfc.nasa.gov/apod/ap000615.html
Cluster of Galaxies, X-ray Observations -- http://www-xray.ast.cam.ac.uk/oday/clusters_demo.html
Gravitational Lensing -- http://csep10.phys.utk.edu/astr162/lect/galaxies/lensing.html
Gravitational Lensing -- http://www.sr.bham.ac.uk/research/lens.html
Most Distance Galaxy -- http://www.eso.org/outreach/press-rel/pr-2004/pr-04-04.html