Constraints on the Galactic bulge using IRAS variable point sources

Abstract

A flux-limited sample of IRAS variable sources is used to probe the 3D structure of the Galactic bulge. No distinct vertically extended bulge component is seen; the spatial distribution is that of a centrally concentrated disk. The 2D distribution of IRAS variables on the sky is well fitted by an exponential disk with scale length a = 4.5 kpc and scale height h = 800 pc and more poorly fitted by an exponential disk plus an R exp 1/4 law. The combined results suggest that any bulge component is quite small and/or flattened with a vertical scale height not significantly larger than the disk itself. The appearance of a bulge in the distribution of sources on the sky can arise from the edge-on projection of a single exponential disk with constant scale height and is further enhanced by preferential extinction of sources in the disk.