The Galactic Bar

Abstract

Like the majority of spiral galaxies, the Milky Way contains a central non-axisymmetric bar component. Our position in the Galactic plane renders it rather hard to see, but also allows us to make measurements of the bar that are completely unobtainable for any other system. This paper reviews the evidence for a bar that can be gleaned from the many extensive surveys of both gas and stars in the Milky Way. We introduce some simplified models to show how the basic properties of the bar can be inferred in a reasonably robust manner despite our unfavorable location, and how the complex geometry can be used to our advantage to obtain a unique three-dimensional view of the bar. The emerging picture of the Galactic bar is also placed in the broader context of current attempts to understand how such structures form and evolve in spiral galaxies.