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The Question

(Submitted July 14, 1998)

What proof do astronomers have when saying our galaxy looks a certain way? All evidence would be indirect I would think. What is this evidence?

The Answer

You have a very good point here: How can we have any idea what our galaxy looks like from the standpoint of an outside observer, when we ourselves are embedded in the middle of it? As you surmised, some detective work is required. If we were to rely only on optical light in trying to formulate an educated guess as to the answer, it would be next to impossible because of all the obscuration due to dust in the plane of our galaxy. When one goes to other wavelengths, however, the project becomes do-able.

The first work which mapped out the spiral structure of our galaxy was done at radio wavelengths by studying the "21 cm" line which is due to the "spin-flip" transition of the hydrogen atom. Basically, this means that the hydrogen atom can have slightly different energy states depending on whether the spin of the nucleus is parallel or anti-parallel with the spin of the whole atom. This change from one state to the other produces this emission, which passes through the dust. By investigating the strength of this emission as a function of point on the sky, early workers (i) determined that we live in a spiral galaxy, and (ii) mapped out the spiral arms. In addition to measuring the strength of the signal at different points of the sky, one also makes use of the Doppler shift of the signal to infer the velocity structure.

More recently it has become feasible to perform this exercise at other wavelengths. For instance, the COMPTEL instrument on the Compton Gamma Ray Observatory has mapped out the spatial distribution of radioactive aluminum 26 which produces an emission line at 1.8 million electron volts. To see what kind of galactic structure they have inferred, take a look at the figure in the following paper. (The paper itself is rather technical, but the figure just shows a picture of the spiral structure of the galaxy in our vicinity.) The paper is just 2 pages, with one figure.

(i) Go to this site

(ii) In the author field enter: Chen, Gehrels, Diehl, Hartmann, then select the Boolean "and" search option, and click "send query". This will get you the following paper:

"On the spiral arm interpretation of COMPTEL 26 Al map features" Astronomy & Astrophysics Supplement Series vol. 120, 315. (1996)

It has just been pointed out to me that it may be difficult for you to access this reference in the manner I suggest if you are on other than a UNIX machine. In that event, it may be necessary to find a university library to get this paper.

Of possible interest, here is the Multiwavelength Milky Way poster site

J.K. Cannizzo, K. Smale, D. Palmer
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