21-Centimeter Radio Astrophysics

Measurement of the Doppler spectrum of interstellar atomic hydrogen and the dynamics of the galactic rotation. A 2.5-meter computer-controlled alt-azimuth parabolic dish antenna, located on a roof of MIT, is used with a heterodyne measurement chain and digital correlator to observe the Doppler spectrum of the 21-cm hyperfine line of interstellar atomic hydrogen in various directions along the Milky Way. Features of the spiral-arm structure of the Galaxy are deduced from the measured radial velocities of the HI clouds in the galactic disc.

Lab Guide (PDF)

A 2.5-meter computer-controlled alt-azimuth parabolic dish antenna. (Image courtesy of Junior Lab staff.)

References

Ewen, H. I., and E. M. Purcell. "Observation of a Line in the Galactic Radio Spectrum." Nature 168 (1951): 356.

Mihalas, Dimitri, and James Binney. "Galactic Rotation and the Spiral Structure of Our Galaxy." Chapter 8 in Galactic Astronomy. San Francisco, CA: W. H. Freeman, 1968.

Kerr, F. J. "Neutral Hydrogen and Galactic Structure." Proceedings of the IEEE 61, no. 9 (September 1973).

Lerner, Rita G., and George L. Trigg. "Radio Astronomy." In Encyclopedia of Physics. Reading, MA: Addison-Wesley, 1981, pp. 41-42. ISBN: 0201043130.

Nobel Prize Lecture for Martin Ryle, and Antony Hewish. Stockholm, Sweden, 1974. (For their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular for the aperture systhesis technique, and Hewish for his decisive role in the discovery of pulsars.)

Shu, Frank H. "Our Galaxy: The Milky Way System." Chapter 12 in The Physical Universe: An Introduction to Astronomy. Mill Valley, CA: University Science Books, 1982. ISBN: 0935702059.
This reference gives a clear description of the interpretation of 21cm spectra in terms of the rotation curve of the Galaxy.

Tuve, M. A., and S. Lundsager. "Velocity Structures in Hydrogen Profiles: A Sky Atlas of Neutral Hydrogen Emission." Carnegie Institution of Washington Publication, no. 630 (1973).

Weaver, H. "Spiral Structure of the Galaxy Derived from the Hat Creek Survey of Neutral Hydrogen." In Symposium on the Spiral Structure of Our Galaxy University of Basel, 1969. Edited by W. Becker and G. Contopoulos. Dordrecht: Reidel, 1970, Symp. no. 38, pp. 126-139, I. A. U. ISBN: 9027701091.