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Perseus Spiral Arm of the Milky Way much closer than thought
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Update time: 2008-09-07
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The Milky Way appears to be made up of four main arms that curve around its centre like a pinwheel. "However, our view from the interior makes it difficult to determine its spiral structure," writes a team led by Ye Xu of the Shanghai Astronomical Observatory (ShAO) in China, in Science.
Figure 1: COVER NASA Spitzer Space Telescope false-color image of a portion of the Perseus spiral arm of the Milky Way. Bright regions are clusters of newly formed stars. Recent observations with the NationaRadio Astronomy Observatory Very Long Baseline Array yielded the distance to a newly formed star (in the bright cluster toward the lower left) with unprecedented accuracy and precisely located the Perseus spiral arm. See Science 311, 54.  

 

 

  Figure 2: This Week in Science Mapping the dimensions of the Milky Way with precision is still a daunting task. Xu et al. (Science 311, 54, published online 8 December 2005; see the cover and the Perspective by Binney) have used precise images of radio sources in a star cluster to fix the distance to the nearest spiral arm from the Sun using trigonometric parallax, the small shift in apparent position as Earth moves between opposing points in its orbit. Using the Very Long Baseline Array, the authors detected this shift for radio sources in a young star cluster that forms part of the Perseus Arm of the Milky Way. The star cluster has extra anomalous motions beyond the simple rotation of our galaxy about its center that may be consistent with spiral density-wave theory.

The Perseus spiral arm, the nearest spiral arm in the Milky Way outside the Sun's orbit, lies only half as far from Earth as some previous results had suggested. An international team of astronomers including scientists from ShAO has recently achieved the most accurate distance measurement ever to the Perseus arm. This was done by use of a vast array of radio telescopes in the USA called the Very Long Baseline Array, observing very bright spots within clouds of gas that contain methyl alcohol in the placental material surrounding a newly formed star called W3OH.

Dr. Xu Ye stated that "we measured distance by the simplest and most direct method in astronomy - essentially the technique used by surveyors called triangulation." Specifically, the team used the changing vantage point of the Earth as it orbits the Sun to form one leg of a triangle. Measuring the change in apparent position of a source, they could calculate the source's distance by simple trigonometry (resulting in 6357±130 light years).

This result resolves the longstanding problem of the distance to this spiral arm. In thje past, different methods of measuring distance have disagreed by more than a factor of 2. Prof. Karl Menten, another member of the team, states that "this confirms distances based on the apparent luminosity of young stars but disagrees with distances based on a model of the rotation of the Milky Way. The reason for the discrepancy is that young stars in the Perseus spiral arm have unexpectedly large motions."

The astronomers found that the young star is not moving in a circular orbit around the Milky Way, but deviates by 10% from circular. It is rotating more slowly and "falling" toward the center of the Milky Way. Team member Zheng Xing-Wu of Nanjing University points out that "the simplest explanation is that the cloud of gas out of which the star formed was gravitationally attracted by excess mass of material in the Perseus spiral arm."

"Studies such as ours are the first steps to accurately map the Milky Way," says Dr. Mark Reid, a member of the team from the Harvard-Smithsonian Center for Astrophysics. "We have established that the radio telescope we used, the Very Long Baseline Array, can measure distances with unprecedented accuracy--nearly a factor of 100 times better than previously accomplished." To get a feeling for this measurement one may visualize a person standing on the moon, holding a torch in his stretched-out hand. Let her turn around herself like an ice scater, but only making a single turn in the course of one year. The VLBA measurement is equivalent to measuring the torch's motion with an accuracy comparable to the torch's size.

Figure 3: Our Milky Way galaxy as an observer located far above its plane would see it. Shown are the known spiral arms. The locations of our solar system and of W3OH are indicated.

 The technique used is Very Long Baseline Interferometry (VLBI), where observations made with many telescopes are combined to achieve the resolution of an extraordinarily large telescope nearly the size of the Earth. The VLBA telescopes stretch from Hawaii over the continental United States to the Virgin Island of St. Croix, producing the resolution of an 8000 km diameter telescope. While the VLBA has extremely high resolution, it requires extremely bright and very compact radio sources such as masers for such measurements (a maser is the microwave equivalent of a laser.) Along with water, methanol is the most widespread maser molecule found in star-forming regions. The methanol spectral line used for the present experiment was discovered in the course of Prof. Menten's dissertation in the 1980s. In 1988, while working with Dr. Reid, they conducted the first VLBI observations of methanol masers; the target then was also W3OH. "Already then we dreamt of observations such as this one" says Menten.

The methanol observations are only the start of a very large-scale project that Reid and Menten have initiated. It will determine distances and motions of methanol masers all over the Milky Way. It has been granted a large block of VLBA observing time. In addition to the motions on the sky these observations also yield the star's velocity toward or away from the observer by measuring the Doppler shift of the methanol lines. The resulting three dimensional motions will deliver unique constraints not only on the rotation of the Milky Way but also on the distribution of the unseen Dark Matter that is postulated to surround it.

Figure 4: Left: At any point in time W3OH as seen from Earth appears at a certain angular position on the plane of the sky which can be measured very precisely with VLBI. Due to the Earth's orbital motion around the Sun this position changes over the course of a year. This effect is called the annual parallax, Pi. The magnitude of the parallax is equal to the radius of the Earth's orbit divided by the distance to W3OH. The distance to W3OH can thus be calculated from the parallax. Right: The actual measurements as a function of time. The curve represents the expected temporal variation of the position displacement. Its shape is determined by the Earth's orbit. The parallax is equivalent to the maximum displacement.

While the method - simple trigonometry - sounds basic, the transformation into practical results requires a comprehensive understanding of VLBA and all aspects of the observations, including thorough modeling of the Earths' atmosphere which affects the incoming radio waves. Dr. Reid has dedicated many years of his life to reach the point were programs such as this one can be performed.

Over the years this truly international effort was supported by a Research Prize granted to Dr. Reid by the Alexander von Humboldt Foundation. The cooperation with Shanghai Observatory is supported by a joint program of the Max Planck Society, the Chinese Academy of Sciences, and the Smithsonian Institution's Visitor Program.

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