First GLIMPSE Results on the Stellar Structure of the Galaxy

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~30 million mid-infrared sources toward the inner Galaxy, 10°<=|l|<=65° and |b|<=1°, 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=N₀(l/l₀)K₁(l/l₀), where l₀ is insensitive to limiting magnitude, band choice, and side of Galactic center: l₀=17°-30° with a best-fit value in the 4.5 μm band of l₀=24°+/-4°. 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 |l|<~30°, with ~25% more stars in the north. For l=10°-30°, 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 φ=44°+/-10° to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l= 26°-28°, 31.5d-34°, and 306°-309° 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.