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A panorama of the Milky Way, which is orbited by dozens of dwarf galaxies — some of which are essentially invisible. (ESO/S.Brunier)

Dwarf Galaxy’s Close Encounter Shakes Up the Milky Way

As it hurtled through space around 300 million years ago, a small, dark matter-dominated galaxy brushed by the Milky Way. It wasn’t exactly a head-on collision, but it shook the gas in the Milky Way’s massive disk, leaving a telltale pattern of ripples at its edges. Today, the dark dwarf is continuing to speed through space at 450,000 miles per hour.

“This thing is really zipping by,” says Sukanya Chakrabarti of the Rochester Institute of Technology, who discussed the work last week at a conference of the American Astronomical Society. “It perturbed our galaxy, exciting ripples that we see now in the gas disk.”

Known as the Norma dwarf, this little galaxy is one of many that buzz around our glittering spiral of stars, some of which are among the most dark matter-dominated objects in the universe.

For years, Chakrabarti has been studying the distribution of dark matter and looking for the fingerprints it can leave on galaxies. Dark matter is a dense, mysterious substance that makes up most of the mass in our universe but doesn’t interact with anything ordinary. That makes it hard to see and detect. Because they can’t see it directly, scientists trying to characterize dark matter in the cosmos have to hunt for its signatures on surrounding, observable matter – and in 2009, Chakrabarti and her colleagues thought they found one of those.

What the team zeroed in on was a set of ripples in our galaxy’s gassy hydrogen disk that had stumped astronomers since their discovery a decade ago. By studying the movement, size and orientation of the warping, Chakrabarti and her colleagues deduced that the culprit may have been a dense, dark dwarf galaxy (sometimes referred to as “Galaxy X”). Their calculations suggested it could contain as much as one percent of the mass of the Milky Way (making it among the most massive of the Milky Way’s dwarf satellites), and be in the direction of the constellation Norma.

So the question was: Where is that small galaxy? And could the team find it?

As one might expect, searching for something you mostly can’t see in a bottomless sky of stars is not an easy task. But last year, Chakrabarti and her colleagues spotted three, pulsing stars that appeared to be zooming away at a remarkable pace. Known as Cepheid variables, these are the kind of stars astronomers use to determine the distances of relatively nearby objects – they have a periodic dimming and brightening that can be used to determine their absolute brightness, and thus their distance.

When Chakrabarti studied the trio, she learned they were about 300,000 light-years from the Milky Way’s core, and were hurtling outward roughly twenty times faster than the average star ambles through the galaxy. Because stars rarely fly through space on their own – let alone in trios – she suspects the Cepheids are embedded in a small, dark matter-dominated dwarf galaxy – just the kind of object that may have jiggled our galaxy as it flew by 300 million years ago.

Chakrabarti and others are hoping that such observations will open up the field of galactoseismology, where the features in galactic disks can be used to detect unseen dark matter.

“Much in the same way as seismologists analyze earthquakes to map out the Earth’s interior, we should be able to analyze the observed disturbances in galactic disks to map out the unseen material in galaxies that is their dark matter content,” she says.

Larry Widrow, of Queen’s University in Kingston, Ontario, concurs. He says it’s possible for unseen dwarf galaxies to trigger all kinds of changes in their massive hosts, perhaps producing bars and spiral arms, as well as disturbances that are perpendicular to the galactic plane. “It may be that the passage of dwarf galaxies through the disk are responsible for a wide range of disk phenomena,” he says.

So, just as galaxy quakes helped scientists find Galaxy X, they could also point to Galaxies Y and Z.

But these ripples don’t stick around forever, though – 300 million years might seem like a lifetime, but it’s barely more than one galactic year. The Milky Way only rotates around itself once every 250 million years or so, and in the not-too-distant future, that cosmic churning will erase the fingerprints left by the Norma dwarf’s close encounter.