by Robert Benjamin
The search for spiral structure in the Milky Way began in 1951 with the presentation of W. Morgan’s first “map” of the nearest spiral arms traced by OB stars/HII regions and the first detection of 21 cm emission. Although optical extinction limited (and continues to limit) the section of the Galaxy that can be observed optically, the same three arms identified by Morgan in 1951-1953 (Perseus, Orion/Local, and Sagittarius) continue to be discernable in modern investigations using parallax measurements of masers and Gaia DR 2 clusters, dust extinction maps, and stellar density maps of upper main sequence stars. A more comprehensive view of the Galaxy was obtained using surveys of HI and eventually CO. These surveys provided evidence for coherent bands of emission in longitude-velocity space. Infrared surveys now allow for the search for spiral overdensity in both young stars—by identifying and estimating distances to clusters associated with HII regions—and old stars—by finding overdensities of red clump giants. I will review some of the areas of progress here, and highlight some key questions that would benefit from the use of VVV and GLIMPSE data.