First GLIMPSE Results on the Stellar Structure of the Galaxy

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point SourceCatalog of ~ 30 million mid-infrared sources towards the inner Galaxy, 10 < |l|< 65 degrees and |b| < 1 degree, was used to determine the distribution ofstars in Galactic longitude, latitude, and apparent magnitude. The countsversus longitude can be approximated by the modified Bessel functionN=N_0*(l/l_0)*K_1(l/l_0), where l_0 is insensitive to limiting magnitude, bandchoice, and side of Galactic center: l_0= 17-30 degrees with a best fit valuein the the 4.5 micron band of l_0=24 +/- 4 degrees. Modeling the sourcedistribution 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 degrees, with ~ 25% more stars in the north. For l=10-30 degrees, there is astrong enhancement of stars of m= 11.5-13.5 mag. A linear bar passing throughthe Galactic center with half-length R_bar=4.4 +/- 0.5 kpc, tilted by phi=44+/- 10 degrees to the Sun-Galactic Center line, provides the simplestinterpretation of this data. We examine the possibility that enhanced sourcecounts at l=26-28 degrees, 31.5-34 degrees, and 306-309 degrees are related toGalactic spiral structure. Total source counts are depressed in regions wherethe counts of red objects (m_K-m_[8.0] >3) peak. In these areas, the counts arereduced by extinction due to molecular gas and/or high diffuse backgroundsassociated with star formation.