July 27 2014
It’s in the room with you now. It’s more subtle than the surveillance state, more transparent than air, more pervasive than light. We may not be aware of the dark matter around us (at least without the ingestion of strong hallucinogens), but it’s there nevertheless.
Composite image of X-ray (pink) and weak gravitational lensing (blue) of the famous Bullet Cluster of galaxies.
X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
Although we can’t see dark matter, we know a bit about how much there is and where it’s located. Measurement of the cosmic microwave background shows that 80 percent of the total mass of the Universe is made of dark matter, but this can’t tell us exactly where that matter is distributed. From theoretical considerations, we expect some regions—the cosmic voids—to have little or none of the stuff, while the central regions of galaxies have high density. As with so many things involving dark matter, though, it’s hard to pin down the details.
CMB per ESA/Planck
Unlike ordinary matter, we can’t see where dark matter is by using the light it emits or absorbs. Astronomers can only map dark matter’s distribution using its gravitational effects. That’s especially complicated in the denser parts of galaxies, where the chaotic stew of gas, stars, and other forms of ordinary matter can mask or mimic the presence of dark matter. Even in the galactic suburbs or intergalactic space, dark matter’s transparency to all forms of light makes it hard to locate with precision.
Despite that difficulty, astronomers are making significant progress. While individual galaxies are messy, analyzing surveys of huge numbers of them can provide a gravitational map of the cosmos. Astronomers also hope to overcome the messiness of galaxies and estimate how much dark matter must be in the central regions using careful observation of the motion of stars and gas.
There’s also been a tantalizing hint of dark matter particles themselves in the form of a signal that may come from their annihilation near the center of the Milky Way. If this is borne out by other observations, it could constrain dark matter’s properties while avoiding messy gravitational considerations. Adding it all up, it’s a promising time for mapping the location of dark matter, even as researchers still build particle detectors to identify what it is.
A (very) brief history of dark matter
In the 1930s, Fritz Zwicky measured the motion of galaxies within the Coma galaxy cluster. Based on simple gravitational calculations, he found that they shouldn’t move as they did unless the cluster contained a lot more mass than he could see. As it turned out, Zwicky’s estimates of how much matter there was were too large by a huge factor. Still, he was correct in the broader picture: more than 80 percent of a galaxy cluster’s mass isn’t in the form of atoms.
Zwicky’s work didn’t get a lot of attention at the time, but Vera Rubin’s later observations of spiral galaxies were another matter. She found that the combined stars and gas had too little mass to explain the rotation rates she measured. Between Rubin’s work and subsequent measurements, astronomers established that every spiral galaxy is engulfed by a roughly spherical halo (as it is called) of matter—matter that’s transparent to every form of light.
The Bullet Cluster
That leads us to the “Bullet Cluster,” one of the most important systems in astronomy.
X-ray photo by Chandra X-ray Observatory of the Bullet Cluster (1E0657-56). Exposure time was 0.5 million seconds (~140 hours) and the scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3. Based on today’s theories this shows the cluster to be about 4 billion light years away. In this photograph, a rapidly moving galaxy cluster with a shock wave trailing behind it seems to have hit another cluster at high speed. The gases collide, and gravitational fields of the stars and galaxies interact. When the galaxies collided, based on black-body temperature readings, the temperature reached 160 million degrees and X-rays were emitted in great intensity, claiming title of the hottest known galactic cluster. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter.
First described in 2006, it’s actually a pair of galaxy clusters observed in the act of colliding. Researchers mapped it in visible and X-ray light, finding that it consists of two clumps of galaxies. But it’s the stuff they couldn’t image directly that ensured the Bullet Cluster is rightfully cited as one of the best pieces of evidence for dark matter’s existence (the title of the paper announcing the discovery even calls it “direct empirical proof”).
Galaxy clusters are the biggest individual objects in the Universe. They can contain thousands of galaxies bound to each other by mutual gravity. However, the stuff within those galaxies—stars, gas, dust—is outweighed by an extremely hot, gaseous plasma between them, which shines brightly in X-rays. In the Bullet Cluster, the collision between the two clusters created a shock wave in the plasma (the shape of this shock wave gives the structure its name).
More dramatically, though, the astronomers who described the cluster used gravitational lensing—the distortion of light from more distant galaxies by the mass within the cluster—to map the distribution of most of the material in the Bullet Cluster. That method is known as “weak gravitational lensing.” Unlike the sexier strong lensing, weak lensing doesn’t create multiple images of the more distant galaxies. Instead, it slightly warps the light from background objects in a small but measurable way, depending on the amount and concentration of mass in the “lens”—in this case, the cluster.
Astronomers found the shocked plasma, which represents most of the mass of the Bullet Cluster, was almost entirely in the region between the two clusters, separated from the galaxies. However, the mass was largely concentrated around the galaxies themselves. This enabled a clear, independent measurement of the amount of dark matter, separate from the mass of the gas.
The results also confirmed some predictions about the behavior of dark matter. Thanks to the shock of the collision, the plasma stayed in the region between the two clusters. Since the dark matter doesn’t interact much with either itself or normal matter, it passed right through the collision without any noticeable change.
It’s a phenomenal discovery, but it’s only one galaxy cluster, and that ain’t enough. Science is inherently greedy for evidence (as it should be). A single example of anything tells us very little in a Universe full of possibilities. We want to know if dark matter always clusters around galaxies or if it can be more widely dispersed. We want to know where all the dark matter is, in all galaxy clusters and beyond, throughout the entire cosmos.
A dark matter census
Weak gravitational lensing provides a method to search for dark matter in other galaxy clusters, too, as well as even larger and smaller structures. Princeton University astronomers Neta Bahcall and Andrea Kulier took a weak lensing census of 132,473 galaxy groups and clusters, all within a well-defined patch of the sky but at a range of distances from the Milky Way. (“Groups” are smaller associations of galaxies; for example, the Milky Way is the second largest galaxy in the Local Group, after the Andromeda galaxy.) While individual galaxy clusters usually can’t tell us much, a large sample allowed the astronomers to treat the problem statistically—weak lensing effects that were too small to spot for a single cluster became obvious when looking at hundreds of thousands.
For example, a typical quantity used in studying galaxies is the mass-to-light ratio. To measure this statistically, Bahcall and Kulier looked at the cumulative amount of light (mostly emitted by stars) and weak lensing (mostly from dark matter), starting from the centers of each cluster and working outward. They found something intriguing: the amount of mass and light increased in tandem and then leveled off together. That means neither the dark matter nor the light extends farther than the other: the stars inside these groups and clusters were a very good tracer for the dark matter. That’s surprising because stars are typically less than two percent of the mass in a cluster, with the balance of ordinary matter made up by gas and dust.
As Kulier told Ars, “The total amount of dark matter in galaxy groups and clusters might be accounted for entirely by the amount of dark matter in the halos of their constituent galaxies.” That’s an average result, though; the details could look quite different. “This does not necessarily imply that the halos are still ‘attached’ to the galaxies,” Kulier said. In other words, when galaxies came together to form clusters, the stronger forces acting on galaxies and their stars could in principle separate them from their dark matter but leave everything inside the cluster thanks to mutual gravity.
Kulier pointed out that these results provide strong support for the “hierarchical” model of structure formation: “smaller structures collapse earlier than larger ones, so that galaxies form first and then merge together to form larger structures like clusters.” The Bullet Cluster is an archetypical example of this, but things could be otherwise. For instance, dark matter could have ended up in the center of clusters, separate from the galaxies and their individual halos.
But that’s not what astronomers see. In their analysis, Bahcall and Kulier also calculated that the total ratio of dark matter to ordinary matter in galaxy clusters matches that of the Universe as a whole. That’s another strong piece of evidence in favor of the standard model in cosmology: maybe most of the dark matter everywhere is in galactic halos.
Every galaxy wears a halo
Computer reconstruction of the location of mass in terms of how it affects the image of distant galaxies through weak lensing.
S. Colombi (IAP), CFHT Team
So what about the halos themselves and the galaxies that wear them? Historically, dark matter was first recognized for its role in spiral galaxies. However, it’s one thing to say that dark matter is present. It’s another to map out where it is—especially in the dense, star-choked inner parts of galaxies.
Spiral galaxies consist of three basic parts: the disk, the bulge, and the halo. The disk is a thin region containing the spiral arms and most of the bright stars. The bulge is the central, densest part, with large populations of older stars and (at its very heart) a supermassive black hole. The halo is a more or less spherical region containing a smattering of stars; it envelops the other regions, extending several times beyond the limit of the disk. To provide an example, the Milky Way’s disk is about 100,000 light-years in diameter, but its halo is between 300,000 to 1 million light-years across.
Because of the relative sizes of the different regions, most of a galaxy’s dark matter is in the halo. Relatively little is in the disk; Jo Bovy and Scott Tremaine showed that the disk and halo contain less than the equivalent mass of 100 Earths in a cube one light-year across. That may sound like a lot, but Earth isn’t that large, and a light-year defines a big volume. That amount isn’t enough to affect the Sun’s orbit around the galactic center strongly. (It’s still enough for a few particles to drift through detectors like LUX, though.)
By contrast, the amount of dark matter increases toward the galaxy’s center, so the density should be much higher in the bulge than anywhere else. For that reason, a number of astronomers look to the central part of the Milky Way for indications of dark matter annihilation, which (under some models) would produce gamma rays. This would occur if dark matter particles are their own antimatter partners, so that their (very rare) collisions result in mutual destruction and some high-energy photons. This winter, a group of researchers announced a possible detection of excess gamma rays originating in the Milky Way’s core, based on data from the orbiting [NASA] Fermi gamma ray observatory.
However, the bulge also has the highest density of stars, making it a tangled mess. Many things in that region could produce an excess of gamma rays. As University of Melbourne cosmologist Katherine Mack told me, “The Galactic Center is a really messy place, and the analysis of the signal is complicated. It’ll take a lot to show that the signal has to be dark matter annihilation rather than some un-accounted-for astrophysical source.” We can’t rule out the possibility of dark matter annihilation, but it’s definitely too soon to break out the champagne.
The difference between the ease of calculating an average density and detecting the presence of dark matter is illustrative of the general problem with mapping dark matter inside galaxies. It’s relatively simple to put limits on how much there is in the disk, since that’s a small fraction of the total volume of a galaxy. The tougher questions include how steeply the density falls off from the galactic center, how far the halo actually extends, and how lumpy the halo is.
For instance, our galaxy’s halo is big enough to encompass its satellite galaxies, including the Magellanic Clouds and a host of smaller objects. But these galaxies also have their own halos in accordance with the hierarchical model. Because they’re denser dark matter lumps inside the Milky Way’s larger halo, the satellites’ halos create a substructure.
Our dark matter models predict how much substructure should be present. However, dwarf galaxies are very faint, so astronomers have difficulty determining if there are enough of them to account for all the predicted substructure. This is known as the “missing satellite problem,” but many astronomers suspect the problem will evaporate as they get better at finding these faint objects.
A hopeful conclusion
So where is the dark matter? Based on both theory and observation, it looks like most of it is in galactic halos. Surveys using weak gravitational lensing are ongoing, with many more planned for the future. These surveys will show where most of the mass in the Universe is located in unprecedented detail.
How dark matter is distributed within those halos is still a bit mysterious, but there are several hopeful approaches. By looking for “dark galaxies”—small satellites with few stars but high dark matter concentrations—astronomers can determine the substructure within larger halos. The [ESA]Gaia mission is working to produce a three-dimensional map of a billion stars and their motions, which will provide information about the structure of the Milky Way and its surrounding satellites. That in turn will allow researchers to work backward, determining the gravitational field dictating the motion of these stars. With that data in hand, we should have a good map of the dark matter in many regions that are currently difficult to study.
Dark matter may be subtle and invisible, but we’re much closer than ever to knowing exactly where it hides.
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