While our optical view of the Milky Way is limited by interstellar dust to just a small fraction of the distance across the Galactic disk, at radio wavelengths emissions from interstellar gas in its various forms can be readily observed throughout the Galaxy. For this reason, radio astronomers have played a leading role in Galactic structure research since the 1950s, when the first large-scale 21 cm surveys of interstellar atomic hydrogen were carried out.
RG astronomers have for many years used the CfA millimeter-wave telescope to study the distribution and motions of dense, star-forming molecular clouds throughout the Galaxy, and have by now obtained a fairly uniform survey of the entire molecular galaxy. This survey is widely used by Galactic astronomers and continues to yield new results on star formation and Galactic structure, including the recent discovery of two new spiral arm features, the Far 3-kpc arm in 2008 and the Outer Scutum-Centaurus arm in 2011.
RG astronomers are pioneering the use of very long baseline inteferometry to measure extremely precise trigonometric parallaxes and proper motions for masers within massive star forming regions, yielding their distances and 3-dimensional motions. They are currently in the middle of a 5-year project called BeSSeL aimed at obtaining such measurements for several hundred masers throughout the Galaxy in order to trace out its spiral structure and make accurate measurements of its overall size and motions.
Tom Dame, Mark Reid, Tony Stark, Patrick Thaddeus