Spiral arms, bar shape and bulge microlensing in the Milky Way

Abstract

A new model for the luminosity distribution in the inner Milky Way is found, using a non-parametric penalized maximum-likelihood algorithm to deproject a dereddened COBE/ DIRBE L-band map of the inner Galaxy. The model isalso constrained by the apparent magnitude (line-of-sight) distributions of clump giant stars in certain bulge fields. An important new feature is the inclusion of a spiral arm model in the disc. Spiral arms make the model appear broader on the sky; thus our bar is more elongated than in previous eight-fold symmetric models. They also lead to a smoother disc model interior to the Sun. The bar length is 3.5 kpc, and its axis ratios are 1: (0.3-0.4): 0.3, independent of whether the spiral arm model is four-armed or two-armed. The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well. With only the surface brightness data, a small model degeneracy is found even for fixed orientation of the bar, amounting to about ′0.1 uncertainty in the in-plane axial ratio. Including the clump giant data removes most of this degeneracy and also places additional constraints on the orientation angle of the bar. We estimate 15° ≤ φ b a r ≤ 30°, with the best models obtained for 20° ≤ φ b a r ≤ 25°. We use our reference model to predict a microlensing optical depth map towards the bulge, normalizing its mass by the observed terminal velocity curve. For clump giant sources at (l,b) = (3°.9, -3°.8) we find τ - 6 ≡ τ/10 - 6 = 1.27, within 1.8σ of the new MACHO measurement given by Popowski et al. The value for all sources at (l, b) = (2°.68, -3°.35) is τ-6 = 1.1, still >3σ away from the published MACHO DIA value. The dispersion of these τ - 6 values within our models is ≃ 10 per cent. Because the distribution of sources is well fitted by the near-infrared model, increasing the predicted optical depths by >20 per cent will be difficult. Thus the high value of the measured clump giant optical depth argues for a near-maximal disc in the Milky Way.