Infrared signatures of the inner spiral arms and bar.

Star counts from the Two-Micron Galactic Survey are used to determine the nature of peaks for 350> 1> 150 in the Galactic plane in the 2- m surface-brightness maps. Low extinction by itself is demonstrated to be insufficient to explain the peaks. The peak at 1=330 is shown to have a different luminosity function from those at 1=210 and 270, the latter being almost certainly caused by very luminous, massive young stars. By far the most likely explanation is that they are associated with a star formation region at the near end of the bar. A simple model which includes the bar is proposed; this accurately predicts the form of the major features in the 2.2- m surface-brightness maps on the Galactic plane for 400> 1> 100 and - 100>1> 400. As a consequence, we find that the most probable position angle for the bar is 750+ 50 that the semimajor axis of the bar is 3.7-4 kpc, that there are few if any luminous stars in the 3-kpc ring, and that the width of the stellar population of the Scutum arm is 300 pc. Key words: stars: formation - Galaxy: stellar content - Galaxy: structure - infrared: stars.