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°, 315-34°, 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.