Experiment Number: 170

Duration: 2 weeks

Galactic Hydrogen Line Profiling

Jump to Section:

This is currently the only experiment set for use with the radio telescope equipment, although the aims of this experiment are rather more open-ended than the experiments set for the optical telescope. It is designed to be more of an investigational experiment where students can make the experiment as in-depth and detailed (particularly in the quality of readings) as time permits.

Download full PDF laboratory script (coming soon!)

Go to top of pageAims of the experiment

The aim of this experiment is to observe the 21cm, 1420MHz radiation emitted by the neutral hydrogen situated in the spiral arms of our galaxy, the Milky Way, and to measure the line profiles for radiation originating from different spiral arms, and different regions of the galaxy.

Go to top of pageObjectives of the experiment

Students will:

Go to top of pageHazards and precautions to be taken

Wind pressure on the dish can damage the steering gear trains if the dish is moved in anything more than a moderate breeze. Always check the wind visually on the TV monitor and consult staff or technicians before releasing the brakes and operating the Alt-Azimuth controls. To read more on the safe and proper operation of the radio telescope, click here.

The following sections on the antenna and electronics (which can be found in full in the laboratory scripts) has been left out as its inclusion is not strictly necessary in this web-based version of the script (which is designed to be a concise over-view of the experiment only).

Go to top of pageMeasurements

There are two possible sets of measurements depending on whether the telescope can be moved from its parked position or not.

  1. Telescope parked due to winds
  2. The altitude co-ordinate will have to remain at 90 degrees throughout measurements. Use the Distant Suns software package to track which sections of the Milky Way pass through the beam (8 degree BWFN) during the observation period. Convert their positions into Galactic co-ordinates, using the computer program provided. Observe and produce printouts of hydrogen line profiles over the section of the Milky Way observable.

  3. Telescope Steerable
  4. Using Distant Suns, decide on several sections of the Milky Way to profile. It will be instructive to sample as wide a range as is observable and, if possible, to work with other laboratory groups to produce as comprehensive map as possible. Obtain the Alt-Az co-ordinates as well as RA-Dec and Galactic co-ordinates for the observing period. Obtain hydrogen line profiles at each of the chosen co-ordinates.

    Refer to the full laboratory script for information on calibration, although in the broader sense of the investigation, this information may be somewhat superfluous. Students may still wish to refer to this information for a greater understanding of the workings of the equipment.

The information that can be gained from the measurements taken will obviously depend on the accuracy of the readings taken. This involves a need for a good handling of system noise and having the optimum control/scan settings in order to obtain the best readings possible. This part of the investigation is for individual students to work out and to decide on how accurate their measurements need to be to allow them to achieve the objectives of the investigation. For advice on how to take the best readings, click here.

Go to top of pageHandling of results

A mathematical treatment of the data that can be obtained from the line profiles is given on page 3 of the laboratory script. Analysis of the line profiles should lead students to a reasonably good picture of the structure of the Milky Way and the temperatures of the gas clouds which make it up.

Profiles obtained from various parts of the Milk Way can be compared with standard profiles available in the radio telescope control room.

Go to top of pageExample Results

Below are a set of profiles obtained from several regions of the Milky Way. These profiles however, should not be taken as a definitive set of results because, as hinted at previously, results can always be improved upon via various means. In fact, if you put your mind to it, you should be able to produce better results than these! It is also the case that atmospheric conditions at the time of measurement and/or solar weather can have some effect on the scans that you take. Such effects may or may not have a significant effect on your scans, and are largely impossible to predict. However, the following scans are reasonable representations of the sort of scans students should be obtaining.

Simple inspection of each of your scans should tell you a great deal about what you are looking at and what each scan (and the peaks contained) means in the grand scheme of things. Questions that you will want to ask for a proper analysis of the scans you obtain are; (a) What do the positions (Doppler Shifts) of the peaks tell me?, (b) What do the widths of the peaks tell me? and (c) What do the heights of the peaks tell me? Information on the analysis of your scans can be found in the laboratory script.

"Clear Sky Scan" - It is important that you perform what is referred to as a "clear sky" scan, i.e. a scan looking perpendicularly to the plane of the galaxy. This should in theory give you a scan showing only system noise, however it may be the case (as shown here) that there is a definite peak in the scan. The position and approximate size of the peak should be noted for comparison with scans of gas clouds within the plane of the galaxy. It could be that a peak in your scan of a particular region in the plane of the Galaxy is not in fact due to the relative velocity of a gas cloud, but due to a systematic error from within the system's electronics.
Galactic Longitude (20,0,0) - As you can see from the graph on the right, there are a number of peaks to consider. The one on far left at doppler shift ~ -437.5 KHz looks very similar to the peak in the clear sky scan above. Thus, it is reasonably justifiable to ignore this peak. There is also a strong peak at Doppler shift ~ 157.4 KHz and it is likely that this represents a large gas cloud in one of the Galaxy's arms. There is also the possibility of two further peaks to the right of the large peak (one to the immediate right and one to the far right), however they are so small that it is neither possible to conclusively say that they represent arms of the Galaxy, nor possible to look at them in a quantitative sense.
Galactic Longitude (50,0,0) - There are two definite peaks on this graph. Although the peak on the left has quite a large negative Doppler shift, it is not close enough to the peak in the clear sky scan to be able to discount it. It is thus reasonable to state that we are looking at two distinct hydrogen gas clouds.
Galactic Longitude (80,0,0) - As you can see, there are two strong peaks in this scan, with one having a large Doppler shift (and thus large cloud velocity). There may also be a hint of a peak being close to the first peak (i.e. two different gas clouds or similar velocity) but unresolvable using this radio telescope.
Galactic Longitude (110,0,0) - The two peaks on this scan appear to be of lower intensity than peaks obtained by looking at different regions of the Milky Way.
Galactic Longitude (160,0,0) - This scan is interesting because it shows a peak which, if experimental uncertainties are taken into account, shows a gas cloud of ~ zero Doppler shift (i.e. neither moving towards or away from us). There also seems to be quite a small, low-intensity peak to the left of this larger peak.

Remember, if your scans do not look exactly like these, you have not necessarily performed the experiment incorrectly. The important thing in this experiment is that you analyse the scans that you have obtained in a qualitative sense as per the hints above and in a quantitative sense as outlined in the laboratory script. This investigation of the structure of the Milky Way requires analysis of evidence (your scans) as well as a discussion on the reliability of your evidence, as in any good investigation.

If however, you are having difficulty obtaining scans which look anything like these and are struggling to make sense of what you are seeing, please contact Dr Pomfret for advice.