by George Rhee
When the surrounding galaxy is too faint to detect and we can only see the star-like central light we call the object a quasar. Since they were originally discovered as radio sources, they were named quasi-stellar radio sources. It turned out that these objects were very distant active galaxies.
Active galaxies present us with a problem. We have to understand how so much energy can come out of a small central region. The answer lies in the fact that super massive black holes lie at the centers of some, if not all, galaxies (including our own). Observations of the centers of galaxies suggest that regions less than a light year across may contain a black hole with a mass between several tens of millions up to several billion times the mass of the Sun.
It seems paradoxical that black holes are responsible for the most luminous objects known in the universe. Shouldn’t black holes be black? The answer to this question is no, because matter on its way into the black hole is accelerated to tremendous speed and gets very hot. The heating is caused by friction. Since hot matter emits lots of light, the region surrounding the black hole can glow very brightly. Once matter gets close enough to the black hole, nothing, not even light can escape, and the black hole is indeed invisible.
The reason the central region of an active galaxy can produce so much energy is linked to the efficiency of energy production. In the nuclear fusion process less than 1 % of the mass involved in the fusion reaction gets converted to energy. However, calculations suggest that as much as 20 % of the total mass of infalling matter can be converted to energy (i.e. light) before the mass is lost forever in a black hole. It is plausible that a star like the Sun could be swallowed by a central massive black hole. This would release enough energy to power an active galaxy for a 1,000 years. Galaxy mergers discussed above could send a star into the central black hole, thereby turning the galaxy into an active galaxy. We have strong evidence that our own galaxy contains a supermassive black hole, so it is possible that it, too, was once a quasar. Surveys of distant quasars indicate that the quasar phenomenon was more frequent in the past. In other words, a higher fraction of all galaxies were quasars at a redshift of one than today. This is possibly due to the fact that collisions and mergers were more frequent in the past. It is another instance of looking back in time and seeing a universe different in appearance from the one we see today.
What Lies Between the Galaxies?
Our best estimate from Big Bang nuclear fusion calculations and observations of the background radiation is that 4.6 % of the density of the universe consists of baryons in the form of neutrons and protons. But where are these baryons located? We estimate that roughly 10 % of the baryons are seen as visible matter in the form of stars in galaxies. Hot gas in clusters of galaxies account for another 5 %. Quasar spectra suggest that another 30 % is in the form of hydrogen clouds located between the galaxies, the so-called Lyman-alpha forest. It is possible that another 10 % lies in the form of warm intergalactic gas. Nevertheless about 50 % of the baryons remain unaccounted for. One idea is that these “missing baryons” may be difficult to detect because they are concentrated in a filamentary web of tenuous warm gas between galaxies that has been continuously heated during the process of galaxy formation.
The hot gas in clusters illustrates how we detect gas between galaxies. This gas is detected because it is hot enough to emit X-rays. Since X-rays are (fortunately for us) absorbed by the atmosphere, X-ray astronomy must be conducted by satellites orbiting the Earth. The role of telescopes is to collect light from distant objects and bring it to a focal point. For optical telescopes, this is done with lenses or reflecting mirrors. X-rays, because of their high energies, tend to pass straight through or be absorbed by the material they strike. The mirrors used to create X-ray images are called grazing incidence mirrors because they focus light that hits their surfacet almost parallel. X-ray telescopes have improved tremendously in the last few years, such that we can now measure redshifts using X-ray data.
When the first X-ray images were taken by a satellite known as the Einstein Observatory, it became clear that clusters of galaxies contained a lot of hot gas with temperatures of about 10 million degrees Kelvin. A substantial fraction, as much as 30 % of the cluster mass, is believed to consist of this gas. A number of questions arise, including, the obvious: “How did the gas get there?” Our current belief is that some of the gas is a relic from the time when the cluster formed. Some of the gas also must have originated in galaxies, because we can detect emission from iron in the cluster X-ray spectrum. Since no iron was created in the Big Bang, the gas must have been processed inside stars, expelled into interstellar space by supernova explosions, and finally blown out of the galaxy during its journey in the cluster. The second question is “Why is the gas so hot?” It has been found that the temperature of the X-ray gas increases for more massive clusters. There is a rule of thumb that the galaxies in clusters that are more massive tend to move faster than galaxies in less massive clusters. By move I mean speed of motion through space, not rotation speed. We shall discuss this more in the next chapter. Galaxies in clusters move at speeds of about 800 km s − 1. From the theory of gases, we can associate a speed with a temperature; the temperature of the gas tells us how fast the molecules constituting that gas are moving. It turns out that the velocity of the gas molecules in clusters inferred from the temperature is comparable to the velocity of galaxies. The idea is that the gas is stripped from galaxies and then stirred up by turbulent motions until it reaches these high temperatures.
The Visible Universe Across the Electromagnetic Spectrum
Let us review the information we get from visible matter by wavelength of light received. We start with radio waves, which have the longest wavelengths, and proceed through visible light, all the way up to gamma rays.
At radio wavelengths, we detect the light from neutral hydrogen atoms. Spiral galaxies contain hydrogen gas, and we can map the distribution and speed of this gas using radio telescopes. Radio telescopes also detect radiation from electrons spiraling in magnetic fields. We see tremendous amounts of this radiation emitted by radio galaxies, which emit most of their light at radio wavelengths. With radio telescopes, we can also detect remnants of supernova explosions known as pulsars. There are several radio telescopes in various countries. A prominent one open to visitors is the Very Large Array, a spectacular array of 27 radio dishes located near Socorro, New Mexico. These dishes are spread out over a distance of several miles on rails.
At shorter wavelengths, we encounter infrared emission, which is mostly associated with warm clouds of gas and dust surrounding star-forming regions. Infrared astronomy can be carried out from high altitude dry observing sites such as Mauna Kea in Hawaii. You may have visited Hawaii and noticed how humid parts of these islands are. The summit of Mauna Kea is very dry because of the high altitude. Satellites also take infrared telescopes above the Earth’s atmosphere. Infrared wavelengths are less strongly scattered than shorter wavelength light, so we can use infrared light to search for objects that would be obscured from view by gas and dust.
Most stars emit most of their energy at visible wavelengths. This is because of their surface temperatures. We can also study gas clouds at visible wavelengths, gathering information about the composition, temperature, and density of the emitted gas. Because the light from distant galaxies gets shifted toward the red, for high redshift galaxies, the light which is emitted as ultraviolet radiation is detected as visible light or even infrared light by the time it reaches the Earth.
Ultraviolet light does not penetrate the Earth’s atmosphere, so we study it using telescopes above the atmosphere, such as the Hubble Space Telescope. Ultraviolet light is emitted by young, very massive, hot stars, among others. It is important to know what nearby galaxies look like at ultraviolet wavelengths so that we can compare like with like when we compare nearby galaxies with high-redshift galaxies. Going on to X-rays, we have mentioned emission from the gas in clusters. The central regions
of active galaxies also emit X-rays.
At the shortest observable wavelengths we detect gamma ray bursts, brief flashes of gamma-ray energy, lasting from a few milliseconds to a few hundred seconds. These short bursts of gamma radiation were observed to be distributed all over the sky. In 1999, a number of these bursts were identified with galaxies at high redshifts, which suggests that the bursts generate enormous power in very short times. The intrinsic power of the first gamma-ray burst identified with a distant galaxy was estimated to be about 1016 (or 10 million billion) times that of our Sun. We think that gamma-ray bursts are supernova events that produced intense narrow jets of radiation. In 2008 a gamma-ray burst went off that was visible to the naked eye for about 30 s. This object is 7.5 billion light years away so that event happened long before the solar system even existed.
Galaxies and Cosmology
We see objects because they shine; that is to say, they emit electromagnetic radiation. Stars are the most obvious example of visible matter in the universe. They emit light because they produce energy in their centers from nuclear fusion. Stars are assembled into larger systems called galaxies. It is the study of these systems that concerns cosmologists. Most of the light emitted by galaxies is produced by stars. Galaxies also contain gas and dust, which we detect through its absorbing properties. Active galaxies produce substantial amounts of energy from a source other than nuclear fusion. These galaxies are believed to contain very massive black holes at their centers. Matter falling into the black hole is believed to be the source of energy for the radiation emitted by these galaxies. When no matter is falling into the black hole the central region is very faint but we still sense the presence of the black hole from its effect on the orbits of nearby stars.
Cosmology seeks to explain and organizet all this information about galaxies. We wish to explain the world of galaxies in terms of the history of their formation. The driving force behind galaxy formation is the gravity of dark matter halos that formed in the early universe. Dark matter is the subject of the next chapter.
Further Reading
Galaxy Collisions. C. Struck, New York, Springer-Praxis, 2011.
Galaxies and the Cosmic Frontier. W. H. Waller and P. W. Hodge, Cambridge, Harvard University Press, 2003.
George RheeAstronomers' UniverseCosmic Dawn2013The Search for the First Stars and Galaxies10.1007/978-1-4614-7813-3_4© Springer Science+Business Media, LLC 2013
4. Dark Matter
George Rhee1
(1)Department of Physics & Astronomy, University of Nevada, Las Vegas, Nevada, USA
Abstract
The issue of the nature and distribution of dark matter in the universe pervades all of modern cosmology. When astronomers carry out a census of the universe they find that stars contribute only 1 % to the total density of the universe. Ordinary matter (such as the atoms the make up our bodies) contributes 4.6 % to the total density of the universe. We know that 95 % of the stuff in the universe is invisible to us. Twenty three percent of the density of the universe consists of dark matter; weakly interacting particles that have not been directly detected on Earth. The remaining 72 % consists of dark energy, an unknown force that counteracts the effects of gravity. These conclusions are remarkable for at least two reasons; firstly the numbers themselves and secondly the undreamed of accuracy with which they have been measured. In this chapter we show how these conclusions were reached using telescopes and astronomical satellites as well as presenting the arguments for and against the existence of dark matter.
…the closest I came to an exam was when one day Ehrenfest asked me to recite and discuss Maxwell’s equations – the fundamental equations of electricity and magnetism – while walking up and down the corridor. “Yes, you have understood some of the music” was his final verdict.
Hendrik Casimir, Haphazard Reality
When astronomers carry out a census of the universe they find that stars contribute only 1 % to the total density of the universe. Ordinary matter (such as the atoms the make up our bodies) contributes 4.6 % to the total density of the universe. We know that 95 % of the stuff in the universe is invisible to us. Twenty three percent of the density of the universe consists of dark matter; weakly interacting particles that have not been directly detected on Earth. The remaining 72 % consists of dark energy, an unknown force that counteracts the effects of gravity. These conclusions are remarkable for at least two reasons; firstly the numbers themselves and secondly the undreamed of accuracy with which they have been measured. In this chapter we show how these conclusions were reached using telescopes and astronomical satellites as well as presenting the arguments for and against the existence of dark matter.
The Discovery of Neptune: Putting Sir Isaac’s Theory to Work
A powerful aspect of Newton’s theory of universal gravitation is that one can make predictions about the behavior of matter. This applies, for example, to motion in the solar system. In 1781, William Herschel discovered the first planet to be found with a telescope. Using Newton’s theory, an accurate orbit for the new planet, named Uranus, was established. Uranus is four times larger than the Earth and about 20 times farther from the Sun. It takes about 3 h for light to travel from the Sun out to Uranus. This planet has a ring and at least 15 moons. Intriguingly, it is tipped on its side. The axis of rotation of the planet lies in the same plane as its orbit. This is possibly the result of a collision that happened long ago. Herschel proposed naming the planet George after King George III. Unfortunately, in this author’s opinion, this name was not adopted. As the planet’s position in the sky was measured during the following few decades, it became apparent that Uranus’s motion was not consistent with the prediction based on Newton’s laws. The difference between the actual and expected position was 20 arc seconds from 1790 to 1830. By 1840, it had increased to about a minute of arc i.e. 60 arc seconds or 1/30 of a moon diameter. I sometimes quote angular measurements in moon diameters, because this is an object we are used to seeing in the sky. Astronomers usually use degrees and minutes and seconds of arc. A minute of arc is 1/60 of a degree. A second of arc is 1/60 of a minute of arc or 1/3,600 of a degree. The Moon, like the Sun has an angular diameter of about half a degree, or 30 min of arc.
When faced with such a discrepancy, astronomers could either dismiss Newton’s laws as having been proven wrong or try to reconcile the known facts with Newton’s laws. They chose the latter course. One reason for this was that Newton’s laws had accurately described the orbit of Halley’s Comet, observed in 1758–1759. The reasonable option was thus to postulate the existence of a perturbing object, whose gravity was influencing the path of Uranus around the Sun. A young Englishman, John Adams, and a well-known Frenchman, Urbain Leverrier, predicted the existence of an undiscovered planet based on the motion of Uranus. Adams requested that a certain part of the sky be searched, but he could not convince the astronomical establishment to carry out the project. His work was subsequently recognized, to the extent that the house I lived in while an undergraduate at Cambridge was located on Adams Road. During Adams’ third year as an undergraduate, he decided to study “the irregularities of the motion of Uranus…in order to find out whether they may be attributed to the actions of an undiscovered planet beyond it.” Such a display of initiative and technique in a 22 year old is really impressive.
In the summer of 1846, Leverrier repeated Adams’s calculation and sent a letter suggesting a search for the planet to Johann Galle at the Berlin Observatory. On the night of September 23 1846, Galle pointed his telescope to the position suggested by Leverrier and saw the planet Neptune. It was a triumph for Newton’s theory. Further discrepancies were discovered in Uranus’ orbit, which prompted a search for yet another planet. Pluto was discovered after an extensive search by Clyde Tombaugh in 1930. I was fortunate to meet him while I was doing postdoctoral research at New Mexico State University in the early 1990s. Arago, another distinguished French scientist, noted of Neptune’s discovery that “In the eyes
of all impartial men, this discovery will remain one of the most magnificent triumphs of theoretical astronomy.”
Leverrier discovered a discrepancy in the motion of Mercury in 1855 and thought this was also due to the existence of an undiscovered planet, which he called Vulcan. In this case, however, it was indeed a shortcoming of Newton’s theory that was responsible. Einstein’s theory of general relativity provided the correct explanation for the discrepancy.
We tell this story, not because of a sudden need to discuss planets in a cosmology book but because it illustrates the central theme of this chapter. The presence of visible matter can be used to detect the presence of other matter. In the case of Neptune the matter was visible. The presence of an invisible or dark object could just as well have been inferred using this method. As one of Leverrier’s colleagues pointed out, “he discovered a planet with the tip of his pen, without any instruments other than the strength of his calculations alone.” This method has been applied to the motion of stars and gas within galaxies, to the motions of galaxies within clusters, and, indeed, to the motion of matter in the universe. In each instance we use the motion of visible matter to infer the existence of invisible matter using the known laws of physics.
