Observations with GCO-SRT

Deprojection of the Data into 3D Space

Joachim Köppen Kiel 2014

The interstellar hydrogen gas of the galactic disk moves in nearly circular orbits around the Galactic Centre. As one finds from the rotation curve the orbital speed is almost independent of the distance from the Centre. If we assume that the speed is constant (or if we use the rotation curve), we can compute for every position of the disk the radial velocity under which this parcel of gas would be observable from our position. Note that the frequency of the line emission is shifted via the Doppler effect only by the radial velocity. For a parcel of gas at a distance R from the Centre and at galactic longitude l one gets

v_rad = (v_rot(R) * R_sun/R - v_sun) * sin l

with the parcel's rotational speed v_rot(R) and the solar distance from the Centre R_sun = 8.5 kpc and its orbital speed v_sun = 230 km/s.

In this manner, we can assign to every position (characterized by galactic longitude l and distance R from the Centre) in the Galactic Plane the observable radial velocity. In the survey data we then look up the intensity of the emission at that radial velocity, and indicate this value (by the colour in the map) on a map of the Galactic Plane, we can get this image of the Galactic Plane:

The Galactic Centre is in the middle (x=y=0), the Sun at top centre (x=0, y=8.5 kpc). There are two artefacts in such a map, because the Doppler effect can only tell us about the radial velocity:

the bright ring around the Centre with a radius of 8.5 kp: this may represent the circular orbit of the sun. However, at these positions one simply finds that the radial velocity will always be zero. Thus this emission is nothing but the local emission, but placed at these positions.
when we look towards the Galactc Centre (longitude 0º) or in the opposite direction (l = 180º) we also seem to have strong emission at a large range of distances. This is not true, because all these regions are regions of near zero radial velocity, as the velocity vector of a circular orbit will be perpendicular to the line of sight.

These areas of confusion are also shown in a model for the galactic disk. The red regions indicate where gas would appear to be red-shifted (to lower frequencies) i.e. moving away from us, blue regions where gas would be bluehifted or seen approaching us. In the grey-greenish area the radial velocity would be close to zero. The sun is marked by a yellow dot, and the white spiral arms are a simple model. Thus, when we look towards l=90º (viz. to the right) we can pick up emission from gas that appears to come towards us; and we would pick up local emission (at zero speed) and two or three spiral arms with negative radial velocity.

To suppress in the maps these regions of confusion, you have two controls: with distance emphasis emission from larger distances is more strongly weighted, and with velocity emphasis the intensities are weighted with the velocity. In the example below, application of the velocity emphasis makes the zero velocity areas less prominent:

Both controls are merely to enhance the view, and permit to bring out better the spiral arms (inside and outside the solar orbit).

The example above was done for the Galactic Plane (i.e. latitude 0º), but one can create these maps for any other galactic latitude.

To get a view of the distribution of gas above and below the Galactic Plane, you can chose to display the deprojection map for a constant Galactic Longitude, for instance at galactic longitude l=90º:

These are the same data as seen in the velocity-latitude map.

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last update: March 2014 J.Köppen DF3GJ