Galaxy Clusters - What Are They Made Up Of & Their Evolution

Galaxies gather together into large groups called "Galaxy Clusters" which are clumped together by their own gravity and are the largest objects in the universe . They comprise a lot of things and objects in huge numbers ⇒ hundreds or thousands of galaxies, lots of hot plasma, and a large amount of invisible dark matter. For example, the Perseus Cluster holds together more than a thousand galaxies and is one of the most shining sources of X-rays in the sky. Galaxy clusters are a domicile to the largest galaxies in the surveyed universe, and furnish the physicists with the information about the structure of the universe on the largest expanses.

The gas-dust and the stars in the galaxies clusters contributes only about 5% of its total mass. And besides, almost 80% of the mass of a galaxies cluster exists as the dark matter, and this dark matter is what clumps everything else together. The enormous amount of dark matter which the galaxy cluster is composed of generates enough gravity to curve the path of light traveling by. This curving which is called "gravitational lensing", turns or makes those clusters into gigantic telescopes, which would magnify the galaxies which would usually be too dim for us to observe. And besides, lensing allows the physicists to trace out where the mass is situated or located in a galaxy cluster.

Hot plasma which is nothing but the gas consisting of ionized atoms where the electrons are removed off from their atoms contributes almost 15% in the total mass of a galaxy cluster and it is the second most largest contributor for the galaxy cluster. This hot plasma occupies most of the empty-space between the galaxies. This gas resplends brilliantly in the X-ray light, and causes an effect on the radio light being emitted through the cluster from the more remote sources. So for this reason, the plasma allows for detecting and studying galaxy clusters in several different ways .

And finally there are the galaxies themselves, which may contribute the least or minimum mass to the galaxy cluster, but are however very crucial. The most gigantic galaxies in the universe are housed in the clusters, and the interactions of the galaxies with one another in a cluster are a reserch lab for comprehending the formation process of these enormous objects. And besides, the galaxies house-in supermassive black holes at their cores, and these black holes show a significant impact on the entire cluster of galaxies.


The Abell 370 is a galaxy cluster made up of several hundred galaxies bound together by mutual gravity. Even though the hot gas and dark matter in the cluster are not visible in this picture, its bulk acts as a gravitational lens, warping pictures of galaxies farther away into streaks of light.
Image credit: NASA, ESA, and J. Lotz and the HFF Team (STScI)

When the bigger samples of galaxies have been collected up, the tendency of galaxies to clump or cluster together has been proved undoubtedly. The UCLA astronomer George Abell produced the first ever modern catalog of galaxy clusters in 1958 and it consisted of 2,712 plentiful clusters of galaxies which were detected by the National Geographic Society-Palomar Observatory Sky Survey. If a cluster consists of more than fifty galaxies, then it is a rich cluster. The recent surveys have traced out the distribution of galaxies further out to distances of several billion light-years. The most detailed one of these is the Sloan Digital Sky Survey (SDSS). This survey has employed the dedicated 2.5-meter optical telescope of the Sloan Foundation located at Apache Point Observatory in New Mexico to produce in-depth, multicolor images extending to a purview of more than a quarter of the sky and to generate 3D (three-dimensional) maps comprising more than 1.2 million galaxies (see, for example, the figure SLOAN DIGITAL SKY SURVEY III MAP below ).


SLOAN DIGITAL SKY SURVEY III MAP
A portion of the three-dimensional map which the Sloan Digital Sky Survey III, which employed the Sloan Foundation's specialized 2.5-meter optical telescope at Apache Point Observatory in New Mexico created. One thousand square degrees of the sky are carved off in the rectangle on the left. The estimated 120,000 galaxies it contains make up around 10% of the entire survey. The optical spectrum measurements of each galaxy, each of which is represented by a dot in this cutout, allow us to glimpse seven billion years into the past by calculating the distance to each galaxy and converting the two-dimensional image into a three-dimensional map (in the center and right). The bright areas on this image indicate regions of the universe that have more galaxies and, therefore, greater mass, which exerts an excessive amount of gravitational pull.
Image credit: NASA

If we project these large-scale surveys of the universe into two dimensions, they look like road maps. Galaxies stay lined up in filaments that traverse or cut across intergalactic space like the superhighways. There are smaller roads linking up these superhighways and there are areas of low aggregation density between them: the cosmic countryside. There are clusters of galaxies (the cosmic megacities) at the junctures where multiple strands or filaments converge together (see the below figures).


ABELL 1689
This massive galaxy cluster is located around 2.3 billion light-years away from Earth and is depicted in a composite image created using data from two different telescopes. The color purple denotes gas in the cluster that the Chandra X-Ray observatory detected to be 100 million degrees Celsius. According to Hubble Space Telescope optical data, galaxies are shown by yellow.
Image credit: NASA


CL J1001+0220 GALAXY CLUSTER
A combination of this hot gas and galaxy concentration 11 billion light-years away from Earth. It is the youngest and furthest galaxy cluster ever found in X-rays. Chandra discovered X-rays (purple), the European Space Observatory's UltraVISTA survey identified infrared emission (red, green, and blue), and the Atacama Large Millimeter/Submillimeter Array (ALMA; green) detected radio emission.
Image credit: NASA


COMA GALAXY CLUSTER
An amalgamation of the Coma galaxy cluster, located approximately 321 million light-years away from Earth. The huge arms of hot gas in the cluster were highlighted in this image by processing; these arms most likely originated when gas was removed from smaller subclusters of galaxies when they merged with Coma. Sloan Digital Sky Survey optical data is shown in white and blue, whereas Chandra data is shown in pink. The picture spans almost two million light-years on either side.
Image credit: NASA

It is intimidating how big these clusters are. Light, which moves at a speed of 186,000 miles per second, takes eight minutes to get from the Sun to Earth and slightly more than a second to get from the Moon. Twenty-five thousand years must pass before light from the Milky Way galaxy's center reaches us. Even that is quick when you consider how long it takes light to travel across a galaxy cluster, which is roughly two million years.

The largest gravitationally bound bodies of the universe are the galaxy clusters. Despite their greater sheer size, the roadlike threads are not cohesive structures held together by gravity. Cosmic megacities, like Earthly metropolises, are built over billions of years from smaller clusters rather than starting as fully developed cities. Gravity is the primary factor behind the formation of galaxy clusters, in contrast to Earth, where economic pressures have a significant impact on the development of cities. Recently, astronomers have discovered that gravity can have unanticipated impacts on a cluster's evolution. The space between the galaxies in a cluster is not a vaccum but rather filled with microwave background radiation and diffuse gas left over as a remnant when galaxies got formed and this gas is heated to tens of millions of degrees by the gradual collapse of the cluster due to gravity. When gravity started to clump together the galaxies and the clouds in which they are embedded to constitue clusters around 11 or 12 billion years ago, the gradual collapse scalded the intergalactic gas up to tens of millions of degrees. This gigantic gas cloud comprises almost five times the total mass of all the stars in all the galaxies in the entire cluster, and this gas is tied up together by the gravity of an even more gigantic nebula of dark matter. Because the hot gas nebulae - which can not be detected by the optical telescopes - are bound to the clusters and emit away their energy extremely slowly, they can retain a record of much of the on-going proceses in the clusters across the last several billion years. The gas continues to keep up the energy as well as the elements interposed into it by past supernova explosions in the galaxies clusters.

The attributes of galaxy clusters position them at another junction: the passage from astrophysics to cosmology which is the study of the universe in a broad sense. The plentifulness and magnitudes of the clusters carry along the impressions and influences of the conditions in the primal (or primordial) background gas from which they bursted forth. Because of the massive repository of gas and the almost closed-box setting, much of what is going on in the cluster remains with in the cluster. This turns the galaxy clusters into ideal testing grounds or regions in which physicists can investigate both the processes running on or else passing off during the organization and evolution of galaxies and the supermassive black holes held with in them.

Analogously to archaeologists, who excavate the past by investigating the artifacts concealed underneath the ground, physicists are utilizing the Chandra and other orbiting X-ray telescopes to discover the rich accumulations of vestiges present in the galaxy clusters and to knit together their ancient past. Part of this ancient past is comparably contemporary, and it informs us about the development of the most gigantic objects in the universe: supermassive black holes enclosed with in the giant galaxies in the clusters. But some of this past history traces back a very long way, much further back than 10 billion years, to the origin of the universe.

Most galaxies are not solitary in the enormous extent of space, but are colligated together to one or more other galaxies by gravity. The very same force that confines us onto the Earth can uphold many separate galaxies connected together. These galaxy clusters can be small like in the case of two galaxies revolving around each other, or large enough such as the rich Coma cluster consisting of thousands of galaxies spreading over for more than ten million light years. These are the most gigantic objects in the known Universe, and there are many attributes that make them great astrophysical research grounds. For example,

→ Clusters undergo changes extremely slowly (it takes up as long as around the age of the Universe for substantial changes to happen in the clusters), thus clusters sustain relics of their formation. This turns them up into a good "trace out" of the history of structure and formation of galaxies.

→ Clusters have the tendency to maintain a holding onto the gas in their systems, unlike galaxies, where the gas is ejected out from supernova explosions. To state it otherwise, clusters are shut-in systems. By investigating the chemical composition of the elements in the clusters, physicists can absolutely get a history of nucleosynthesis in the Universe.

→ The gravitational force that confines clusters together araises for the most part from dark matter, marking the clusters as an excellently suitable object to research the dark matter in the Universe.

What is surprising is that the most perceptible or seeable part of galaxy clusters, that is all of the stars in all of the galaxies that constitute the cluster, is only a small fraction of the total mass of the cluster, and is likely the slightest intriguing part of the cluster. For example, physicists survey the X-ray emission from galaxy clusters. The X-rays are emitted from the hot (10 to 100 million degrees) gas confined by the gravitational force of the cluster. This gas composes a much bigger part of the total mass of the cluster than the stars, but is entirely imperceptible to our eyes!

Clusters are constituted of two fundamental kinds of matter: lambent matter (like stars and hot gas) and dark matter. Dark matter does not have the tendency to luminesce on its own, and the only manner in which we can detect its presence is by its gravitational affect on lambent matter. If we have to calculate what is the amount of dark matter that exists in the entire Universe, we need to investigate something that is symbolic and typical of the entire Universe. Something big, that is. Clusters of galaxies are the most gigantic objects known in the Universe, which are observed to be big enough that they comprise the same amount of dark matter as does the entire Universe. One of the areas of the research in clusters is directed at using X-ray observations to comprehend how much lambent matter and dark matter is resident in the clusters.


X-ray image of Virgo Cluster
Image credit: NASA


Visible image of virgo cluster
Image credit: NASA

Much of the lambent matter in the clusters is inherently present in the form of hot gas most likely in between the galaxies. The gas, which has been heated up to a temperature of 10 to 100 million degrees, emits away X-rays. What is the amount of the hot gas in a cluster is plainly colligated to the total X-ray luminance that can be detected from the cluster. In this way, physicists can gauge the amount of the lambent matter directly from the X-ray observations of the galaxies clusters. Please take a look at the above images of Virgo cluster - one taken with visible light spectrum and the other with X-Rays spectrum.

What is the amount of the dark matter in a galaxy cluster, however, has to be derived from the detectable, lambent matter. It is manageable to do because the galaxy clusters are "relaxed" organizations, that means, there exists a balance between the dark matter and the pressure exerted by the cluster. The pressure from the cluster is colligated to the X-ray emitting gas (which can be observed), so, by presuming a dynamic balance between the two, physicists can account for the amount of the dark matter.

Chemical Copiousness in Clusters: Other Cogent Evidence

By making use of the X-ray spectra , scientists can determine what types of elements the gas-dust resident in between the galaxies consists of. The results from these investigations can then be utilized to test the cluster models and their evolution further.

The lambent matter in a cluster is not entirely unvarying . Some part of it is primordial hydrogen and helium generated during the Big Bang; some other part of it is the heavier elements such as the oxygen, magnesium, silicon, neon and sulfur. These subsequent elements were produced by the stars with fusion processes, or in supernova explosions. Theoretical models that can explicate how these elements were released out of the stars that are resident inside the galaxies in the clusters and into the gas-dust occupying the space between the galaxies in the cluster might also explicate why there is relatively more dark matter in some of the clusters than in others.