Something about the structure of the Galaxy

We analyse a sample of 507 evolved (OH/IR) stars in the region (10 degrees > l >-45 degrees), (\ b \< 3 degrees). We derive average ages for subsets of this sample, and use those sets as beacons for the evolution of the Galaxy. In the bulge, the oldest OH/IR stars in the plane are 7.5 Gyr (1.3 M-.), and in the disc 2.7 Gyr (2.3 M-.). The vertical distribution of almost all AGB stars in the disc is found to be nearly exponential, with scaleheight increasing from 100 pc for stars less than or similar to 1 Gyr old to 500 pc for stars greater than or similar to 5 Gyr old. There may be a small, disjunct population of OH/IR stars. The radial distribution of AGB stars is dictated by the metallicity gradient. Unequivocal morphological evidence is presented for the existence of a central bar, but parameters can be constrained only for a given spatial-density model. Using a variety of indicators, we identify the radii of the inner ultraharmonic (2.5 kpc) and corotation resonance (3.5 kpc). We show that the 3-kpc arm is likely to be an inner ring, as observed in other barred galaxies, by identifying a group of evolved stars that is connected to the 3-kpc H I filament. Also, using several observed features, we argue that an inner-Lindblad resonance exists, at similar to 1-1.5 kpc. The compositions of OH/IR populations within 1 kpc of the Galactic Centre give insight into the bar-driven evolution of the inner regions. We suggest that the bar is similar to 8 Gyr old, relatively weak (SAB), and may be in a final stage of its existence.