Infrared signatures of the inner spiral arms and bar

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Abstract
Star counts from the Two-Micron Galactic Survey are used to determine the nature of peaks for 35° > l > 15° in the Galactic plane in the 2-μm surface-brightness maps. Low extinction by itself is demonstrated to be insufficient to explain the peaks. The peak at l = 33° is shown to have a different luminosity function from those at l = 21° and 27°, 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-μm surface-brightness maps on the Galactic plane for 40° > l > 10° and – 10° > l > – 40°. As a consequence, we find that the most probable position angle for the bar is 75° ± 5°, 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.

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