Science is an endeavor that’s necessarily filled with dead ends, left turns to nowhere, detours, delays, and – fortunately – the occasional accidental discovery.
That’s exactly what happened earlier this year, when a team of astronomers serendipitously observed a population of 47 Milky Way-size, weirdo galaxies living very far from home.
Unexpectedly large and surprisingly dim, the star-poor galaxies appear to reside in the immense Coma Cluster, located roughly 320 million light-years away. It’s a region that has been observed and photographed countless times, but many of the most powerful telescopes aren’t able to see such sparsely populated objects with very low surface brightness. “You essentially kind of look through them – they’re so big and diffuse,” says Yale University astronomer Pieter van Dokkum, who described the strange galaxies in a manuscript posted to the arXiv.
Van Dokkum and his colleagues still aren’t sure exactly what’s going on with these galaxies. But the team suspects it’s seeing a population of extreme, dark matter-dominated galaxies that have somehow grown to be about the size of the Milky Way. If that is what’s going on, then ideas about how galaxies form and evolve will need to be re-jiggered a bit, since most theories suggest such dim, poorly populated galaxies should be small.
“Small galaxies can more easily get rid of their stars and gas, as they have lower gravity,” van Dokkum says. “The fact that we see these galaxies, that they still exist, means that they must have a lot of mass to hold them together.“
These Are Not the Objects You’re Looking For
The discovery took van Dokkum and his colleagues by surprise.
One day in March, the team interrupted an ongoing survey to stare at the Coma Cluster, hoping to study faint, intergalactic starlight – the illumination produced by homeless stars that have left their galaxies and now live in the intervening voids. Measuring that light helps astronomers reconstruct a cluster’s history, because it offers some clues about the ancient mergers and collisions that punted stars into intergalactic space.
For 26 hours spread over 25 nights, the team observed the 1,000-galaxy cluster using a small telescope in New Mexico called the Dragonfly Telephoto Array. The array is kind of like an insect’s compound eye, except it’s made of eight Canon telephoto lenses. It’s been optimized to detect objects with low surface brightness, such as dim dwarf galaxies and dust grains illuminated by starlight.
The team’s observations wrapped up in May, but it wasn’t until later in the summer that van Dokkum finally got around to looking at the images he’d gathered. In them, he noticed some faint, fuzzy blobs. Forty-seven blobs, to be precise. They looked rather large, and they really weren’t supposed to be there – at least, not according to the catalogues keeping track of galaxies in the cluster.
“Once you saw them, it was hard to un-see them,” van Dokkum says. “You notice one, and then another one, and then another one…”
Many Galaxies, Far, Far Away
Suspecting the objects might be clumps of galaxies too small for Dragonfly to resolve, van Dokkum and his colleagues began searching through archival images of the region shot by a telescope that should be able to parse giant blobs into smaller, constituent objects.
But the images taken by the Canada-France-Hawaii Telescope contained the same faint blotches. How bizarre, van Dokkum thought. “Most of the things that by eye had seemed like these fuzzy blobs,” he says, “Actually are fuzzy blobs. Surprise, surprise.”
But how far away were these faint galaxies? Were they in the foreground and closer to Earth, instead of in the Coma Cluster? If that were true, the population would make a lot more sense because it would fit with existing observations of dim, diffuse dwarf galaxies that live near the Milky Way.
To estimate the galaxies’ distance, the team traced the blobs’ distribution on the sky. It matched that of the Coma Cluster’s galaxies, suggesting the 47 objects were indeed a part of the cluster. Then, the team again turned to archival images — this time, those shot by Earth’s sharpest eye in the sky, the Hubble Space Telescope, which had observed the Coma Cluster many times before.
Completely by chance, Hubble had captured three of the faint galaxies. One of the images was good enough to work with.
In that image, there’s a faint, blurry galaxy hanging out with Coma’s classic inhabitants. But the galaxy’s individual stars aren’t resolved by Hubble, which suggests the blob is very far away indeed. The team simulated Hubble images taken at varying distances, and calculated that in order for the individual stars to be invisible, the galaxy needed to be at least 160 million light-years away.
Which means, van Dokkum says, the blob – and most of its friends – are probably in the Coma Cluster.
“We conclude that most or all of the low surface brightness galaxies are, in fact, at the distance of the Coma cluster and are resolved in the Dragonfly data because they are intrinsically very large,” the authors write.
Come to the Dark Side
If that’s true, calculations suggest these things are at least the size of the Milky Way, Earth’s home galaxy. And that’s a bit of a problem for scientists thinking about galaxy formation and evolution.
“Dark matter models predict lots more small dark matter halos than we actually see, so the natural explanation is that they end up as very dim/diffuse galaxies which we cannot pick up normally in optical surveys,” writes Karen Masters, an astronomer at the University of Portsmouth who studies galaxy structures and evolution. “What’s a bit surprising is how large these galaxies are – but then the environment of the Coma cluster is quite extreme, so it may be related.”
Stretching more than 100,000 light-years from end-to-end, the Milky Way is a bright, easily identifiable spiral galaxy, lit with the starlight of several hundred billion stars.
These 47 galaxies, though, contain a tiny fraction of the Milky Way’s stars. More observations are needed to discern the exact nature of these galaxies, but van Dokkum suggests the only way they could still exist is if they contain a huge amount of dark matter, roughly 98 percent of the galaxy’s mass. “That would make them among the most dark matter-dominated objects we know,” he says.
But, he notes, it’s also possible these large, diffuse galaxies have been spotted in the process of breaking apart, and that rather than being a stable population, these represent an unanticipated endpoint of galaxy evolution. There’s only been one other observation of anything even remotely resembling these galaxies. In the late 1980s, a different team of astronomers saw some diffuse, super dim galaxies in the Virgo cluster, another conglomeration of many thousands of galaxies.
Van Dokkum has a number of follow-up observations on his wish-list, some of which are possible, some of which might not be. Measuring the spectrum of the galaxies should help pin down a more accurate distance; measuring the motions of the stars in the galaxies should help determine the precise dark matter fraction. And returning to the Coma Cluster with an even more sensitive telescope might reveal more hidden galaxies, lurking near its heart.
Hubble Space Telescope close-up of one of the Coma Cluster’s beautiful spiral galaxies, NGC 4911. (NASA/ESA/Hubble Heritage Team)
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