Something about the structure of the Galaxy

Abstract

We analyse a sample of 507 evolved stars in the inner galactic Plane. 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, in the Disk 2.7 Gyr. The vertical distribution of almost all AGB stars in the Disk is found to be nearly exponential, with scaleheight increasing from 100 pc for stars of \lsim 1 Gyr to 500 pc for stars of \gsim 5 Gyr. 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 ultra-harmonic (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 HI filament. Also, using several observed features, we argue that an inner-Lindblad resonance exists, at $\sim$1-1.5 kpc. The compositions of OH/IR populations within 1 kpc from the galactic Centre give insight into the bar-driven evolution of the inner regions. We suggest that the Bar is $\sim$8 Gyr old, relatively weak (SAB) and may be in a final stage of its existence.