New detailed images from NASA’s Spitzer Space Telescope are shedding light on the true structure of the Milky Way, revealing that it has just two major arms of stars instead of the four it was previously thought to possess. Meanwhile, astronomers using ground-based telescopes discover the missing 3-kiloparsec arm.
Astronomers have long believed that the Milky Way has four major star-forming arms: Norma, Scutum-Centaurus, Sagittarius and Perseus, with other partial arms – such as the Orion Arm where our Sun lies – and additional bands of gas and dust running through the central bar of the galaxy.
With the advent of large infrared sky surveys during the 1990s, which allowed astronomers to pierce through the Milky Way’s central dusty cloak in better detail than ever imaginable, major revisions to the standard models of the Galaxy’s structure came about. In 2005, Robert Benjamin of the University of Wisconsin, and colleagues, used Spitzer's infrared detectors to obtain detailed information about the Milky Way's central bar, and found that it extends farther out from the centre of the galaxy than previously thought, up to several thousand light years either side of the the galactic centre.
"Spitzer has provided us with a starting point for rethinking the structure of the Milky Way," says Benjamin.
Astronomers have discovered that the Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a dense central bar of stars. The artist impression also includes a new spiral arm – the far 3 kiloparsec arm – discovered via a radio survey of gas in the Milky Way. The 3 kpc arms lie along the bar of the Galaxy. Image: Robert Hurt (SSC/JPL/Caltech).
In the new survey, the team used star-counting software to measure the density of stars in different regions of the Milky Way. They found an increase in population in the direction of the Scutum-Centaurus Arm, as would be expected for a spiral arm, which contains the greatest densities of both young, bright stars and older red giant stars. But, when they looked in the direction where they expected to see the Sagittarius and Norma arms, there was no jump in the number of stars, although they were found to contain occasional small pockets of young stars. The fourth arm, Perseus, wraps around the outer portion of our Galaxy and cannot be seen in the new Spitzer images, but the two major arms seem to connect up nicely with the near and far ends of the Galaxy's central bar.
"Now, we can fit the arms together with the bar, like pieces of a puzzle," says Benjamin, "And, we can map the structure, position and width of these arms for the first time."
The announcement of the Milky Way having just two major arms comes at the same time another group of astronomers reveal a new minor spiral arm on the far side of the galactic centre from the Earth, which is a virtual twin of a known arm on the near side, completing the picture of a beautifully symmetric galaxy and answering a long standing mystery.
Fifty years ago, radio astronomers discovered an unusual spiral arm 10,000 light-years from the centre of the Milky Way, along our line of sight. It was nicknamed the 3 kiloparsec (kpc) arm since 3 kpc equals 10,000 light years. Astronomers always suspected that a similar arm might exist on the far side of the Milky Way, but until now, the arm remained elusive.
Tom Dame and Patrick Thaddeus of the Harvard-Smithsonian Centre for Astrophysics analysed data obtained using a 1.2 metre diameter millimeter-wave telescope located at Cerro Tololo Inter-American Observatory in Chile and discovered a spiral arm exactly where the far 3-kpc arm should be, with properties like radius, expansion velocity, mass, and brightness that were mirror images of the near 3-kpc arm. They suspect that the 3-kpc spiral arms are linked to the galactic bar in the centre of the Milky Way.
"The 3-kiloparsec arms are a natural result of the stellar bar," explained Thaddeus. "We expected that the bar should drive symmetric structure. Now, we have proof that it does."
Both of these results were presented at the 212th American Astronomical Society meeting held in St Louis, Missouri this week.