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Radio studies of HII regions and galactic structure

Published online by Cambridge University Press:  14 August 2015

M. M. Komesaroff  and
G. Westerhout
M. M. Komesaroff
Affiliation:
CSIRO Radiophysics Laboratory, Sydney
G. Westerhout
Affiliation:
University of Maryland

Extract

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Radio studies of galactic HII regions are best carried out at the two ends of the radio spectrum. At high frequencies, of hundreds or thousands of megacycles per second, HII regions are seen by virtue of their thermal emission against a weak nonthermal background. Since radio waves are unaffected by the obscuration along the plane, it is possible in principle to see right through the Galaxy, and the high resolution which can be achieved in the thousands of megacycles range enables us to study at least the nearer regions in considerable detail. At low frequencies, below about 20 Mc/s, ionized hydrogen is seen in absorption against a bright nonthermal background. Since quite tenuous regions may be almost opaque at the lower frequencies, the technique provides quite a sensitive method of detecting them. The absorption increases with decreasing frequency so that studies at different frequencies enable us to see to varying depths along the line of sight and could permit the derivation of rough distance estimates.

Type
Section 1: The Galaxy
Information
Symposium - International Astronomical Union , Volume 20: The Galaxy and the Magellanic Clouds , 1964 , pp. 116 - 122
Copyright
Copyright © Australian Academy of Science 1964 

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