The Local Arm of the Milky Way galaxy, where the Earth resides, is larger than previously believed. Caught between the Sagittarius Arm on the inside of the spiral and the Perseus Arm on the outside, the Local Arm has always been thought of as a small structure.
New observations made using the Very Long Baseline Array (VLBA) radio telescope network show that the Local Arm is larger than previously measured, resembling one of major arms of the spiral. The VLBA is a network of 10 radio telescopes, each measuring 82 feet across, spanning from Hawaii to the Caribbean.
"Our new evidence suggests that the Local Arm should appear as a prominent feature of the Milky Way," Alberto Sanna, of the Max-Planck Institute for Radio Astronomy, said.
The difficulty in determining the exact shape of our galaxy is the fact that we lie within it. The estimated shape of the Milky Way galaxy is determined by measuring the distances to stars and mapping out where they are located, creating a map.
But distances to the stars are difficult to measure, especially for the more-distant specimens that you most want to measure to determine shape. Trying to map by this method is like walking through a room with plexiglass panels standing like a house of mirrors and estimating the shape the barriers make as seen from above.
The VLBA was pointed at star-forming regions of the Milky Way for four years, from 2008 to 2012. In these regions, water and methanol (wood alcohol) amplify radio waves that are created there, forming a bright spot of radio waves in the sky called masers. With the best resolution of any telescope or array in the world, the VLBA could theoretically read a street sign in New York City from Los Angeles.
"Based on both the distances and the space motions we measured, our Local Arm is not a spur. It is a major structure, maybe a branch of the Perseus Arm, or possibly an independent arm segment," Sanna said.
The observations also reveled more information about the processes that create masers. The research was part of the Bar and Spiral Structure Legacy (BeSSel) survey, which aims to use the VLBA to further refine our map of the Milky Way.
In the Hitchhiker's Guide to the Galaxy, sci-fi comedy writer Douglas Adams described our location as "...the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy." Now, it appears that is not the case after all.
