Stellar patrons of Milky Way bar identified
Astronomers now have the clearest understanding yet of the bar at the centre of the Milky Way.
Scientists with the Sloan Digital Sky Survey III (SDSS-III) have announced the discovery of hundreds of stars rapidly moving together in long, looping orbits around the centre of our galaxy.
"The best explanation for their orbits is that these stars are part of the Milky Way bar," David Nidever from the University of Michigan said.
"We know that the bar plays an important role in determining the structure of the galaxy, so learning more about these stars will help us understand the whole galaxy, even out here in the spiral arms," he said.
The team's discovery came from accurately measuring the speeds of thousands of stars near the centre of the Milky Way. The centre of our galaxy is 30,000 light-years away -- close by cosmic standards -- yet we know surprisingly little about it, because the galaxy's dusty disk hides it from view.
In spite of this blind spot, though, we do know a key fact about our galaxy: like many spiral galaxies, the Milky Way has a 'bar' of stars that orbit together around the galactic centre.
"We know of the bar's existence from many separate lines of evidence," Gail Zasowski from The Ohio State University said.
"What we don't know is which stars are part of the bar, and what the velocities of those stars are. That information will help us understand how the bar formed, and how its stars relate to the stars in the rest of the galaxy," Zasowski said.
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 the same neighbourhood. 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 ones are part of the bar.
Enter SDSS-III's new Apache Point Galactic Evolution Experiment (APOGEE). APOGEE uses a custom-built high-resolution infrared spectrograph attached to the 2.5-meter Sloan Foundation Telescope in New Mexico, and is capable of measuring the velocities and chemical compositions of up to 300 stars at once.
"What separates APOGEE from previous spectroscopic surveys is that we are studying the galaxy using infrared light," Nidever said.
APOGEE began observations in June 2011 and has already observed more than 48,000 stars all over our galaxy.
In a new study, a worldwide team of scientists including Nidever and Zasowski used data from the first few months of APOGEE observations to measure the velocities for nearly 5,000 stars near the galactic centre.
With these velocity measurements, they assembled a picture of how these stars orbit the centre of the Milky Way.
However, quite unexpectedly, they found that a substantial fraction of stars in the inner galaxy are moving away from us quickly -- about 10 percent of the total stars in their sample are moving at more than 200 kilometres per second (400,000 miles per hour) away from the Earth.
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 midplane of the galaxy -- suggesting that these measurements of fast central stars are not just a statistical fluke, but really are a feature of our galaxy.
The team then compared their observations with the predictions of the 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 fast-moving stars are part of the bar," Nidever said.
"I was actually quite surprised that they showed up so clearly in our survey," Nidever added.
The study has been published in Astrophysical Journal Letters.