Some of the strongest evidence for dark matter to date has been discovered – and ironically, that's thanks to its absence. In a pair of studies published this week, astronomers have shed new light on dark matter through close observation of a galaxy previously found to have very little of the stuff, while the same team found a new example of a similar oddball galaxy.
The idea that the universe is full of mysterious, invisible matter first arose in 1933. Swiss astrophysicist Fritz Zwicky noticed that a cluster of galaxies was moving faster than should be possible, based on the mass of the visible matter in them. He calculated that there had to be far more mass in the cluster than they could see, and the concept of dark matter was born.
In the decades since then, dark matter has all but been confirmed indirectly, although the actual particles themselves still elude direct detection. Last year a Yale University team discovered one of the most compelling clues to date: a galaxy with almost no dark matter at all.
It's generally believed that galaxies are held together through the gravitational influence of clumps of dark matter, so to find a galaxy with little to no dark matter was a surprise. And while it might sound like a strike against the theory, it actually ends up supporting it.
Dark matter was "invented" to plug a hole between calculations and observations, and some argue that it's our understanding of the laws of physics that needs a revision instead. But if that were the case, then this same discrepancy should apply to every galaxy. Finding a galaxy that's light on dark matter shows that it is a separate substance that can vary in amount, rather than a mathematical mistake.
Now, two new studies have backed up the earlier work. In the first, astronomers took a closer look at the galaxy, known as NGC 1052-DF2 (or DF2 for short), and clocked the speed of its stars. Sure enough, the galaxy is moving at roughly 6.5 million km/h (4 million mph), which is consistent with the expected speed based on the mass of its visible matter. If the galaxy had the usual amounts of dark matter, it would be moving much faster.
The second study found that DF2 isn't a unique anomaly. Some of the scientists behind the discovery of the first dark matter-deprived galaxy have now found a second example, dubbed DF4. This galaxy has a similar size, shape and surface brightness as the first – and of course, a similarly-slow speed for the movement of its stars.
"Discovering a second galaxy with very little to no dark matter is just as exciting as the initial discovery of DF2," says Pieter van Dokkum, an author of both studies. "This means the chances of finding more of these galaxies are now higher than we previously thought. Since we have no good ideas for how these galaxies are formed, I hope these discoveries will encourage more scientists to work on this puzzle."
The researchers plan to continue scanning the sky for more examples using the Dragonfly Telephoto Array. Once candidates have been identified, they can be studied in closer detail by telescopes at the Keck Observatory.
Both studies were published in The Astrophysical Journal.
Source: Yale University
