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New Tool Is Probing the Structure of the Milky Way's Heart

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19 December 2012

The discovery that hundreds of stars are rapidly moving together in long, looping orbits around the center of our Galaxy has been announced by a team of scientists including a Penn State University astronomer and others collaborating in the Sloan Digital Sky Survey III (SDSS-III). "The stars near the nucleus of the Milky Way appear to form a bar-like structure thousands of light years in size," said Donald Schneider, a Distinguished Professor of Astronomy and Astrophysics at Penn State and a coauthor of the study. "In this investigation, we were able to accurately measure the motions of a set of stars located in the Galaxy's bar and to gain some insight into the dynamics of the central, dense regions of the Milky Way." Schneider is the Survey Coordinator and Scientific Publications Coordinator for the SDSS-III.

An artist's impression of what the Milky Way might look like viewed from above. The small blue dot is where we are on Earth (not to scale). The solid red arrows show the high-speed stars moving away from Earth that were discovered by SDSS-III. The dashed arrows show the stars moving toward Earth that are expected to be seen by the fourth-generation Sloan Digital Sky Survey.
Credit: Jordan Raddick (Johns Hopkins University) and Gail Zasowski (The Ohio State University / University of Virginia). Milky Way artist's concept by NASA/JPL-Caltech/R. Hurt (SSC-Caltech).

The center of our Galaxy is 30,000 light-years away -- close by cosmic standards -- but we know surprisingly little about it because the Galaxy's dusty disk hides the Galaxy's center from our view. Although interstellar dust blocks nearly all visible light, longer infrared wavelengths can partially shine through. To detect this infrared light, the SDSS-III team's new Apache Point Galactic Evolution Experiment (APOGEE) added a new custom-built high-resolution infrared spectrograph to the 2.5-meter Sloan Foundation Telescope in New Mexico. The astronomers then focused the new instrument on the center of our Galaxy, where it is capable of simultaneously measuring the velocities and chemical compositions of 300 stars.

"The best explanation for their orbits is that these stars are part of the Milky Way bar," says David Nidever, a Dean B. McLaughin Fellow in the Department of Astronomy at the University of Michigan and lead scientist on the paper. "Studying the bar is a key piece of the puzzle to understanding the whole galaxy, even out here in the spiral arms."

APOGEE began observations in June 2011 and has since observed 48,000 stars in our Galaxy. Nidever's team 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 how these stars orbit the center of the Milky Way. The results are published recently in the Astrophysical Journal.

The team's observations showed an unexpectedly large number stars moving quickly away from us -- about 10 percent of the stars in the sample are moving at more than 400,000 miles per hour away from the Earth. When the scientists compared their observations with predictions made with computer models of the Galaxy, they found that 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 says.

Graph with regions of purple, blue, green, red, yellow, and orange
A map of the innermost Milky Way, with circles marking the regions explored by the SDSS-III APOGEE project. Circles marked with "X" show places where the project found high-speed stars associated with the Milky Way's bar moving away from Earth. The lighter regions marked with dots on the other side of the Galactic Center show places where the fourth-generation Sloan Digital Sky Survey hopes to find counterpart bar stars moving toward the Earth.
Credit: David Nidever (University of Michigan / University of Virginia) and the SDSS-III Collaboration. Background image from the Two-Micron All Sky Survey Image Mosaic (Infrared Processing and Analysis Center/Caltech & University of Massachusetts).

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," says Steve Majewski, a professor of astronomy at the University of Virginia 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 spiral arms where we live, but this effect is not well understood. With our new sample of bar stars, we should be able to learn more about exactly how the bar mixes up our galaxy."

However, the team's discovery only tells half the story because, so far, APOGEE has observed only one side of the bar -- the side where the stars are moving away from the Earth. The SDSS-III telescope is inconveniently placed to measure the other side where the stars must be moving toward Earth because that side is visible only from Earth's southern hemisphere. Seeing the other side of the bar is one of the motivations for a fourth-generation Sloan Digital Sky Survey to begin in 2014. Part of this project will place a duplicate of the APOGEE instrument on a 2.5-meter telescope in Chile, where it will be able to observe the rest of the inner Milky Way.

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U. S.Department of Energy Office of Science.

[ Jordan Raddick / Barbara K. Kennedy ]

The scientific paper is published in the online version of The Astrophysical Journal Letters.

The SDSS-III press release is online at sdss3.org/press/milkywaybar.php. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Penn State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

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