Ultra-diffuse galaxies: a 'Rosetta Stone' for galaxy formation?

Astronomers report finding nearly 1,000 'dark' galaxies at the heart of the Coma Cluster. Once dismissed as uninteresting, these 'tiny, fuzzy blobs' were, in fact, large galaxies some 321 million light-years away.

Dr. Hideaki Fujiwara/Subaru Telescope/NAOJ
The International Space Station passing over the Subaru Telescope in this photo provided by the National Astronomical Observatory of Japan (NAOJ). Japan's Subaru Observatory and the twin Keck observatory domes line a ridge atop Mauna Kea, Hawaii, widely considered to be the best spot on the planet for ground-based astronomy.

For nearly 30 years, a small but growing collection of incredibly faint, diffuse collections of stars have been classified as uninteresting outcasts in the grand sweep of objects filling large clusters of galaxies.

No longer. Astronomers have identified nearly 1,000 of these objects within an enormous galaxy cluster some 321 million light-years away.

The researchers have dubbed these ultra-diffuse galaxies. Taken together, they could prove to be "a Rosetta Stone for galaxy formation," says Jin Koda, an astronomer at Stony Brook University in Stony Brook, N.Y., and the lead author of a paper announcing the discovery of 854 of these in the Coma Cluster.

Many of these galactic Wiffle balls are as large as the Milky Way. The galaxies are devoid of gas, the raw material for new stars. The stars they host appear to be as ancient as the old-timers in the globular star clusters around the Milky Way. And they hold far fewer stars.

Stars in these galaxies are spaced about 30 light-years apart on average, Dr. Koda estimates. In the Milky Way, roughly 1,000 stars would fall within 30 light-years of any single star.

The discovery of such large numbers of these galaxies, published today in the journal Astrophysical Journal Letters, follows close on the heels of results published in May announcing the identification of 47 of them by a team led by Yale University astronomer Pieter van Dokkum.

Other researchers studying faint objects in galaxy clusters actually had detected these, Dr. van Dokkum explains. But they were larger than expected.

"They thought an object this large cannot be real galaxies, certainly not real galaxies at the distance of the Coma Cluster," he says. Galaxies that dim would be tiny, the thinking went. Anything as large as the ultra-diffuse galaxies either would be a foreground object or a glitch in the images. So researchers cut the galaxies out of their catalogs of objects in the cluster.

Van Dokkum and colleagues detected them as well, using a collection of 10 professional-grade 400-millimeter telephoto lenses – assembled in the researchers' spare time – to act like an insect's compound eye. Van Dokkum and colleagues, too, were looking for very faint light within the Coma Cluster.

In this case, they were trying to measure the collective, faint light given off from stars that populate intergalactic space within the cluster. The galaxies shed the stars as they move through the cluster. Galaxies in clusters don't shine as brightly as theory predicts. A diffuse glow from intergalactic stars might explain the discrepancy.

To van Dokkum and colleagues, the ultra-diffuse galaxies "were tiny, fuzzy blobs" that taxed the detection limits of their "Dragonfly Telephoto Array."

They mined past observations of the cluster made using data from the 3.6-meter Canada-France-Hawaii Telescope on Hawaii's Mauna Kea, as well as a Hubble Space Telescope image, to show that their tiny blobs and these overlooked galaxies were the same thing. They confirmed that the objects belonged to the Coma Cluster, using the 10-meter Keck Telescope, on the same mountain.

The new specimens, uncovered from archived images of the Coma Cluster taken using Japan's 8.2-meter Subaru Telescope, also on Hawaii's Mauna Kea, "are just beautiful big galaxies," he says. "They are quite obvious."

Indeed, van Dokkum's team's work inspired Koda and colleagues to expand the search, Koda says.

For now, Koda's team classifies the galaxies it's found as "candidates." He says the team needs to measure the motions of the stars to estimate the amount of dark matter holding the galaxies together. Dark matter is the scaffolding that frames the structure of the cosmos – from individual galaxies and galaxy clusters to vast filaments and sheets of galaxies.

No one has directly detected dark matter yet. Astronomers infer its presence by its gravitational influence on matter researchers can detect. 

Measuring the stellar motions in these galaxies likely will have to wait for an new generation of larger ground-based telescopes, currently under construction.

Assuming these are galaxies, the amount of dark matter that keeps them intact must be enormous, Koda says, because the galaxies are so large and their stars so diffuse.

Indeed, that presents a mystery, he suggests. If dark matter's gravitational tug is large enough to hold such large, diffuse galaxies together, what accounts for the galaxies' missing gas? Why wouldn't gravity have kept the gas needed to form stars corralled as well?

It's likely that the movement of these galaxies through the cluster could do the job, van Dokkum offers. The space between galaxies is filled with hot gas. As galaxies move around in the cluster, they plow through this hot gas, which heats the cold gas within galaxies and strips it away. Over time this reduces a galaxy's reserve of gas available for star formation. The process is evident in gas tails scientists have detected trailing spiral galaxies moving around in clusters.

One question with intriguing implications, van Dokkum says: What would these diffuse galaxies have looked like if they hadn't been part of a cluster?

If they would have evolved into something like the Milky Way, rather than a dwarf galaxy of some sort, that would suggest that they have the same content of dark matter as the Milky Way.

"That's a very exciting idea," he says, because it could challenge one of the pillars of the theory behind galaxy evolution. A central tenet of galaxy formation is that "as we count the number of stars, we can predict the amount of dark matter" holding the galaxy together, he explains.

"If you have two galaxies, the Milky Way and one of these things, which have a very different number of stars but the same amount of dark matter, that simple idea doesn't hold."

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