Spiral Arms of the Milky Way Galaxy

Joachim Köppen, Strasbourg, 2011/12

Observations

Follow the procedures to switch-on the ESA-Dresden telescope

• Click Record to start recording the data
• The best positions in the Galactic Plane are: G30, G40, … , and G150 (observe for 5 min each) or G80,…, G110 (for 8 min).
• Use a frequency span of 1.5 or 2 MHz, and set the centre frequency so that you get the entire Galactic feature
• Observe each position for 5 or 8 min

Interpretation (more details):

• Import the text file with the data into Excel
• For each position (Galactic longitude l) do this
  • get the average spectrum of all the spectra taken here, and plot it in a Scatter Plot
  • Convert the frequencies to radial speeds, by
    vrad = - (f/1420.406 – 1) * 299792.458 - VLSR
    where VLSR is the value of the 6th column, the correction to the Local Standard of Rest for the direction we had observed
  • Identify the individual features with their radial velocities. You might do it simply by visual inspection or more carefully by matching the observations with a sum of Gaussian components (example)
• Match these features in the neighbouring position
• By assuming a ‘flat’ rotation curve vrot(R) = 210 km/s we can use Oort’s formula to compute for any direction l in the Galactic Plane from the observed radial velocity
  r(l, vrad) = rsun * vrot/(vsun + vrad/sin l)
  The circle about the Galactic Centre with radius r then intersects the line of sight (a straight line with y = ysun-x*tan l with ysun = rsun = 8.5 kpc and vsun = 210 km/s) in maximal two possible positions. In this way we can construct the spiral arms in the (x,y)-plane.

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  last update: Sept. 2011 J.Köppen