21-cm Line
The spectral line given off by atomic hydrogen with a wavelength of 21 cm ( or frequency of 1.4 Gigahertz).
21-cm line - A spectral line of neutral hydrogen at the radio wavelength of of 21 cm.
21-cm line radiation emission line in the radio band by cool, neutral atomic hydrogen that is used to map the structure of the Milky Way and other galaxies because it passes easily through dust in the interstellar medium.
The 21-cm line in hydrogen is potentially a means of studying this period, as well as the "dark ages" that preceded reionization.
Because hydrogen is a major constituent of the interstellar medium, the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way.
The 21-cm line emission from neutral hydrogen is also observed throughout the Galaxy. Small changes in the observed wavelength of the 21-cm line are caused by the motion of the hydrogen clouds toward or away from an observer.
It is the 21-cm line of cold, neutral hydrogen in the galactic disk. (Sec. 18.4) It is used in preference to optical lines because (1) optical radiation is strongly absorbed by dust in the disk under study and (2) the 21-cm line is normally very ...
Tully-Fisher Relation: An empirical relationship between the width of the 21-cm line of hydrogen emissions from spiral galaxies, and the mass of the galaxy.
The wavelength interval within which a feature (e.g., the 21-cm line) is measured (cf. comparison band).
Signal-To-Noise Ratio
Signature ...
The wavelength interval measured in the continuum outside a spectral feature - e.g., the 21-cm line. [H76]
Complex Analytic ...
 See also: Astronomy, Galaxy, Light, Earth, Emission
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