Does the Milky Way have a Maximal Disk?

The Milky Way is often considered to be the best example of a spiral for which the dark matter not only dominates the outer kinematics, but also plays a major dynamical role in the inner galaxy: the Galactic disk is therefore said to be “sub-maximal.” This conclusion is important to the understanding of the evolution of galaxies and the viability of particular dark matter models. The Galactic evidence rests on a number of structural and kinematic measurements, many of which have recently been revised. The new constraints indicate not only that the Galaxy is a more typical member of its class (Sb-Sc spirals) than previously thought, but also require a re-examination of the question of whether or not the Milky Way disk is maximal. By applying to the Milky Way the same definition of “maximal disk” that is applied to external galaxies, it is shown that the new observational constraints are consistent with a Galactic maximal disk of reasonable M/L. In particular, the local disk column can be substantially less than the oft-quoted required Σ ≈ 100 M pc — as low as 40 M pc in the extreme case — and still be maximal, in the sense that the dark halo provides negligible rotation support in the inner Galaxy. This result has possible implications for any conclusion that rests on assumptions about the potentials of the Galactic disk or dark halo, and in particular for the interpretation of microlensing results along both LMC and bulge lines of sight. Subject headings: Galaxy: structure — Galaxy: kinematics and dynamics — galaxies: kinematics and dynamics — galaxies: dark matter Submitted to the Astrophysical Journal on 25 August 1996 Accepted for publication on 20 January 1997