Matters of Dark Matter

By Josh Simon, with excerpts taken from an interview by George Musser for Scientific American in January, 2008

Listen as Dr. Simon describes the history of dark matter research.

One of the most important astronomical discoveries in recent decades is that galaxies are much more massive than would be expected from simply looking at them. It is relatively straightforward to count all the stars in a galaxy, add up their masses, and include a little bit extra to account for the gas and dust. But this total mass is nowhere close to being enough to explain how fast the stars and gas within galaxies are moving! Apparently, galaxies also contain a whole lot of additional material that we can’t see—- dark matter.

While we know that dark matter feels the force of gravity, its nature is a mystery to physicists and astronomers. Dark matter is probably made of a new kind of subatomic particle, but even though there are many ideas about what those particles could be, we have not yet succeeded in actually identifying a dark matter particle or detecting one in a laboratory. Whatever it is, there is about 6 times as much dark matter in the universe as ordinary atoms and molecules. A wide variety of evidence indicates that dark matter is the major building block of the largest structures in the universe: galaxies and giant clusters of galaxies.

When dark matter was first discovered, astronomers considered several different types of this new material, which they classified according to how fast the individual dark matter particles zoom around us. Heavy, slow-moving particles are called “cold dark matter,” while very light particles moving at close to the speed of light are referred to as “hot dark matter.” One of the main differences between the two is that the slow speed of the cold dark matter particles allows galaxies to clump together more easily. Rapidly moving hot dark matter particles, on the other hand, act something like little electric mixers in preventing clumps of matter from accumulating. The effects that these different theoretical types of dark matter would have on the universe are still under active investigation.

The cold dark matter theory, which agrees much better with observations than hot dark matter, predicts that at least 100, and possibly as many as 500, dwarf galaxies should be in our local neighborhood. Until 2005, however, only 11 dwarf galaxies near the Milky Way had been found. My research seeks to investigate this serious disagreement, known as the “missing satellite problem,” in detail to help prove or disprove the theory.

To provide general context for this dwarf galaxy research, the Milky Way galaxy is tens of thousands of light years across, with a mass of approximately a trillion times the mass of the Sun. Dwarf galaxies in general are about one tenth the size of the Milky Way and smaller. The dwarf galaxies we know about tend to range from one millionth to one tenth the size of the Milky Way. They orbit the Galaxy in elliptical or circular patterns, just as planets orbit around the Sun. Two examples of the brightest and most massive of our local dwarf galaxies are the Large and Small Magellanic Clouds.

There have been many suggestions regarding potential causes for the discrepancy between the theory and current observations. One possibility could be that dark matter does not even exist, and instead our understanding of gravity is the problem, though most astronomers today agree that dark matter is real. A second possibility is that there are indeed hundreds of dwarf galaxies orbiting the Milky Way but nobody has managed to find them yet. The third possibility is that hundreds of low mass clumps of dark matter really are out there as the theory predicts, but for some reason most of them happen not to form any stars, so we can’t see them. Rather, they would be completely invisible clouds of dark matter, and obviously it would be very difficult to detect such objects—- or even prove that they exist. The general theoretical thinking leans toward this third possibility.

Over the last five years, the Sloan Digital Sky Survey has observed about one fifth of the sky and discovered a new population of very faint groups of stars that look like tiny dwarf galaxies. The reason that Sloan was able to find these objects is that it employed a bigger telescope and a better camera than previous sky surveys, so it could see much fainter stars. Starting in 2005, a number of teams of researchers led by Beth Willman of Harvard University and Dan Zucker and Vasily Belokurov of the University of Cambridge used this method to find 13 new candidate dwarf galaxies near the Milky Way. This series of discoveries was remarkable because over the entire history of astronomy before Sloan there were only 11 known dwarf galaxies orbiting our Galaxy! It was obvious that these newly found objects could dramatically impact the missing satellite problem, so we knew it was important to determine whether the new objects were really dwarf galaxies. To start this investigation, my collaborator Marla Geha and I traveled out to Hawaii to study these dwarf candidates with the Keck telescopes and the DEep Imaging Multi-Object Spectrograph (DEIMOS).

DEIMOS was built in 2002 for the DEEP Extragalactic Imaging Probe 2 (DEEP2) survey, and was a collaboration between astronomers at the University of California and other institutions. W. M. Keck Observatory support astronomer Greg Wirth describes what sets DEIMOS apart from spectrographs on other big telescopes.

According to Wirth, “DEIMOS is the most powerful spectrograph on the world,s leading telescope. Three things make this instrument special. First, it has a wide field of view—- over twice as large as our other spectrographs at Keck—- which allows it to observe more objects at once. Second, instead of observing objects one by one, DEIMOS uses special inserts called slitmasks to view the light from hundreds of objects at one time, multiplying the power of the telescope enormously and producing more scientific results per night. Third, DEIMOS has an image stabilization system which allows it to take spectra that are sharper than its competitors and thus measure velocities more precisely, which are crucial for the science problems we,re addressing at Keck.”

A three night observing run revealed some exciting news when we observed stars in eight newly discovered dwarf galaxy candidates. We obtained spectra of over 800 stars in total, which we used to measure with high accuracy the velocities with which the stars are moving.

These measurements enabled us to determine the velocity dispersion, or the range of velocities, that is present in each of these dwarf galaxies. The typical velocity dispersion we found is about 5 kilometers per second, which means that if you pick a random star in any of the galaxies, there is a good chance that it will have a velocity within 5 km/s of the overall average velocity. This allows us to measure the mass of the galaxies, in the same way that we use the velocities of planets moving around the Sun to figure out the Sun,s mass. Prior to this study we didn,t know whether these systems were actually dwarf galaxies or globular clusters. But their velocity dispersions tell us there must be large quantities of dark matter along with the stars in these galaxies. If they had been globular clusters, with only stars and no dark matter, the velocity dispersions would have been perhaps a few tenths of a km/s, a factor of ten or more lower than what we measured.

Our observations therefore confirmed not only that these newly discovered objects really are galaxies but also that they contain at least one hundred times as much dark matter as stars. Integrated over the entire universe there is about six times as much dark matter as normal matter, and normal galaxies like the Milky Way have a ratio of ten to twenty to one of dark matter over normal matter. So these tiny dwarf galaxies turned out to be the most dark matter-dominated galaxies that have ever been found.

By measuring the masses of these galaxies we were also able to study them in the context of the missing satellite problem. Because we could go down to a specific mass, we now know how many dwarf galaxies have at least 1 million or 10 million times the mass of the Sun, and that is exactly the quantity that is predicted by the cold dark matter theory. Then we could compare our observations on a one to one basis with the theory. What we found is that even though we doubled the number of dwarf galaxies, the total number is still smaller than what is predicted by the theory. The initial conclusion was that the missing satellite problem remains; there just do not seem to be enough satellite galaxies orbiting the Milky Way.

Based on these results, the only way that the cold dark matter theory can be correct is if there are hundreds of clouds of nearly pure dark matter surrounding our Galaxy, but we can,t see them because they have no stars in them and don,t give off any light. We therefore started looking at various scenarios proposed by theorists to explain why these low mass dark matter clumps were not able to form any stars. There were several ideas that we tested. The most important of these possibilities requires a review of the evolution of the universe since the Big Bang.

Initially, after the Big Bang everything was ionized because the universe was extremely hot. As it expanded and cooled, after about 300,000 years protons and electrons combined into neutral atoms like Hydrogen (H) and Helium (He). Over the next epoch large clouds of H and He came together and began to collapse into stars and galaxies. At some point in this process the first stars (and perhaps also massive black holes) turned on, releasing a huge amount of ultraviolet radiation and ending the so-called Dark Ages. This blast of radiation reionized the universe, heating up the gas in dwarf galaxies and thereby preventing it from becoming cool enough to form more stars.

This is our current best guess for why we don,t see hundreds of nearby dwarf galaxies. Our Milky Way has a couple dozen small dwarf galaxies surrounding it, each with some stars but mostly containing dark matter. A much larger number of completely dark clouds of dark matter are probably orbiting the Galaxy. Now the challenge will be to get definitive evidence to confirm this picture, which could come from actually identifying some of these dark, starless, galaxies. But obviously, if dark galaxies do not have any stars in them, they are going to be pretty hard to find!

Determining whether there really are hundreds of additional clouds of dark matter around the Milky Way is an extremely difficult problem for which we still do not have a definite solution. One possibility is that even though these clouds do not contain any stars, they could have some hydrogen gas in them that would be visible with radio waves. Alternatively, if there are hundreds of these objects orbiting around the Milky Way some of them will hit the disk of the Galaxy and might produce observable effects on the matter in the Milky Way. Another possibility is that there are some dwarf galaxies, most notably the Sagittarius dwarf, that are being tidally disrupted by the Milky Way. This dwarf is being torn apart into enormous streams of stars that wrap around our Galaxy. Again, if there are large numbers of small clouds of dark matter zooming around, some of the dark halos should run into the tidal streams and could distort the path that those streams are following.

Finally, one very intriguing idea is that it is possible that dark matter is able to annihilate with itself—- whenever you put matter and anti matter together they emit a flood of gamma rays. Whatever particle makes up dark matter may be able to do exactly the same thing, thus one of the exciting possibilities in the next few years will be to use the newly completed gamma ray telescopes to look for gamma rays coming from apparently blank regions of the sky. These gamma rays might allow us to see otherwise invisible clumps of dark matter.

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