Rotation of the Milky Way Galaxy
Joachim Köppen Kiel Feb 2017
This tool shows the radial velocity, as would be measured by an observer in the solar
system (small yellow circle), of HI clouds in the plane of the Milky Way galaxy, as they
move on circular orbits around the Galactic Centre (small white circle).
If we look from the Sun into the direction of galactic longitude gl a parcel of gas
on this line of sight but at radius r from the centre and with circular velocity
Vrot(r) appears to have a radial velocity with respect to our Sun's
Vrad = (Vrot(r) *rsun/r - vsun) * sin gl
A simple form of the rotation curve is assumed, specified by the speed at the galactocentric distance of the Sun (usually 220 km/s) and the radial gradient dvdr. The left hand map shows the radial velocity from each position in the Galactic Plane, colour-coded from negative values (blue) to positive values (red). Moving the mouse to a position displays the relevant numerical values.
The plot on the right displays the radial velocity of gas clouds on the chosen line of sight as a function of their distance from the Sun. The button single curve/overplot allows to superpose curves from various lines of sight for comparison.
Along lines of sight towards the inner part the radial velocity shows a maximum value. This easily measurable feature is used to determine the rotational velocity at the radius to whose circle the line of sight is a tangent. In this way one obtains the rotation curve of the Milky Way. Since lines of sight towards the exterior parts of the Galaxy have a monotonic behaviour of the radial velocity with distance, the outer rotation curve has to be derived in other, less direct ways.
Note that from central Europe and North America the Galactic Plane is visible only between longitudes 0..250°.