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 is also 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 disk. 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 disk model interior to the Sun. The bar length is approx. 3.5kpc and its axis ratios are 1:(0.3-0.4):0.3, independent of whether the spiral arm model is 4-armed or 2-armed. The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well. For the bar angle we estimate 15^o <= phi <= 30^o, with the best models obtained for 20^o <= phi <= 25^o. We use our reference model to predict a microlensing optical depth map towards the bulge, normalising its mass by the observed terminal velocity curve. For clump giant sources at (l,b)=(3.9,-3.8) we find tau_{-6}:=tau/10^{-6}=1.27, within 1.8*sigma of the new MACHO measurement given by Popowski et al. The value for all sources at (l,b)=(2.68,-3.35) is tau_{-6}=1.1, still >3*sigma away from the published MACHO DIA value. The dispersion of these tau_{-6} values within our models is 10%. Because the distribution of sources is well-fit by the NIR model, increasing the predicted optical depths by >20% will be difficult. Thus the high value of the measured clump giant optical depth argues for a near-maximal disk in the Milky Way. (abridged)