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17. Optical investigations of radio sources: Introductory Lecture

  • R. Minkowski (a1)

Extract

Loose agreement of a radio position of low accuracy with that of some object listed in the NGC is not sufficient to provide the identification of a radio source. Even satisfactory coincidence of a precise position with that of an astronomical object requires supporting evidence. Agreement of the size of the source with that of the visible object, at least in order of magnitude, is an important argument in favour of an identification; exact agreement of sizes can be expected only where radio and optical emission are physically connected. The radio spectrum, the optical spectrum, and the physical characteristics of the visual object also have to be taken into account. Observations of the radio spectrum should be particularly useful to support the identification of sources with H 11 regions which can be recognized from their thermal emission even if they are obscured and optically inaccessible. If all data are available, satisfactory agreement exists between optical and radio observations. The best example of this kind at the moment is perhaps NGC 2237, the Rosette nebula, reported as a source by Ko and Krauss (1955) [1] and also observed by Mills, Little and Sheridan (1956 [11]; see also paper 18).

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References

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1. Ko, H. C. and Kraus, J. D. Nature , 176, 221, 1955.
2. Mills, B. Y. Aust. J. Sci. Res. A , 5, 266, 1952.
3. Mills, B. Y. Aust. J. Sci. Res. A , 5, 456, 1952.
4. Shklovsky, I. S. A.J. U.S.S.R. 30, 30, 1953.
5. de Vaucouleurs, G. Observatory , 73, 252, 1953.
6. Baade, W. and Minkowski, R. Observatory , 74, 130, 1954.
7. Baade, W. and Minkowski, R. Ap. J. 119, 215, 1954.
8. Seyfert, C. K. Ap. J. 97, 195, 1943.
9. Shklovsky, I. S. Reports of the Academy, U.S.S.R. 90, 983, 1953.
10. Baade, W. and Minkowski, R. Ap. J. 119, 206, 1954.
11. Mills, B. Y., Little, A. G., and Sheridan, K. V., Aust. J. Phys. 9, 218, 1956.
12. Photographs with the 48-inch Schmidt telescope and a plate-filter combination with a narrow passband for Hα have shown that the filaments are indeed part of an extremely faint nebulosity which coincides in position and size with the radio source HB 21 (Hanbury Brown, R. and Hazard, C., M.N.R.A.S. 113, 123, 1953).
13. Mills, B. Y., Aust. J. Phys. 9, 368, 1955.
14. The number of identified galactic non-thermal sources has now increased to nine.
15. An extremely faint nebulosity which is the remnant of Tycho's nova has recently been found. The approximate centre of the nebulosity follows by about 30 sec. the accepted position of the nova. It now appears probable that this position is less reliable than was formerly assumed and that the source is to be identified with Tycho's nova. Since a radio source in the position of Kepler's nova of 1604 has been found by Mills, Little and Sheridan [11] the remnants of all three known galactic supernovae of type 1 are now identified radio sources.
16. The nebulosity with which the source HB 21 has been identified [12] also belongs in this group.
17. If changes of shape of the diffuse filaments with time make a noticeable contribution to the random components of the proper motions, the distance of 540 parsec is only a lower limit for the true distance.

17. Optical investigations of radio sources: Introductory Lecture

  • R. Minkowski (a1)

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