How to Determine the Thickness of the Galactic HI Disc

Marissa Rosenberg, Joachim Köppen Strasbourg 2010

If we had eyes to look at the sky in the light of the 21 cm line of neutral hydrogen we would see like in the all-sky image below (from National Radio Astronomy Observatory / Associated Universities, Inc. / National Science Foundation) that the emission in strongly concentrated along the great circle of the Milky Way: The hydrogen gas is essentially found in the Galactic Plane.

It forms a rather thin disc, a bit like a pizza, with a diameter of about 30 kpc and a thickness of about 200 pc. The gas is in the form of clouds (with a mean density of about 1 atom per cubic centimeter and above) that fill the vast space between the stars. It is from these clouds that eventually new stars are formed when such a cloud becomes massive and compact enough to be able to cool and form molecules, and finally collapses under its own gravity.

The gas disc is substantially thinner than the disc in which the stars are located. From the above false-colour image we also get the impression that the thickness is not the same in all directions. For example, towards the Galactic Centre - in the centre of the image - it looks thinner than in the direction of the anti-centre. Let us measure the thickness of the gas disc, and find out more about the gas in our own Galaxy. Below we describe how to do it as a workshop activity:

Objectives:

Materials:

Observational Procedures

Once you started up the system, as described here you are ready to observe. Here we will expose you to two different types of observation. First it is important to try the manual version. It is good to know what you are looking for and you will get a real feeling of what it is like to use a telescope. We hope that seeing how easy it is to use will motivate you to do some of your own observing. We also want to provide some hints on what you should be looking for while you’re observing. First, watch the waterfall plot. The variation you see between each line should be slight and is due to the fluctuations of the noise in the signal. You may click on the yellow fields to adjust the range of the values represented by the colours. Since the galactic emission is concentrated to a narrow frequency range, you should be able to discern eventually a vertical band of slightly higher signal. This is also seen in the frequency plot to the upper left: the black curve shows the current spectrum, the red curve is the accumulated one, so that after a while the galactic features would become more distinct. After you had tried the manual observing, we will encourage you to run batch files while you are in class or doing other things. This will allow for much easier, less tedious data acquisition and hopefully permits you to accumulate as much data as you may need.

Manual Observations:

  1. Find the four places along the Galactic Plane labeled G20, G90, G150, and G180, indicating their galactic longitude. You will notice there are multiple other locations associated with them (G20.5, G90.10n, etc). These are the different galactic latitudes at which we will make observations (above and below the Plane).
  2. Find which point is currently visible and not close to the horizon.
  3. Make sure that your frequency is centered at 1420.4 MHz and your width is 1500 kHz span. Also, if not already done, plug in the PointCorr: Az =-0.5 El = -3.5 which are corrections necessary for the proper positioning in the sky.
  4. Move the telescope to G__.0, with __ indicating what ever longitude you had chosen, by clicking on that point. It is good to start with the position in the Plane, as the signal is strongest. The name of the source will be displayed on the right hand panel, and the telescope will execute the move. The actual position is displayed on the top right, with a background that blinks red (while moving in azimuth) and blue (elevation).
  5. When it is finished moving, have a look that the galactic feature is somewhere in the middle of the range. If not, you can change the central frequency by entering it in the corresponding field and hitting the return key. At the lower right, an appropriate message should appear in blue letters.
  6. Press the Record button and time this observation for about 5 minutes.
  7. Everytime you give a command, record it in the logbook, as well as any notes that you may have on the spectrum.
  8. After 5 minutes, continue recording but move the telescope to the next latitude position
  9. Repeat steps 8 and 7 for all the latitudes
  10. After your last observation, stop recording by pressing the Record button again.
  11. in case neither you nor someone else wants to use the telescope, stow it by pressing Stow on the top bar. Then shut down the system as described here.
  12. You find your recorded data in the text file at: C:/SRTcassi, named: year/month/day/time.txt for example: 1004141630.txt if I observed starting at 16:30 on April 14th 2010 (all times are given in UTC). Copy the file to your USB stick or flash drive to transfer onto your computer.
Batch Observations:
  1. Find which galactic longitudes are visible.
  2. Click cmdfil and open the .txt file for whichever G coordinate you want to observe. If you want to observe G20, open G20.txt. Each batch file will go from G__.15n to G__.15 observing each latitude for 500 seconds (to be tried and decided).
  3. In your first batch observations you might want to follow what is going on for a while. As each position takes about 8 minutes, you have time to observe how well the features in the red curve build up. The final spectrum will be close to what you will obtain in your analysis.
  4. After about one hour the batch is completed, you can find the file in the same place as above, step 12.

Analysis
Basic Operations

  1. To import your text file to Microsoft Excel, go to: File -> Import-> Text File -> Select your Text File -> Delimited Text file -> Delimiter is spaces -> Finish
  2. Each row represents a single spectrum taken by the telescope at a certain time instant. The columns from left to right represent: Time, Azimuth, Elevation, offsets in azimuth and elevation, velocity with respect to the Local Standard of Rest (LSR), the first frequency in the spectrum, the frequency increment, an integer number indicating the frequency mode, the number of frequency points in the spectrum, followed by the flux at the first frequency all though to the flux at the last frequency.
  3. It is very convenient to separate each block of data corresponding to a latitude value into its own worksheet. If you observed from b=-15° to +15°, you should have 7 separate worksheets.

    Treatment of Each Latitude

  4. Let us start with one worksheet: Insert at least 4 rows in between the top of the page and your first row of data.
  5. Insert two columns in between K and J.
  6. Above the first flux measurement, start an array that tells the frequency for each column. You can do this by starting with the frequency given in column G and adding the given increment (column H) to each subsequent column.
  7. Next, let us examine the time-averaged spectrum: Create another array below the frequency array, and put in the averages of the values of the fluxes measured during the time we stayed at this position. Now plot frequency versus flux and inspect the results.

    Removal of the Baseline

  8. As we had noticed in Step 7, there is a high level of background noise in our measurements, mainly from the noise produced in the electronic devices of the receiver itself. This is called the baseline, above which any genuine external emission is seen. As the noise is fairly independent of frequency, we can subtract this background, either as a constant value or by assuming that is a straight line, so that the baseline flux varies linearly with frequency.
  9. Using the plot we made in Step 7, select left and right of the feature of interest an empty spectral portion and take average values from both sides. The graph below shows an example where it is easy to find suitable portions to define the baseline, which is rather constant. Put this average value in some convenient cell next to the averaged spectrum.
  10. Below the time-averaged flux array, create an identical flux array with the baseline flux subtracted from the original flux values. Now plot these reduced fluxes against frequency, like in Step 7. In this plot the flat portions of the spectrum should be close to zero. If not, you can adjust your choice of the value for the baseline flux.
  11. If you notice that the flat and low portions of the spectrum do not meet the horizontal axis in the same manner, you have to deal with a sloping baseline. You must model a line using this equation:
    F(f) = Fa + (Fb-Fa)/(b-a)*(f-a)
    where you estimate the frequencies a and b and the fluxes Fa and Fb that best fit the background near the galactic feature. Make sure that it fits the whole profile as well as a zoomed in picture of the galactic feature.

    Finishing

  12. Repeat steps 4-11 for each galactic latitude (b)

    Determination of the Vertical Profile

  13. Open a new Excel worksheet and copy and paste the Frequency and Flux–Baseline fields from the top of each measurement. When you paste be sure to “Paste Special -> Values”.
  14. We essentially need to integrate the total flux received for each observed latitude. If we plot the total flux by the galactic latitude we should see a peak of total flux when looking directly in the Plane and it should decrease as we look further from the plane. To integrate the flux, SUM the flux values at each time-averaged spectrum, ignoring negative values. Make sure that you integrate only over the galactic feature, and not include any in-band interference signals. The red tick marks in the figure below give an example of how to place the integration limits. On the left hand side you see the strong interference we had named the “Twin Towers”. Notice how the genuine galactic emission was dwarfed by them.
  15. Now let's plot the total flux as a function of latitude. Do your results match the expected outcome?

From the Leiden/Dwingeloo Survey of the HI disc - done with a 25 m diameter radio telescope - one obtains these vertical profiles, after degrading the angular resolution to 6°, the HPBW of our telescope: l=20°

90°

150°

and 180°

Interpretation and Modeling We may interpret the obtained vertical profiles in various ways:

Modelling tool for the Galactic Disc

Latitude-Velocity Diagrams

When making the vertical profiles, we throw away the information from the radial velocities. Since the frequency of a feature in the spectrum relates to the radial speed with with this packet of gas moves away from us or moves towards us, we can learn also something about how the gas motions in the disc: The easiest way is to make a map - similar to the waterfall plot:

Below are the latitude-velocity maps from the original Leiden/Dwingeloo Survey data, for l=20°

90°

150°

and 180°

The white horizontal line marks the Galactic Plane. Our plots cannot show structures as fine or as faint as these, but the essential features will be there! What do they mean?

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last update: Feb. 2010 J.Köppen