Mapping the Inner Boxy Bulge of the Milky Way

Historically, star count analyses have been vitiated by heavy extinction in the visible, but even in the infrared Galactic structure (especially for the inner ∼4 kpc) is especially complicated by the multiplicity of distinct stellar and gaseous/dusty components (a typical line of sight will include spiral arms, rings and bulge or bar). We have adopted a Bayesian strategy devised by Lucy [5] to extract bulge parameters from the data rather than making prior guesses at the parameters. We used the Lucy algorithm to invert the equation of star counts in order to recover 3D information on the structure of the inner Galaxy. This method enabled us to obtain both the stellar density distribution and the luminosity function with no need for prior assumptions [3]. Using 2MASS star counts [4], we obtained the 3D density distribution and K-band luminosity of the Galactic bulge. The result is a boxy bulge with axial ratios 1:0.5:0.4, the major axis making an angle of 20–35 deg w.r.t. the Sun–Galactic Centre radius vector. We set our results for the bulge into the wider context of the Tenerife model of the inner Galaxy, in which what has sometimes been referred to as the Galactic bar is identified with the boxy bulge described here in contradistinction to the long Galactic bar ([2], [1], [6]).