The Interstellar Medium, Part III, the Neutral Gas
radiation from being seen in the lab. It takes one-hundred trillion atoms to see one photon per second, but those atoms have to be in a very sparse (~ 0.1 atom per cubic centimeter) and cool (~ 100 K) environment. Otherwise, collsions with other atoms would dominate this subtle effect. Since such a vacuum has never been created on the earth, 21-cm. radiation was not discovered until the 1940s, when the discovery itself was called the "most important achievement in the astronomical study of the ISM."
The extreme subtlety of such radiation has a very important advantage, however. Because the transition probability is so low, the radiation is not absorbed by hydrogen between us and the emitting region. Such a situation is called "optically thin." Another example of optically thin radiation is one candle behind another. You can see the flame of the rear candle through the flame of the near candle and thus perceive that there are two candles. Similarly, we can see one neutral hydrogen cloud behind another and perceive that there are two. This may seem trivial, but we could not perceive whether there was another star behind the sun just by looking, and the same is true of observing gas in many other ways. Because the gas is optically thin in this transition we can see it all and, so, measure the total amount of neutral hydrogen in any galaxy our radio telescopes are powerful enough to reach.
Aside from taking a census of the total amount of neutral gas in a galaxy, a very important measurement one can make from the neutral hydrogen is the rotation velocity of a galaxy as a function of radius, seen here (Rots & Shane 1975; contours are in kilometers per second).
This measurement contributes vitally to three very exciting aspects of cosmology. First, the rotation curve measures the total mass (in gas, stars, dark matter, etc.) in the galaxy and so tells us how much dark matter the galaxy contains. Second, the measurement gives us clues about any funny dynamics going on that might suggest a recent merger of two galaxies, which gives us some evidence about the way galaxies form. Third, the rotation velocity can be used as one component of the Tully-Fisher relation, which relates the rotation velocity tightly to the total luminosity. If we know the total luminosity and the amount of light we get back here on earth, we know its distance. If we then measure the redshift of the galaxy, we put a constraint on the Hubble constant. Massive surveys to do just this have been
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