what role did interstellar dust play in the quest to determine the shape and size of our galaxy?

Learning Objectives

Past the end of this section, you will be able to:

• Explain how astronomers know that the solar system contains very little night matter
• Summarize the bear witness for dark affair in almost galaxies
• Explicate how we know that milky way clusters are dominated past dark thing
• Relate the presence of nighttime thing to the average mass-to-low-cal ratio of huge volumes of space containing many galaxies

So far this chapter has focused almost entirely on matter that radiates electromagnetic energy—stars, planets, gas, and dust. But, every bit we have pointed out in several earlier chapters (particularly The Milky Way Galaxy), information technology is now clear that galaxies contain large amounts of dark matter equally well. In that location is much more dark affair, in fact, than matter we can run across—which means information technology would be foolish to ignore the effect of this unseen fabric in our theories almost the structure of the universe. (As many a ship captain in the polar seas establish out too tardily, the function of the iceberg visible above the ocean's surface was non necessarily the only office he needed to pay attention to.) Dark affair turns out to be extremely important in determining the development of galaxies and of the universe equally a whole.

The idea that much of the universe is filled with dark matter may seem like a baroque concept, but we can cite a historical example of "night matter" much closer to home. In the mid-nineteenth century, measurements showed that the planet Uranus did not follow exactly the orbit predicted from Newton'due south laws if ane added upward the gravitational forces of all the known objects in the solar organisation. Some people worried that Newton's laws may merely not piece of work so far out in our solar organization. But the more straightforward interpretation was to attribute Uranus' orbital deviations to the gravitational effects of a new planet that had non yet been seen. Calculations showed where that planet had to exist, and Neptune was discovered just almost in the predicted location.

In the aforementioned manner, astronomers now routinely decide the location and corporeality of dark matter in galaxies by measuring its gravitational effects on objects nosotros can see. And, by measuring the way that galaxies motion in clusters, scientists have discovered that night thing is also distributed among the galaxies in the clusters. Since the environment surrounding a milky way is important in its development, dark matter must play a central role in milky way evolution besides. Indeed, information technology appears that night affair makes up nearly of the matter in the universe. Simply what is night matter? What is it made of? Nosotros'll await adjacent at the search for dark matter and the quest to determine its nature.

Nighttime Matter in the Local Neighborhood

Is in that location dark matter in our ain solar arrangement? Astronomers have examined the orbits of the known planets and of spacecraft as they journeying to the outer planets and across. No deviations have been plant from the orbits predicted on the basis of the masses of objects already discovered in our solar arrangement and the theory of gravity. We therefore conclude that in that location is no evidence that in that location are big amounts of dark matter nearby.

Astronomers have likewise looked for evidence of dark affair in the region of the Milky Manner Galaxy that lies inside a few hundred low-cal-years of the Dominicus. In this vicinity, most of the stars are restricted to a sparse deejay. It is possible to calculate how much mass the disk must contain in gild to keep the stars from wandering far in a higher place or below it. The full matter that must be in the deejay is less than twice the amount of luminous matter. This means that no more half of the mass in the region nearly the Sun can be dark matter.

Dark Matter in and around Galaxies

In dissimilarity to our local neighborhood virtually the Sun and solar organisation, at that place is (as we saw in The Milky way Galaxy) aplenty evidence strongly suggesting that nigh xc% of the mass in the unabridged galaxy is in the form of a halo of dark matter. In other words, in that location is apparently about 9 times more nighttime matter than visible affair. Astronomers have constitute some stars in the outer regions of the Milky way beyond its bright disk, and these stars are revolving very rapidly around its center. The mass contained in all the stars and all the interstellar thing we tin discover in the milky way does not exert enough gravitational forcefulness to explicate how those fast-moving stars remain in their orbits and practise non fly away. Only by having large amounts of unseen thing could the galaxy be holding on to those fast-moving outer stars. The same result is institute for other screw galaxies as well.

Figure i is an instance of the kinds of observations astronomers are making, for the Andromeda milky way, a fellow member of our Local Grouping. The observed rotation of spiral galaxies similar Andromeda is ordinarily seen in plots, known as rotation curves, that show velocity versus distance from the galaxy center. Such plots suggest that the dark matter is found in a big halo surrounding the luminous parts of each milky way. The radius of the halos around the Galaxy and Andromeda may be as large as 300,000 calorie-free-years, much larger than the visible size of these galaxies.

Effigy 1: Rotation Indicates Night Matter. We come across the Galaxy'south sister, the spiral Andromeda galaxy, with a graph that shows the velocity at which stars and clouds of gas orbit the galaxy at different distances from the middle (scarlet line). Equally is true of the Galaxy, the rotational velocity (or orbital speed) does not decrease with distance from the heart, which is what you would expect if an assembly of objects rotates around a common heart. A calculation (blue line) based on the total mass visible as stars, gas, and dust predicts that the velocity should be much lower at larger distances from the centre. The discrepancy betwixt the ii curves implies the presence of a halo of massive dark matter extending outside the purlieus of the luminous affair. This dark matter causes everything in the galaxy to orbit faster than the observed thing solitary could explain. (credit background: modification of work by ESO)

Dark Matter in Clusters of Galaxies

Galaxies in clusters besides motility around: they orbit the cluster's center of mass. It is not possible for us to follow a milky way around its entire orbit because that typically takes about a billion years. It is possible, however, to measure the velocities with which galaxies in a cluster are moving, and and so approximate what the total mass in the cluster must exist to continue the individual galaxies from flying out of the cluster. The observations indicate that the mass of the galaxies alone cannot continue the cluster together—some other gravity must again be nowadays. The full amount of night matter in clusters exceeds past more than ten times the luminous mass contained within the galaxies themselves, indicating that dark matter exists betwixt galaxies too as inside them.

There is another arroyo nosotros can take to measuring the amount of dark matter in clusters of galaxies. As we saw, the universe is expanding, just this expansion is not perfectly uniform, thank you to the interfering paw of gravity. Suppose, for example, that a galaxy lies outside but relatively shut to a rich cluster of galaxies. The gravitational forcefulness of the cluster will tug on that neighboring galaxy and boring downwards the rate at which information technology moves away from the cluster due to the expansion of the universe.

Consider the Local Grouping of galaxies, lying on the outskirts of the Virgo Supercluster. The mass concentrated at the center of the Virgo Cluster exerts a gravitational force on the Local Group. As a issue, the Local Group is moving abroad from the center of the Virgo Cluster at a velocity a few hundred kilometers per second slower than the Hubble police predicts. Past measuring such deviations from a polish expansion, astronomers can approximate the full corporeality of mass contained in big clusters.

There are two other very useful methods for measuring the corporeality of dark matter in galaxy clusters, and both of them have produced results in general agreement with the method of measuring galaxy velocities: gravitational lensing and X-ray emission. Let'due south take a look at both.

Every bit Albert Einstein showed in his theory of full general relativity, the presence of mass bends the surrounding fabric of spacetime. Lite follows those bends, so very massive objects can curve light significantly. You saw examples of this in the Gravitational Lensing department in the previous department. Visible galaxies are non the but possible gravitational lenses. Night affair can also reveal its presence past producing this effect. Figure two shows a galaxy cluster that is acting like a gravitational lens; the streaks and arcs you see on the motion-picture show are lensed images of more distant galaxies. Gravitational lensing is well enough understood that astronomers can use the many ovals and arcs seen in this image to calculate detailed maps of how much matter there is in the cluster and how that mass is distributed. The result from studies of many such gravitational lens clusters shows that, like private galaxies, galaxy clusters contain more than ten times as much nighttime matter as luminous matter.

Figure 2: Cluster Abell 2218. This view from the Hubble Space Telescope shows the massive galaxy cluster Abell 2218 at a distance of about 2 billion calorie-free-years. Most of the yellowish objects are galaxies belonging to the cluster. Merely discover the numerous long, thin streaks, many of them bluish; those are the distorted and magnified images of even more distant background galaxies, gravitationally lensed past the enormous mass of the intervening cluster. Past carefully analyzing the lensed images, astronomers tin construct a map of the dark affair that dominates the mass of the cluster. (credit: modification of piece of work by NASA, ESA, and Johan Richard (Caltech))

The third method astronomers use to detect and measure dark matter in galaxy clusters is to image them in the light of 10-rays. When the first sensitive Ten-ray telescopes were launched into orbit around Earth in the 1970s and trained on massive milky way clusters, information technology was quickly discovered that the clusters emit copious 10-ray radiation (encounter Figure 3). Most stars do not emit much X-ray radiation, and neither does nearly of the gas or dust between the stars within galaxies. What could be emitting the X-rays seen from most 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. The particles in these huge reservoirs of gas are non just sitting still; rather, they are constantly moving, zooming effectually under the influence of the cluster'southward immense gravity like mini planets around a behemothic sun. As they move and bump against each other, the gas heats up hotter and hotter until, at temperatures as high as 100 one thousand thousand M, 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 Ten-rays. Astronomers summate that the mass present to induce those motions must be well-nigh 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 night matter.

Effigy three: X-Ray Image of a Galaxy Cluster. This composite image shows the galaxy cluster Abell 1689 at a distance of 2.iii billion light-years. The finely detailed views of the galaxies, most of them yellowish, are in visible and near-infrared lite from the Hubble Space Telescope, while the diffuse purple haze shows Ten-rays as seen by Chandra X-ray Observatory. The abundant 10-rays, the gravitationally lensed images (sparse curving arcs) of background galaxies, and the measured velocities of galaxies in the clusters all testify that the total mass of Abell 1689—most of information technology dark matter—is about 1015 solar masses. (credit: modification of piece of work past NASA/ESA/JPL-Caltech/Yale/CNRS)

Mass-to-Low-cal Ratio

We described the utilise of the mass-to-light ratio to narrate the matter in galaxies or clusters of galaxies in Properties of Galaxies. For systems containing by and large sometime stars, the mass-to-light ratio is typically x to 20, where mass and calorie-free are measured in units of the Dominicus'due south mass and luminosity. A mass-to-calorie-free ratio of 100 or more is a signal that a substantial amount of night matter is present. Table one summarizes the results of measurements of mass-to-low-cal ratios for various classes of objects. Very large mass-to-calorie-free ratios are found for all systems of galaxy size and larger, indicating that dark matter is nowadays in all these types of objects. This is why we say that dark matter apparently makes up almost of the full mass of the universe.

Table i: Mass-To-Lite Ratios
Type of Object	Mass-to-Low-cal Ratio
Sun	1
Thing in vicinity of Sun	2
Total mass in Milky Way	10
Small groups of galaxies	l–150
Rich clusters of galaxies	250–300

The clustering of galaxies can be used to derive the full amount of mass in a given region of infinite, while visible radiation is a skillful indicator of where the luminous mass is. Studies evidence that the night matter and luminous thing are very closely associated. The dark matter halos do extend beyond the luminous boundaries of the galaxies that they environment. Nonetheless, where at that place are big clusters of galaxies, y'all will also discover large amounts of nighttime matter. Voids in the galaxy distribution are too voids in the distribution of night affair.

What Is the Nighttime Matter?

How do nosotros get about figuring out what the dark affair consists of? The technique we might utilize depends on its composition. Permit's consider the possibility that some of the dark matter is made up of normal particles: protons, neutrons, and electrons. Suppose these particles were assembled into black holes, brown dwarfs, or white dwarfs. If the black holes had no accession disks, they would be invisible to the states. White and brown dwarfs do emit some radiation but take such low luminosities that they cannot be seen at distances greater than a few thousand light-years.

We can, even so, look for such compact objects because they can deed as gravitational lenses. (Meet Gravitational Lensing from The Distribution of Galaxies in Infinite.) Suppose the dark matter in the halo of the Milky way were made up of blackness holes, brown dwarfs, and white dwarfs. These objects have been whimsically dubbed MACHOs (MAssive Meaty Halo Objects). If an invisible Macho passes directly betwixt a distant star and Earth, it acts as a gravitational lens, focusing the lite from the distant star. This causes the star to appear to burnish over a fourth dimension interval of a few hours to several days before returning to its normal brightness. Since nosotros can't predict when any given star might brighten this fashion, we have to monitor huge numbers of stars to catch one in the human action. There are non enough astronomers to keep monitoring so many stars, but today's automated telescopes and figurer systems tin can exercise it for united states.

Enquiry teams making observations of millions of stars in the nearby milky way called the Big 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). However, in that location are not enough MACHOs in the halo of the Milky way to account for the mass of the dark matter in the halo.

Figure 4: Large and Small Magellanic Clouds. Here, the two small galaxies we call the Large Magellanic Cloud and Small Magellanic Deject tin can be seen above the auxiliary telescopes for the Very Large Telescope Assortment on Cerro Paranal in Chile. You can see from the number of stars that are visible that this is a very dark site for doing astronomy. (credit: ESO/J. Colosimo)

This consequence, forth with a multifariousness of other experiments, leads u.s. to conclude that the types of matter we are familiar with tin make up only a tiny portion of the night matter. Another possibility is that dark matter is equanimous of some new type of particle—one that researchers are at present trying to detect in laboratories hither on Globe (run into The Big Bang).

The kinds of nighttime matter particles that astronomers and physicists have proposed generally autumn into ii primary categories: hot and cold dark thing. The terms hot and cold don't refer to true temperatures, but rather to the average velocities of the particles, analogous to how nosotros might call back of particles of air moving in your room right now. In a cold room, the air particles movement more slowly on boilerplate than in a warm room.

In the early on universe, if dark matter particles hands moved fast and far compared to the lumps and bumps of ordinary matter that somewhen became galaxies and larger structures, nosotros call those particles hot dark matter. In that case, smaller lumps and bumps would be smeared out past the particle motions, meaning fewer small-scale galaxies would get fabricated.

On the other hand, if the dark thing particles moved slowly and covered simply small distances compared to the sizes of the lumps in the early universe, we call that cold night matter. Their dull speeds and free energy would mean that fifty-fifty the smaller lumps of ordinary matter would survive to grow into pocket-sized galaxies. By looking at when galaxies formed and how they evolve, we can utilize observations to distinguish between the two kinds of nighttime matter. And then far, observations seem most consistent with models based on cold dark matter.

Solving the night matter problem is one of the biggest challenges facing astronomers. Afterwards all, nosotros tin can hardly understand the development of galaxies and the long-term history of the universe without agreement what its most massive component is made of. For instance, we need to know merely what role dark affair played in starting the higher-density "seeds" that led to the formation of galaxies. And since many galaxies have large halos made of night matter, how does this touch their interactions with one some other and the shapes and types of galaxies that their collisions create?

Astronomers armed with various theories are working hard to produce models of milky way structure and evolution that accept dark affair into account in just the right way. Even though we don't know what the dark thing is, nosotros do accept some clues about how it affected the germination of the very showtime galaxies. Equally we will see in The Big Bang, careful measurements of the microwave radiation left over later the Big Bang have allowed astronomers to gear up very tight limits on the actual sizes of those early seeds that led to the germination of the big galaxies that we see in today's universe. Astronomers accept also measured the relative numbers and distances between galaxies and clusters of unlike sizes in the universe today. And so far, most of the show seems to weigh heavily in favor of cold dark matter, and most current models of galaxy and big-scale construction formation use cold dark matter every bit their main ingredient.

Equally if the presence of nighttime matter—a mysterious substance that exerts gravity and outweighs all the known stars and galaxies in the universe only does non emit or absorb light—were non enough, at that place is an fifty-fifty more inexplainable and as important constituent of the universe that has only recently been discovered: we have called it nighttime free energy in parallel with night matter. We will say more than virtually it and explore its effects on the evolution of the universe in The Big Bang. For now, we tin consummate our inventory of the contents of the universe by noting that it appears that the entire universe contains some mysterious free energy that pushes spacetime apart, taking galaxies and the larger structures made of galaxies forth with it. Observations evidence that dark energy becomes more and more important relative to gravity equally the universe ages. Equally a event, the expansion of the universe is accelerating, and this acceleration seems to be happening mostly since the universe was virtually half its electric current age.

What we see when nosotros peer out into the universe—the light from trillions of stars in hundreds of billions of galaxies wrapped in intricate veils of gas and dust—is therefore actually merely a sprinkling of icing on meridian of the cake: every bit we volition meet in The Big Bang, when nosotros wait outside galaxies and clusters of galaxies at the universe as a whole, astronomers observe that for every gram of luminous normal matter, such as protons, neutrons, electrons, and atoms in the universe, there are about 4 grams of nonluminous normal matter, mainly intergalactic hydrogen and helium. There are virtually 27 grams of dark matter, and the energy equivalent (remember Einstein's famous E = mc ²) of about 68 grams of nighttime energy. Dark matter, and (equally nosotros will see) fifty-fifty more so night energy, are dramatic demonstrations of what we have tried to emphasize throughout this volume: science is always a "progress report," and we often run into areas where nosotros have more questions than answers.

Let's side by side put together all these clues to trace the life history of galaxies and large-scale structure in the universe. What follows is the current consensus, but inquiry in this field is moving chop-chop, and some of these ideas will probably be modified as new observations are made.

Primal concepts and summary

Stars motility much faster in their orbits around the centers of galaxies, and galaxies effectually centers of galaxy clusters, than they should co-ordinate to the gravity of all the luminous matter (stars, gas, and dust) astronomers tin can find. This discrepancy implies that galaxies and milky way clusters are dominated by dark matter rather than normal luminous thing. Gravitational lensing and X-ray radiation from massive galaxy clusters ostend the presence of dark affair. Galaxies and clusters of galaxies contain about 10 times more dark matter than luminous matter. While some of the dark thing may be made upward of ordinary matter (protons, neutrons, and electrons), maybe in the course of very faint stars or black holes, nigh of it probably consists of some totally new type of particle not nonetheless detected on Globe. Observations of gravitational lensing effects on distant objects take been used to look in the outer region of our Galaxy for whatever nighttime matter in the grade of compact, dim stars or star remnants, but not plenty such objects have been institute to account for all the dark matter.

Glossary

cold dark affair: slow-moving massive particles, not yet identified, that don't absorb, emit, or reflect low-cal or other electromagnetic radiations, and that make up most of the mass of galaxies and milky way clusters

dark energy: an free energy that is causing the expansion of the universe to advance; the source of this energy is non withal understood

hot night matter: massive particles, not still identified, that don't absorb, emit, or reflect light or other electromagnetic radiation, and that brand upwardly most of the mass of galaxies and galaxy clusters; hot night affair is faster-moving material than cold dark thing

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