It is now clear that galaxies also contain large amounts of dark matter. In fact, there is far more dark matter than we can see, meaning it would be foolish to ignore the effects of this invisible material in our theories about the structure of (as many captains of ships discovered in the polar seas) too late, the on The part of the iceberg visible on the surface of the sea was not necessarily the only part to look out for). It turns out that dark matter extremely important to the evolution of galaxies and the universe as a whole.
The idea that much of the universe is filled with dark matter may seem like a strange concept, but we can cite a historical example of “dark matter” much closer. Measurements in the middle of the 19th century showed that the planet Uranus does not exactly follow the orbit predicted by Newton’s laws if one adds the gravitational forces of all known objects in the solar system. Some people were concerned that Newton’s laws just didn’t work that far in our solar system. But the simplest interpretation was to attribute Uranus’ deviations in orbit to the gravitational effects of a new planet that had not yet been seen. The calculations showed where this planet must be, and Neptune was discovered almost at the predicted location.
In the same way, astronomers now routinely determine the location and amount of dark matter in galaxies by measuring its gravitational effects on objects we can see. And by measuring the movement of galaxies, the scientists found that dark matter also affects the evolution of galaxies. Since the environment around a galaxy is important to its evolution, dark matter must also play a central role in the evolution of galaxies. In fact, dark matter seems to make up most of the matter in the universe. But what is dark matter? What is it made of?Next, we consider the search for dark matter and determine its nature.
Dark Matter in the Local Neighborhood
Is there dark matter in our own solar system? Astronomers have studied the orbits of known planets and spaceships as they travel to the outer planets and beyond. Due to the already discovered object masses of our solar system and the theory of gravity, no deviations from the predicted orbits were found. From this we conclude that there is no evidence that large amounts of dark matter nearby.
Astronomers have also searched for evidence of dark matter in the Milky Way Galaxy within a few hundred light years of the sun. In this neighborhood, most of the stars are confined to a thin disk. It can be calculated how much mass the disk must contain so that the stars do not drift too far above or below it. The total matter that must be in the disk is less than twice the luminous matter. That means no more than half of the luminous mass. the mass in the region near the Sun can be dark matter.
credit background: modification of work by ESO
Dark Matter in and Around Galaxies
Unlike our neighborhood near the Sun and the Solar System, there is (as we saw in The Milky Way Galaxy) abundant evidence that about 90% of the mass in the entire galaxy is in the a halo form of dark matter. In other words, there is apparently about nine times more dark matter than visible matter. Astronomers have found some stars in the outer regions of the Milky Way beyond its bright disk, and these stars spin around their center very quickly. The mass of all stars and all interstellar matter that we can discover in the galaxy does not exert enough gravitational force to explain how these fast-moving stars stay in their orbit and not fly away. Only with large amounts of invisible matter could the galaxy look at these fast-moving outer stars. The same result is found for other spiral galaxies as well.
the Andromeda Galaxy, a member of our local group. The observed rotation of spiral galaxies like Andromeda is usually seen in graphs called rotation curves, which show velocity as a function of distance from the center of the galaxy. These graphics suggest that dark matter resides in a large halo that surrounds the glowing parts of each galaxy. The radius of the halos around the Milky Way and Andromeda can be up to 300,000 light years, much larger than the visible size of these galaxies.
Dark Matter in Clusters of Galaxies
Galaxies in cluster also move-around: they orbit the cluster’s center of mass. It is not possible for us to track a galaxy in its entire orbit because it usually takes about a billion years to do so. However, it is possible to measure the speeds at which the galaxies move in a cluster and then estimate what total mass must be in the cluster to prevent individual galaxies from flying out of the cluster. Observations suggest that the mass of the galaxies alone cannot hold the cluster together; another gravity must be present again. The total amount of dark matter in clusters exceeds ten times the luminous mass of the galaxies themselves, suggesting that dark matter exists both between and within galaxies.
There is another approach to measuring the amount of dark matter in galaxy clusters. As we have seen, the universe is expanding, but this expansion is not entirely uniform thanks to the interfering hand of gravity. Suppose a galaxy is outside of, but relatively close to, a rich cluster of galaxies. The cluster’s gravitational force will attract the neighboring galaxy, slowing the rate at which it is moving away from the cluster due to the expansion of the universe.
Consider the local group of galaxies located on the edge of the Virgo supercluster. The mass concentrated in the center of the Virgo cluster exerts a gravitational force on the local group. As a result, the local group moves away from the center of the Virgo cluster. at a speed a few hundred kilometers per second slower than Hubble’s law predicts. By measuring such deviations from smooth expansion, astronomers can estimate total mass in large clusters.
There are two other very useful methods of measuring the amount of dark matter in galaxy clusters, and both have produced results that are broadly in line with the method used to measure galaxy velocities: gravitational lenses and x-ray emission both.
As Albert Einstein showed in his general theory of relativity, the presence of mass bends the surrounding structure of spacetime. Light follows these curves so that very massive objects can bend the light significantly. Visible galaxies are not the only possible gravitational lenses. Through this effect, dark matter can also reveal its presence. A cluster of galaxies that acts as a gravitational lens; The streaks & arcs you see in the picture are lens images of more distant galaxies. Gravitational lenses are known enough that astronomers can use the many ovals and arcs in this image to compute detailed maps of how much matter is in the cluster and how that mass is distributed. The study results of many of these gravitational lens clusters show that galaxy clusters as individual galaxies contain more than ten times more luminous matter than dark matter.
The third method astronomers use to detect and measure dark matter in galaxy clusters is to image it in the light of X-rays.. When the first sensitive X-ray telescopes were brought into orbit in the 1970s and formed in massive galaxy clusters, it was quickly determined that the clusters were emitting copious X ray radiation. Most stars do not emit much X-rays, and neither does most of the gas or dust between the stars within the galaxies. What could the X-rays from practically all massive galaxy clusters emit?
It turns out that galaxies spread gas among their stars, galaxy clusters spread gas between their galaxies. The particles in these huge gas reservoirs are not just sitting still; rather, they are constantly moving and, under the influence of the immense gravity of the cluster, rotate like mini-planets around a giant sun. As they move and collide with each other, the gas gets hotter and hotter until it glows at temperatures of up to 100 million K. bright at x-ray wavelengths. The more mass cluster has, the faster the movements, the hotter the gas and the brighter the X-rays. Astronomers calculate that to trigger these motions, the existing mass must be about ten times the mass they can see in clusters, including all galaxies and all gases. Again, this is evidence that clusters of galaxies appear to be dominated by dark matter.
credit: modification of work by NASA/ESA/JPL-Caltech/Yale/CNRS
Mass to Light Ratio
We describe the use of the mass to light ratio to characterize matter in galaxies or galaxy clusters in properties of galaxies. For systems that contain mostly old-stars, the mass-to-light ratio is typically 10 to 20, where mass and light are measured in units of sun-mass & luminosity. A mass-to-light ratio of 100 or more is a sign that a significant amount of dark matter is present. In the given table below summarizes the results of measurements of mass-to-light ratios for different classes of objects. Very large mass-to-light ratios are found for all galaxy size systems and larger, suggesting that dark matter is present in all of these object types. This is why we say that dark matter appears to make up most of the total mass in the universe.
| Type Of Object | Mass To Light Ratio |
| Sun | 1 |
| Matter in vicinity of Sun | 2 |
| Total mass in Milky Way | 10 |
| Small groups of Galaxies | 50-150 |
| Rich clusters of Galaxies | 250-300 |
The cluster of galaxies can be used to infer the total mass in a given region of space, while visible radiation is a good indicator of where the luminous-mass is. Studies show that dark matter & luminous matter are very closely related. Dark-mass halos extend beyond the luminous boundaries of the surrounding galaxies. However, where there are large clusters of galaxies, there are also large amounts of dark matter. Gaps in the distribution of galaxies are also gaps in the distribution of dark matter.
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