A new study of six young, star-forming galaxies suggests they're less influenced by dark matter than expected. But the results may say more about galaxy evolution than about the nature of dark matter.

If only Vera Rubin had lived another year: I wonder what she would have made of the news today. Rubin and her colleague at the Carnegie Institution of Washington, Kent Ford, achieved astronomical fame when they measured the rotation of our neighbor Andromeda Galaxy 47 years ago. Their work served as a crucial piece of evidence for the existence of dark matter.

Now, in the journal Nature, Reinhard Genzel (Max Planck Institute, Garching, Germany) and colleagues report similar measurements of six distant galaxies — with a result surprisingly opposed to Rubin’s historic find.

Discovering Dark Matter

When Rubin and Ford collected spectra of ionized hydrogen in Andromeda almost half a century ago, they measured the speed of 67 gas clouds as they whirled about the galaxy’s center with far greater precision than ever before. What the astronomers found was at the time quite curious: beyond 15,000 light-years or so from the galaxy’s center, the clouds’ velocities didn’t slow down — the outermost clouds whirled just as fast as those much closer to the center. Either Andromeda Galaxy was in the midst of flying apart (not likely) or there was some additional matter in the galaxy’s outer reaches that we just couldn’t see.

This groundbreaking result, though not the first to suggest the existence of dark matter, encouraged scientists to start taking the matter seriously. And even though physicists still struggle to detect dark matter particles in the lab, astronomers have had enormous success in supporting their existence.

Since Rubin and Ford’s 1970 publication, scientists have found multiple lines of evidence for dark matter, such as the rotations of galaxies within clusters, weak gravitational lensing, and incredibly large-scale computer simulations of the distribution of galaxies in the universe. These observations suggest that galaxies and even galaxy clusters are ensconced in gigantic, massive dark matter halos, which started coming together before the stars began to shine.

That’s why the six galaxies studied by Genzel’s team proved so surprising.

Missing Halos

Genzel and colleagues observed several hundred star-forming galaxies in the distant universe (2.5 billion to 8 billion years after the Big Bang) using the European Southern Observatory’s Very Large Telescope. The galaxies are Milky Way-mass or more, which is pretty massive considering that we’re looking billions of years back in time. The galaxies are also forming 50 to 200 Suns’ worth of stars a year, a typical rate of star formation for this cosmic era.

Like Rubin and Ford, Genzel’s team measured the motion of hydrogen gas clouds. Unlike Rubin and Ford, the new measurements showed that toward the edge of six massive, star-forming galaxies, the clouds did slow down. Averaged data from 97 other (fainter) galaxies show the same result.

That’s not to say there isn’t some dark matter there — just not as much as expected. The dark matter cushions these galaxies lounge in appear to be rather threadbare.

Evolution of Galaxies and Halos

It turns out these results may say more about the path of galaxy evolution than about the nature of dark matter. In fact, computer simulations of dark matter may even have predicted what Genzel and colleagues observed.

One possibility, says Mark Swinbank (Durham University, UK), who authored an opinion piece accompanying the Nature article, is that the dark matter halos of these galaxies are still in the process of growing. But that would fundamentally change how we view galaxy evolution, where the standard picture says that the halos are largely in place before the gas and stars come together.

Another possibility is that we’re simply viewing these galaxies during a crucial era. Genzel’s team chose to observe massive, star-forming disk galaxies during “cosmic noon,” the universe’s peak in star formation. These are the ancient precursors to “red and dead” elliptical galaxies we see nearer the Milky Way, so nicknamed for their redder color and their low rates of star formation. Recent computer simulations by Adi Zolotov (The Hebrew University, Israel, and Ohio State University) and colleagues, show that virtually all such massive galaxies take the fast track toward evolution, their journey instigated by a single event.

Whether it be a merger with another galaxy or gas flows entering the galaxy from the larger cosmic web, this event triggers a burst of star formation in the galaxy’s center. As a result, massive, star-forming galaxies during this cosmic era will look a lot more compact than they actually are — “blue nuggets,” as Zolotov and colleagues refer to them. So measuring nuggets’ rotation won’t reveal the full dark matter halo around them, because observations would only cover the parts of the galaxies that are dominated by normal matter.

“[Genzel and colleagues’] declining rotation curves in massive star-forming galaxies are just what the high-resolution zoom-in galaxy simulations by my collaborators and I predicted,” says Joel Primack (University of California, Santa Cruz), a coauthor on Zolotov’s paper.

A Matter of Resolution

It’s worth noting that other simulations, such as Illustris and Eagle, don’t make the same prediction, but Primack points out that this could be due their fuzzier view. Simulating an entire universe is a battle between resolution, volume, and time covered versus computing time. While the Illustris and Eagle simulations can see elements down to 3,000 light-years across (they can’t make out star formation regions, for example), the more computationally expensive simulations that Primack and Zolotov are involved in can see details as fine as 60 light-years.

“Both are useful,” Primack says, “but to find out what’s really going on inside these galaxies, you really have to simulate these high-resolution environments.”