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Milky Way a swifter spinner and more massive



Posted: 06 January, 2009

New high precision measurements of the Milky Way suggest that our home Galaxy is spinning a dizzy 100,000 miles per hour faster than previously believed.

According to Mark Reid of the Harvard-Smithsonian Center for Astrophysics, the increased spin means that the Milky Way must be 50 percent more massive, bringing it in line with the mass of the Andromeda Galaxy. "No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy in our Local Group family," he says. But the greater mass comes with a hefty price, for its greater gravitational pull increases the likelihood of collisions with the Andromeda galaxy or smaller nearby galaxies.

Artist's Conception of our Milky Way Galaxy: Blue, green dots indicate distance measurements. Image: Robert Hurt, IPAC; Mark Reid, CfA, NRAO/AUI/NSF.

Reporting at the American Astronomical Society's meeting in Long Beach, California yesterday, Reid’s team described their work conducted with the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope to remap the Milky Way, in particular to measure distances and motions in our Galaxy. The VLBA comprises 10 radio telescope antennas stretching from Hawaii to New England and the Carribbean, and has the resolving power equivalent to being able to read a newspaper in Los Angeles from the distance of New York. The new observations indicate that our Solar System, located 28,000 light years from the galactic centre, is moving at about 600,000 miles per hour around the Galaxy, up from the previous estimate of 500,000 miles per hour.

The scientists also observed regions of intense star formation across the Galaxy, which define the spiral arms of the Milky Way and therefore provide a yardstick for mapping the Galaxy's spiral structure. In areas within these regions, gas molecules are strengthening naturally occurring radio emission in the same way that lasers strengthen light beams. These areas, called cosmic masers, serve as bright landmarks for the VLBA. By observing these regions repeatedly at times when the Earth is at opposite sides of its orbit around the Sun, the astronomers can measure the slight apparent shift of the object's position against the background of more distant objects.

"The new VLBA observations of the Milky Way are producing highly-accurate direct measurements of distances and motions," says Karl Menten of the Max Planck Institute for Radio Astronomy in Germany, a member of the team. "These measurements use the traditional surveyor's method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies."

The VLBA is a system of ten radio-telescope antennas spanning over 5,000 miles, each with a dish 25 metres in diameter. Image: NRAO/AUI/SeaWiFS Project NASA/GSFC and ORBIMAGE.

The astronomers found that their direct distance measurements differed from earlier, indirect measurements, sometimes by as much as a factor of two. "These direct measurements are revising our understanding of the structure and motions of our Galaxy," says Menten. "Because we're inside it, it's difficult for us to determine the Milky Way's structure. For other galaxies, we can simply look at them and see their structure, but we can't do this to get an overall image of the Milky Way. We have to deduce its structure by measuring and mapping.”

Thanks to the VLBA’s accurate measurements, the astronomers could also determine the three-dimensional motions of star-forming regions, revealing that most star-forming regions do not follow a circular path as they orbit the Galaxy but instead move more slowly than other regions and on elliptical orbits. The astronomers think this is due to ‘spiral density wave shocks’, which can take gas in a circular orbit, compress it to form stars, and cause it to go into a new, elliptical orbit. This, they explained, helps to reinforce the spiral structure.

Reid and his colleagues found other surprises, too. Measuring the distances to multiple regions in a single spiral arm allowed them to calculate the angle of the arm. "These measurements indicate that our Galaxy probably has four, not two, spiral arms of gas and dust that are forming stars," announced Reid. Recent surveys by NASA's Spitzer Space Telescope suggest that older stars reside mostly in two spiral arms, raising a question of why the older stars don't appear in all the arms. But answering that question will require more measurements and a deeper understanding of how our Galaxy works.

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