Why does the gas held in a cluster of galaxies

From: cluster of galaxies in A Dictionary of Astronomy ». Subjects: Science and technology — Astronomy and Cosmology. View all related items in Oxford Reference ». Search for: 'cluster of galaxies' in Oxford Reference ».

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Most stars do not emit much X-ray radiation, and neither does most of the gas or dust between the stars inside galaxies. What could be emitting the X-rays seen from virtually all massive galaxy clusters? It turns out that just as galaxies have gas distributed between their stars, clusters of galaxies have gas distributed between their galaxies. As they move and bump against each other, the gas heats up hotter and hotter until, at temperatures as high as million K, it shines brightly at X-ray wavelengths.

The more mass the cluster has, the faster the motions, the hotter the gas, and the brighter the X-rays. Astronomers calculate that the mass present to induce those motions must be about ten times the mass they can see in the clusters, including all the galaxies and all the gas.

Once again, this is evidence that the galaxy clusters are seen to be dominated by dark matter. This composite image shows the galaxy cluster Abell at a distance of 2. The finely detailed views of the galaxies, most of them yellow, are in visible and near-infrared light from the Hubble Space Telescope, while the diffuse purple haze shows X-rays as seen by Chandra X-ray Observatory. The abundant X-rays, the gravitationally lensed images thin curving arcs of background galaxies, and the measured velocities of galaxies in the clusters all show that the total mass of Abell —most of it dark matter—is about solar masses.

We described the use of the mass-to-light ratio to characterize the matter in galaxies or clusters of galaxies in Properties of Galaxies.

A mass-to-light ratio of or more is a signal that a substantial amount of dark matter is present. Table 1 summarizes the results of measurements of mass-to-light ratios for various classes of objects. Very large mass-to-light ratios are found for all systems of galaxy size and larger, indicating that dark matter is present in all these types of objects. This is why we say that dark matter apparently makes up most of the total mass of the universe.

The clustering of galaxies can be used to derive the total amount of mass in a given region of space, while visible radiation is a good indicator of where the luminous mass is. Studies show that the dark matter and luminous matter are very closely associated.

The dark matter halos do extend beyond the luminous boundaries of the galaxies that they surround. However, where there are large clusters of galaxies, you will also find large amounts of dark matter. Voids in the galaxy distribution are also voids in the distribution of dark matter.

How do we go about figuring out what the dark matter consists of? The technique we might use depends on its composition. Suppose these particles were assembled into black holes, brown dwarfs, or white dwarfs. If the black holes had no accretion disks, they would be invisible to us. White and brown dwarfs do emit some radiation but have such low luminosities that they cannot be seen at distances greater than a few thousand light-years.

We can, however, look for such compact objects because they can act as gravitational lenses. Suppose the dark matter in the halo of the Milky Way were made up of black holes, brown dwarfs, and white dwarfs. If an invisible MACHO passes directly between a distant star and Earth, it acts as a gravitational lens, focusing the light from the distant star. This causes the star to appear to brighten over a time interval of a few hours to several days before returning to its normal brightness.

Research teams making observations of millions of stars in the nearby galaxy called the Large Magellanic Cloud have reported several examples of the type of brightening expected if MACHOs are present in the halo of the Milky Way Figure 4.

Figure 4: Large and Small Magellanic Clouds. You can see from the number of stars that are visible that this is a very dark site for doing astronomy. Without star formation in the cloud there are no young, massive, luminous stars to do this. No other source seems to be nearby, either. Maybe a nearby galaxy, its central black hole sending out high-energy radiation …?

No one knows for sure. Maybe this one just happens to be the brightest of a collection of more common objects that are too dim to spot. Hopefully more of these can be found to help us understand them… or that this really is rare and deeper observations are made. The intracluster medium controls how stars form inside galaxies in the cluster, and how the galaxies themselves behave on large scales.

Sign Up For Free to View. Planetary nebulae have nothing to do with planet formation, but get their name because through a small telescope they look like planetary discs. They appear during the final stage in the life of a Sun-like star. The outer layer of the star is expelled and a white-dwarf star remains at the centre of the nebula. IOP Publishing Jobs. Sign in Register. Enter e-mail address Show Enter password Remember me. Enter e-mail address This e-mail address will be used to create your account.

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