Enormous, Faint Intergalactic Clouds Offer a Clue to Galaxy Formation

It's hard to fathom just how big the millions of parsecs of intergalactic space that separates galaxies actually is. It's truly astounding that as our universe expands and drags our swirling galaxies apart, there's this massive fabric of near-nothing filling in the blanks. But according to new research from the National Radio Astronomy Observatory (NRAO), intergalactic space—at least in one instance—may not be as empty as we thought.

The artist render above shows two nearby galaxies, Andromeda (M31, upper right) and Triangulum (M33). The space between them was once relatively empty. But after imaging the area in high resolution using the NRAO's Green Bank Telescope (GBT), massive clouds of hydrogen gas appeared.

According to Felix Lockman, an NRAO astronomer and co-author of the report recently published in Nature, the dwarf galaxy-sized clouds are difficult to image despite their size. Now that they've been found, it raises the possibility that other fain galactic features will be discovered as well.

"What's tantalizing is that they're so faint," he said in a phone call. "They contain a lot of mass by terrestrial standards, but they're so far away. We've only just recently been able to see them, and we've only looked at a small part of the sky."

The clouds consist of neutral atomic hydrogen, which emits a known radio frequency that can be detected by radio telescopes like the GBT. Hydrogren exists in great quantities all over the universe, but finding faint clouds in intergalactic space isn't easy. The clouds between M31 and M33 have been thought to exist for some time, and were confirmed by the NRAO last year. But only now have the cloud formations been more clearly imaged. The trick, then, is to find out what it's doing there.

"These clouds are interesting because they're pretty far from the other galaxies," Lockman said. "Are they clouds that are condensing out of hot gases, or are they remnants of something from the past?"

As the authors note in their paper, galaxies must continue to suck in gases in order to create stars, which has been observed, and some of that may be from intergalactic gases. But considering how far these clouds are from neighboring galaxies, it's not clear if they may form new formations, or were spit out by old ones.

"One of the big puzzles that vexes contemporary astronomers is where galaxies come from and how they grow," Lockman said. "We see evidence that some galaxies, like the Milky Way, have captured smaller galaxies in the past. In fact, we can see that right now; we can see the Milky Way in the process of dismembering its neighbors."

No matter where they came from, it appears that the clouds remain relatively independent. The NRAO found that they're moving at a similar velocity to their neighboring galaxies, as opposed to having been flung off from one of them, and their cloud shape could even be indicative of their being held together by ever-elusive dark matter.

"These observations suggest that they are independent entities and not the far-flung suburbs of either galaxy," Lockman said in a release. "Their clustered orientation is equally compelling and may be the result of a filament of dark matter. The speculation is that a dark-matter filament, if it exists, could provide the gravitational scaffolding upon which clouds could condense from a surrounding field of hot gas."

All of the questions raised by the discovery aside, the idea that there is a lot more matter to be found in the universe is compelling. It's pretty intuitive, too. Our telescopes can only see so much, and as technology improves, they'll see stuff we've never seen before. And when it comes to the stuff galaxies are made of, every new find is exciting.

@derektmead