Ripples in galaxy help locate dark-matter satellites of Milky Way

Three bright pulsating stars on the outskirts of the Milky Way galaxy could be beacons from an invisible dwarf galaxy that astronomers predicted was there based on its effects on the gas in our galaxy.

Supercomputer simulation of the collision of a dwarf, dark-matter galaxy with the Milky Way over the course of the last billion years. The collision created ripples in the hydrogen gas (left, blue) that were used to pinpoint the location of the dwarf galaxy today. The Milky Way’s stars (right, colored by density) also were affected by the collision. Sukanya Chakrabarti video, edited by Roxanne Makasdjian and Stephen McNally.

The prediction was the first to come out of the new field of galactoseismology, which uses ripples in the distribution of hydrogen gas in the plane of the Milky Way to infer the presence of invisible satellite galaxies, thousands of which may be buzzing around or through the Milky Way. The technique was pioneered by former UC Berkeley postdoctoral fellow Sukanya Chakrabarti, now an assistant professor of astronomy at the Rochester Institute of Technology, and her UC Berkeley mentor, Leo Blitz, a professor of astronomy.

Chakrabarti discussed the stars, so-called Cepheid variables, and their connection to the predicted galaxy at a media briefing on Jan. 7 at the American Astronomical Society meeting in Kissimmee, Florida. She is lead author of a paper about the Cepheid variables submitted to Astrophysical Journal Letters.

While some of the Milky Way’s unseen satellite galaxies are hidden from view by dust, many are invisible because they’re composed mostly of dark matter, a so-far mysterious substance that dominates the matter in the universe: 85 percent of all matter in the universe is dark matter. Where it concentrates, normal matter – mostly gas – congregates and condenses into stars and galaxies that can be seen. While the normal matter in the Milky Way is large enough to produce hundreds of billions of bright stars, however, the normal matter in dark matter-dominated galaxies is apparently too small to produce enough stars to be visible over large distances.

Chakrabarti thought of looking for the effects these galaxies have on the gas distribution in the galaxy, and using this to pinpoint their location. Just as seismologists analyze waves traveling through the earth to infer properties of our planet’s interior, she uses waves in the galactic disk to map the interior structure and mass of galaxies.

“We have made significant progress into this new field of galactoseismology, whereby you can infer the dark matter content of dwarf galaxies, where they are, as well as properties of the interior of galaxies by looking at observable disturbances in the gas disk,” she said.

Cepheid variables as yardsticks

In 2009, Chakrabarti and Blitz used these techniques to predict the existence of a dwarf satellite galaxy in the direction of the constellation Norma, and last year she and her team used the Gemini South Telescope in Chile and Magellan telescopes to search for stars in that region that might be part of the galaxy. They found three pulsating stars called Cepheid variables, typically used as yardsticks to measure distance, that are at approximately the same distance from the sun: 300,000 light-years.

stars and gas in Milky Way shortly after interaction with dwarf galaxy

Simulation of the distribution of stars (top) and gas in the Milky Way galaxy just after the dwarf satellite (lower left) perturbed it. Sukanya Chakrabarti image.

Using spectroscopic analysis, they were able to show that the stars also have about the same velocity and that they are moving too fast to be part of our galaxy. They are racing away from the center of the galaxy at 450,000 miles per hour (200 kilometers per second), whereas the average Milky Way star has a radial velocity of only about 25,000 miles per hour (12 kilometers per second).

Most likely, these stars mark the location of a dark matter-dominated dwarf galaxy, Chakrabarti said, far beyond the edge of the Milky Way disk, which terminates at 60,000 light-years.

“The radial velocity of the Cepheid variables is the last piece of evidence that we’ve been looking for,” she said. “You can immediately conclude that they are not part of our galaxy.”

“These observations basically confirm that the galaxy Sukanya predicted but couldn’t see is there,” Blitz said.

Chakrabarti’s team also included Rodolfo Angeloni of the Gemini South Telescope; Ken Freeman of the Mount Stromlo Observatory of the Australian National University in Canberra; former UC Berkeley postdoc Josh Simons, now of the Carnegie Institution of Washingto; RIT research scientist Benjamin Sargent; and RIT graduate student Andrew Lipnicky.

For more detail, link to the Gemini Observatory press release and an earlier UC Berkeley story about Chakrabarti’s prediction technique.

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