The Milky Way has long been believed to harbour a central bar, and observations during the 1990's have provided strong evidence to support this. However, our vantage point with respect to the Galactic Centre makes it difficult to accurately determine the size & shape of this bar and the surrounding arms. Results from the Spitzer Space Telescope-GLIMPSE survey give the most detailed picture to date of the inner region of the Milky Way. Yet many unanswered questions still remain regarding the formation process, age, metal content, mass distribution and kinematics of the bulge. Gaia will make a significant contribution to addressing these issues by providing precision astrometry, photometry and radial velocity measurements for a significant part of the inner region of the Galaxy.
The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) point source catalogue (containing approximately 30 million mid-infrared sources at 10° <| l | < 65° and | b | <1° ) was used to determine the distribution of stars in l, b and apparent magnitude, m. A strong north/south asymmetry in source counts for |l| <30° and a distinct hump in the number of star at apparent magnitude m ~12.5 over the range l =10-30° suggests the detection of the Galactic bar. The authors note that the data is best interpreted by a linear bar passing through the Galactic Centre with half-length Rbar = 4.4 ± 0.5 kpc, tilted by phi = 44 ± 10° to the Sun-Galactic Centre line. This is longer and at a different orientation to previous results which estimated Rbar = 3.1 - 3.5 kpc and phi = 15-35°. While this is the simplest interpretation of the results, the authors caution that other configurations should be explored.
The results of this investigation are published in First GLIMPSE Results on the Stellar Structure of the Galaxy by R.A. Benjamin et al., accepted for publication in Ap. J. Letters (preprint available as astro-ph/0508325).
This artist's impression shows a view of the Milky Way and the central bar relative to our Sun's position and viewed from above.
High-resolution image available from the Spitzer Space Telescope web site.
Image copyright: NASA/JPL-Caltech/R. Hurt (SSC)