The nature of the Galactic Bulge

The bulge of the Galaxy as seen by IRAS forms a spatially concentrated system of long-period cool variable stars with circumstellar dust shells. It is approximately round with an axis ratio of ≳ 0.7 and a vertical scale of ∼ 370 pc, approximately an order of magnitude more compact than the globular cluster system, but similar to the distribution of late M giants. By modelling the IRAS sampling properties we have deduced the period distribution of the long-period variable stars which outline the Galactic Bulge in the IRAS Point Source Catalog. The majority of these sources have periods from ∼ 400 day (90 per cent confidence lower limit) to ∼ 1400 day. The derived period distribution corresponds to a main sequence initial mass in excess of ∼ 1.3 $$M_\odot$$, and a corresponding age of less than about 10 Gyr, on a scale on which the globular clusters have an age of ∼ 16 Gyr. Thus the centrally concentrated stellar distribution making up the IRAS bulge comprises an intermediate age population of stars which is not obviously part of the extended stellar spheroid.