First glimpse results on the stellar structure of the galaxy

The GLIMPSE ( Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of similar to 30 million mid-infrared sources toward the inner Galaxy, 10 degrees <= vertical bar l vertical bar <= 65 degrees and vertical bar b vertical bar <= 1 degrees, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N = N-0(l/l(0)) K-1(l/l(0)), where l(0) is insensitive to limiting magnitude, band choice, and side of Galactic center: l(0) = 17 degrees - 30 degrees with a best-fit value in the 4.5 mu m band of l(0) = 24 degrees +/- 4 degrees. Modeling the source distribution as an exponential disk yields a radial scale length of H-* = 3.9 +/- 0.6 kpc. There is a pronounced north-south asymmetry in source counts for vertical bar l vertical bar <= 30 degrees, with similar to 25% more stars in the north. For l = 10 degrees - 30 degrees, there is a strong enhancement of stars of m = 11.5 - 13.5 mag. A linear bar passing through the Galactic center with half-length R-bar = 4.4 +/- 0.5 kpc, tilted by phi 44 degrees +/- 10 degrees to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l p 26 degrees - 28 degrees, 31 degrees.5 - 34 degrees, and 306 degrees - 309 degrees are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m(K) - m([8.0]) > 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.