The Warped and Flaring Layer of Atomic and Molecular Gas in the Outer Galaxy

Abstract

The Milky Way serves in several regards as a prototype of a warped galactic system. Our embedded perspective is, in fact, ideal for studying the morphology of the outer Galaxy. The improved sensitivity and positional- and velocity-coverage of recent HI data, together with improved confidence in the velocity field at large R, led to an improved description of the HI warp, although the essentials of the description dating from the late 1950’s remain unchanged. In the outer Galaxy, the v-to-R transformation is single valued, so the heliocentric (l, b, v) data cube can be transformed to the more directly relevant galactocentric (R, θ, z) coordinates. HI can be traced to distances greater than 3R ₀, and molecular clouds can be traced to more than 2R ₀. Both tracers yield a quantitative description of the deviation of the density midplane from the equator plane b = 0°, the flare to increasing thickness at larger R, the location of the line of nodes of the warp, and the radial variation of the projected surface densities. The line of nodes of our Galaxy’s warp is quite straight, showing no evidence of precession or of winding under rotational shear. The molecular clouds found with embedded IRAS heat sources at large R indicate that star formation occurs well beyond the conventionally-defined optical disk. The ensemble of molecular clouds shows the same warped shape and flaring thickness as shown by the outer-Galaxy HI layer. Preliminary evidence suggests that the chemical and physical circumstances in the warped outer reaches of the Galaxy differ from those in the inner regions. The parameters specifying the global, flaring warp in M31 are remarkably similar to those specifying the warp in our own Galaxy. Restrictive viewing requirements suggest that even substantial, regular external warps might be difficult to recognize from the projected shapes alone, without kinematic information. It seems difficult to argue against the proposition that all galactic disks are globally warped.