Jul 11, 2015
Xaq Rzetelny
About 321 million light-years away from us is the Coma Cluster, a massive grouping of more than 1,000 galaxies.
A Sloan Digital Sky Survey/Spitzer Space Telescope mosaic of the Coma Cluster in long-wavelength infrared (red), short-wavelength infrared (green), and visible light. The many faint green smudges are dwarf galaxies in the cluster.
Credit: NASA/JPL-Caltech/GSFC/SDSS
Some of its galaxies are a little unusual, however: they’re incredibly dim. So dim, in fact, that they have earned the title of “Ultra-Dark Galaxies” (UDGs). (The term is actually “Ultra-Diffuse Galaxies”, as their visible matter is thinly spread, though “ultra-dark” has been used by some sources and, let’s face it, sounds a lot better). This was discovered earlier this year in a study that identified 47 such galaxies.
Dimness isn’t necessarily unusual in a galaxy. Most of a galaxy’s light comes from its stars, so the smaller a galaxy is (and thus the fewer stars it has), the dimmer it will be. We’ve found many dwarf galaxies that are significantly dimmer than their larger cousins.
What was so unusual about these 47 is that they’re not small enough to account for their dimness. In fact, many of them are roughly the size of our own Milky Way (ranging in diameter from 1.5 to 4.6 kiloparsecs, compared with the Milky Way’s roughly 3.6) but have only roughly one thousandth of the Milky Way’s stars. The authors of the recent study interpret this to mean that these galaxies must be even more dominated by dark matter than are ordinary galaxies.
Finding the dark
Intrigued by this tantalizing observation, a group of researchers constructed a more detailed study. Using archival data from the 8.2-meter Subaru telescope, they examined the sky region in question and discovered more UDGs—854 of them. Given that the images they were working with don’t cover the full cluster, the researchers estimated that there should be roughly 1,000 UDGs visible in the cluster altogether.
NAOJ/Subaru
There are a lot of small caveats to this conclusion. First of all, it’s not certain that all these galaxies are actually in the Coma Cluster, as some might just be along the same line of sight. However, it’s very likely that most of them do lie within the cluster. If the UDGs aren’t part of the cluster, then they’re probably a typical sample of what we’d observe in any patch of sky the same size as the Subaru observation. If that’s true, then the Universe has an absurdly high number of UDGs, and we should have seen more of them already.
In this particular patch of sky, the concentration of UDGs is stronger towards the center of the Coma Cluster. While that doesn’t prove they’re part of the cluster, it’s strongly suggestive.
Dark tug-of-war
The dim galaxies’ relationship to the cluster probably has something to do with the mechanism that made the UDGs so dark in the first place. These galaxies would have had an ample supply of gas with which to make stars, so something must have prevented that from happening. This could be because the gas was somehow stripped from its galaxy or because something cut off a supply of gas from elsewhere.
The dense environment in the cluster might be responsible for this. Gravitational interactions can pull the galaxies apart or strip them of their gas. These encounters can also deplete the gas near the galaxies, cutting off the inflow of new material. Since there are plenty of galaxies swirling around in the dense cluster, there are plenty of opportunities for this to happen to an unfortunate galaxy. The victims of these vampiric attacks might become dark, losing their ability to form stars. Neither living nor dead, these bodies still roam the Universe, perhaps waiting to strip unsuspecting galaxies of their gas.
But unlike those bitten by movie vampires, the galaxies have a way to fight back. Rather than letting their blood (or in this case gas) get sucked away, the galaxy’s own gravity can hang onto it. And since most of a galaxy’s mass comes in the form of dark matter, the mysterious substance is pretty important in the tug-of-war over the galaxy’s star-forming material. The more dark matter a galaxy’s got, the more likely it will be able to hold onto its material when other galaxies pass by.
“We believe that something invisible must be protecting the fragile star systems of these galaxies, something with a high mass,” said Jin Koda, an astrophysicist with Stony Brook University and the paper’s lead author. “That ‘something’ is very likely an excessive amount of dark matter.”
The role dark matter plays in this struggle is useful for researchers here on Earth. If they want to find out how much dark matter one of these UDGs has, all they have to do is look at how much material the galaxy has held onto. While the results of an encounter between galaxies are complicated and dependent on many factors, this technique can at least give them a rough idea.
Close to the core
Near the core of the Coma Cluster, there’s a higher density of galaxies, and so many more opportunities for galaxies to lose their gas in encounters. Tidal forces are much stronger there, and as such it takes more dark matter to continue to hold onto material.
The earlier study’s smaller sample of UDGs didn’t see any of them very close to the core, and it seemed safe to assume any potential UDGs deeper in had been ripped apart entirely. That provided a clue as to the amount of dark matter these galaxies contain: not enough to hold them together in the core. The authors of that study used this information to put an upper limit on the percentage of dark matter in the UDGs, but it was very high—up to 98 percent. But even galaxies with 98 percent dark matter shouldn’t survive in the rough center of the cluster.
Thus, in the new study, researchers didn’t expect to find UDGs any closer to the core. But they did. These galaxies are less clearly resolved because, in the cluster’s center, more interference from background objects mucks up the view. But assuming they have been correctly identified, they’ve got even more dark matter than the previous estimate: greater than 99 percent. There can be no doubt these UDGs live up to their (unofficial) name, as everything else the UDG includes—stars, black holes, planets, gas—make up less than one percent of the galaxy’s mass.
Into the dark
The discovery of so many dark galaxies in the Coma Cluster is a stride forward in the exploration of these objects. (Note: some of the objects included in the study had been previously discovered and were included in galaxy catalogs, but they were inconsistently classified, with many of them not identified as UDGs at all). The study’s large sample size compared strengthens its conclusions and also provides a more detailed picture of how these dark galaxies come to be.
Many questions remain for future work to address, however. It’s still not known exactly how many of the objects identified in the study are actually part of the Coma Cluster, though it is likely that most are. Another question is whether the Coma Cluster’s UDG distribution is typical of other clusters, which will determine how well the findings of this study can be extrapolated elsewhere in the Universe. Modeling should also provide a more detailed look into the complex interactions of galaxies in the cluster, including the exact mechanisms responsible for the creation of UDGs.
And crucially, UDGs offer an excellent opportunity to observe and study dark matter. Situations like this one, where dark matter’s interactions with baryonic (ordinary) matter can be observed, are ripe for study.
“This discovery of dark galaxies may be the tip of the iceberg,” said Dr. Koda. “We may find more if we look for fainter galaxies embedded in a large amount of dark matter, with the Subaru Telescope and additional observations may expose this hidden side of the Universe.”
The Astrophysical Journal Letters, 2015. DOI: 10.1088/2041-8205/807/1/L2 (About DOIs)
Suprisingly, the institution responsible for this research is not named, nor are we given the names of the team members and their affiliations.
See the full article here.
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