Spiral Arms of the Milky Way Galaxy
Joachim Köppen Kiel Nov 2020

This is a simple modeling of the spiral arms of the Milky Way galaxy. Given a prescription for the galactic rotation curve, and their parameters (density, pitch angle [kpc/turn], initial radial offset, and arm width) two spiral arms are shown in a view onto the Galactic Plane (left map). Parts that are visible from the Sun (specified by minimum and maximum distances and a range of galactic longitude) are rendered white. Note that from central Europe and North America the Galactic Plane is visible only between longitudes 355..240°. HI clouds in the plane of the Milky Way galaxy are assumed to move on circular orbits around the Galactic Centre. If we look from the Sun (small yellow/red circle) into the direction of galactic longitude long a parcel of gas on this line of sight but at radius r from the centre (small white circle) and thus with circular velocity V_rot(r) appears to have a radial velocity with respect to our Sun's motion:

The background image of the map indicates whether a position in the Galactic Plane appears as a red- or blue-shifted object. The Mouse position gives the relevant data values.

The right hand map displays how these visible parts of the arms appear in the galactic longitude vs. radial velocity map. The yellow curves give the limiting values for the radial velocity (maximum in the first quadrant (0 < long < 90°) and minimum in the fourth quadrant). To make the features brighter, increase the density of the arms.