Wednesday, September 17, 2014

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Astronomers Identify Stellar Patrons of Milky Way Bar

Forget the restaurant at the end of the Universe – astronomers now have the clearest picture yet of the bar at the center of the Milky Way.

University of Virginia scientists associated with the Sloan Digital Sky Survey III, or SDSS-III, have announced the discovery of hundreds of stars rapidly moving together in long, looping orbits around the center of our galaxy.

“The best explanation for their orbits is that these stars are part of the Milky Way bar,” said David Nidever, a U.Va. Ph.D. astronomy alumnus and recent postdoctoral researcher who now is a Dean B. McLaughin Fellow in astronomy at the University of Michigan. “Studying the bar is a key piece of the puzzle to understanding the whole galaxy, even out here in the spiral arms.”

Nidever conducted much of the research for this study while at U.Va. working with U.Va. co-investigators Gail Zasowski, Steven Majewski, Rachael Beaton, John Wilson, Michael Skrutskie, Robert O’Connell, Ana Elia Garcia Perez and Fred Hearty in the College of Arts & Sciences.

The discovery came from measuring the speeds of thousands of stars near the center of the Milky Way. The center of our galaxy is 30,000 light-years away – relatively close by cosmic standards – but astronomers know surprisingly little about it, because the galaxy's dusty disk hides it from view. In spite of this blind spot, though, scientists do know a key fact about the galaxy: Like many spiral galaxies, the Milky Way has a 'bar' of stars that move together in long, thin orbits in the inner part of the galaxy.

“We know of the bar's existence from many separate lines of evidence,” said Zasowski, a National Science Foundation postdoctoral fellow at The Ohio State University and a recent U.Va. Ph.D. graduate in astronomy who also worked on the study while at U.Va. “What we don't know is which stars are part of the bar, and what the velocities of those stars are. That will help us understand how the bar formed, and how its stars relate to the stars in the rest of the galaxy.”

The trouble is that there is no obvious way to tell a star in the Milky Way's bar apart from any other star in that neighborhood. Instead, the key to finding “bar stars” is to measure the velocities of many stars, then see whether some of those stars are moving together in some unusual pattern. Although interstellar dust blocks nearly all visible light, longer infrared wavelengths can partially shine through. So a survey of stellar positions and velocities that operates in infrared light could finally pierce the veil of dust and collect data from enough stars in the innermost Milky Way to firmly identify which are part of the bar.

 

Enter SDSS-III's new Apache Point Galactic Evolution Experiment, or APOGEE. It uses a custom-built, high-resolution infrared spectrograph built in the U.Va. astronomy labs and attached in April 2011 to the 2.5-meter Sloan Foundation Telescope in New Mexico. The instrument is capable of measuring the velocities and chemical compositions of up to 300 stars at once. APOGEE began observations in June 2011 and has since observed 48,000 stars all over the Milky Way.

“What separates the APOGEE survey from previous surveys is that we are studying the galaxy using infrared light,” Nidever said.

In a paper published recently in the Astrophysical Journal, a worldwide team of scientists, including the U.Va. investigators, used data from the first few months of APOGEE observations to measure the relative velocities for nearly 5,000 stars near the galactic center. With these velocity measurements, they assembled a picture of how these stars orbit the center of the Milky Way.

They found an unexpectedly large number of stars moving quickly away from our solar system – about 10 percent of the total stars in their sample are moving at more than 400,000 miles per hour. The observed pattern of these fast stars is similar in many different parts of the inner galaxy, and is the same above and below the disk of the galaxy – suggesting that these measurements of fast central stars are not just a statistical fluke, but really are a feature of the Milky Way.

 

The team then compared its observations with the predictions of bar stars from the latest computer models of the galaxy, and the observations matched the predictions closely. “Based on the evidence from the model comparisons, I am now confident that these stars are part of the bar,” Nidever said. “I was actually quite surprised by this result.”

APOGEE's identification of which stars are part of the bar will allow astronomers to study how stars in the bar and in the rest of the galaxy react to one another.

“The bar is like a giant mixer for our galaxy,” said Majewski, a U.Va. astronomy professor and the principal investigator for the APOGEE project. “As the bar rotates, it churns up the motions of nearby stars. Over time, this mixing should have a big effect on the disk of our galaxy, including in spiral arms out where we live, but how and to what extent is not well understood. This new sample of definitively-identified bar stars gives us a unique opportunity to learn more about exactly how this giant blender mixes up our galaxy.”

But the team’s discovery only tells half the story. So far, APOGEE has observed only one side of the bar – the side where the stars are moving away from the Earth. On the other side, the stars must be moving toward Earth. But unfortunately, the SDSS-III telescope is inconveniently placed: the other part of the Milky Way bar is visible only from Earth's Southern Hemisphere.

 

Seeing the other side of the bar is one of the motivations for a fourth-generation SDSS, set to begin in 2014. Part of this successor project, led again by U.Va. astronomers, will implement the same techniques using a 2.5-meter telescope in Chile to observe the rest of the inner Milky Way.

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