Ripples in gas at the outer disk of our galaxy have puzzled astronomers since they were first revealed by radio observations a decade ago. Now, astronomers believe they have found the culprit - a dwarf galaxy, containing dark, unseen material, which skimmed the outskirts of our galaxy a few hundred million years ago.

The research, led by Sukanya Chakrabarti of the Rochester Institute of Technology, presents the first plausible explanation for the galactic ripples. "It's a bit like throwing a stone into a pond and making ripples," said Chakrabarti at a conference at the 227th meeting of the American Astronomical Society in Kissimmee, Florida.

"Of course we aren't talking about a pond, but our galaxy, which is tens of thousands of light-years across, and made of stars and gas, but the result is the same - ripples!"

Chakrabarti adds that this work is part of a new discipline called galactoseismology: "This is really the first non-theoretical application of this field, where we can infer things about the unseen composition of galaxies from analyzing galaxy-quakes."

To reach their conclusion the research team studied a trio of stars, called Cepheid variables, which are part of the likely dwarf galaxy now estimated to lie about 300,000 light-years away from our galaxy in the direction of the constellation Norma.

"We have a pretty good idea of the distance to these stars because the intrinsic brightness of Cepheid variable stars depends on their period of pulsation, which we can measure," says Chakrabarti.

"But the stars' distances were already estimated before this work - what I wanted to know was how fast this speeding bullet was going when it passed by our galaxy - with that information we can begin to understand the dynamics, and ultimately how much unseen dark matter is there."

To do that, Chakrabarti and her team focused on three Cepheids in the tiny galaxy. Using spectroscopic observations obtained at the Gemini Observatory (as well as the Magellan Telescope, and the WiFeS spectrograph) the researchers found that the stars are all speeding away at similar velocities - about 450,000 mph (~200 kilometers/second).

"This really implicates these stars as being part of an organized, fast-moving system which we believe is a dwarf galaxy. It's also very likely that this dwarf satellite brushed our galaxy millions of years ago and left ripples in its wake," said Chakrabarti.

"This new, potentially powerful way to study how stars, gas and dust are distributed in galaxies is really quite exciting," said Chris Davis, program director at the U.S. National Science Foundation that funds roughly 65% of Gemini as part of its international partnership, as well as this research program.

"Known as galactoseismology, it can trace both visible and invisible materials, including the elusive dark matter. It's a great way to better understand how galaxies and neighboring satellite dwarf galaxies interact as well."

Gemini Observatory astronomer Rodolfo Angeloni oversaw the observations at the Gemini South telescope in Chile. He adds that Gemini South is uniquely well-equipped to make these types of observations.

"The combination of Gemini's silver-coated mirror and the versatility of the infrared spectrograph Flamingos-2 really made this work possible." However, he continues, "These were especially faint and remote targets - we really had to push the limits."

The team plans to continue this work by looking for more Cepheid variable stars in our galaxy's halo. "There could be a population of yet undiscovered Cepheid variables that formed from a gas-rich dwarf galaxy falling into our galaxy's halo," said Chakrabarti.

"With the capabilities of today's telescopes and instruments we should be able to sample enough of the Milky Way's halo to make reasonable estimates on dark matter content - one of the greatest mysteries in astronomy today!"

The international research team includes Rodolfo Angeloni, Ken Freeman, Leo Blitz, among others, and RIT research scientist Benjamin Sargent and Andrew Lipnicky, a graduate student in the astrophysical sciences and technology program.