External galaxies exhibit the well-known “red shift”, a decrease of the observed frequency dependent on the distance of the galaxy. Receivers used at present allow successful studies to be made at intensities below 0.01 jansky *. Most normal spiral galaxies at this flux density level have radial velocities of 4000 to 6000 km/s, and therefore are observed at frequencies down to 1390 MHz. The number of normal galaxies for which hydrogen-line data are available is approximately 500; this number is increasing rapidly, and is being augmented by investigations of nearby dwarf galaxies which were heretofore undetectable. These studies are important for determining the gross properties of galaxies such as distance, mass and hydrogen content. Data on a wide variety of objects will facilitate the development of theories of galactic formation and evolution. Concurrently, detailed synthetic aperture observations of the larger nearby galaxies are being made to confirm theories of spiral structure. * One jansky (Jy) = I flux unit (f.u.) = IO-26 W/m’ per Hz. ** The frequencies in this Report are the rest frequencies for the radiation concerned. Some of the most exciting and important present day radio observations involve detection of the observed hydrogen line in distant continuum sources. These have been accomplished for the radio galaxy Perseus A [De Young eta/., 1973] and for the quasi-stellar objects 3C 286 [Brown and Roberts, 1973] and AO 0235 + 164 [Roberts et a!., 1976]. The detection of red-shifted 21 cm radiation from AO 0235 -l- 164 is of great cosmological significance. Since red-shifted optical lines (Mg + ) had been previously detected in this source it became possible to make a direct comparison between the radio and optical red-shifts over a large cosmological distance. The fact that the radio and optical red-shifts agreed to within the experimental uncertainties (1 part in 5000 for the optical measurement, and 1 part in 50 000 for the radio), allows one to place upper limits on the time variations of the physical constants which enter the atomic theory of spectroscopy. Historically, the first line to be detected in the radio spectrum, the 21-cm hydrogen line, continues to grow in importance. The development of sensitive receivers has broadened the scope of extra-galactic studies to include objects with large Doppler shifts. Current observational programmes are directed towards obtaining spectra of large numbers of galaxies having emission frequencies at and below 1370 MHz at intensities of about 0.01 jansky. At the same time, detailed studies of our own and nearby galaxies are continuing between 1415 and 1427 MHz. REFERENCES BROWN, R. L. and ROBERTS, M. S. [1973] Astrophys. Journ., 184, L7. BURTON, W. B. [1974] Galactic and extragalactic radio astronomy. Eds. G. L. Verschuur and K.L. Kellermann, Springer- Verlag, 82. DE YOUNG, D. S., ROBERTS, M. S. and SASLAW, W. C. [1973] Astrophys. Journ., 185, 809. EWEN, H. 1. and PURCELL, E. M. [1951] Nature, 168, 356. HEILES, C. and WRIXON, G. [1976], Methods of experimental physics, 13C, ed. M. L. Meeks (Academic Press: New York), 58. KERR, F. J. [1969] Annual Review of Astron. Astrophys., 7 39. ROBERTS, M.S., BROWN, R. L., BRUNDAGE, W. D., ROTS, A. H„ HAYNES, M.P. and WOLFE, A M. [1976] Astron. Journ., 81, 293. VERSCHUUR, G. L. [1971] Astrophys. Journ., 165, 651. ANNEX II THE OH LINES IN RADIOASTRONOMY 1. Introduction Radio-frequency spectral lines due to the hydroxyl molecule (OH) were first detected and measured in the laboratory in 1959 [Ehrenstein et a!., 1959], and in interstellar space in 1963 [Weinreb et al., 1963]. Absorption of radio-frequency radiation from the radio source Cassiopeia A was observed at frequencies ** which correspond to those of the two principal ground state lines, at 1665.401 and 1667.358 MHz. Shortly afterwards, even stronger absorption lines were found from the region of the galactic centre [Bolton et al., 1964a; Robinson et al., 1964; Goldstein et al., 1964], and two expected subsidiary ground-state lines, arising from alternative configurations of spins within the OH molecule, were detected and their rest frequencies determined at 1612.231 and 1720.527 MHz [Goss, 1968]. The very much lower intensity absorption line at 1639 MHz has been detected in the direction of the galactic centre [Rogers and Barrett, 1966]. This line is due to the OIXH molecule where the oxygen is the mass-18 isotope of the normal O16. Investigation of narrow-band emissions from regions of ionized hydrogen in the galaxy [Weaver et al., 1965; Zuckerman eta!., 1965] identified them as arising from OH. This emission is unexpectedly intense, producing increases in antenna temperature of as much as 150 K, and it has, surprisingly, been found to have a circularly polarized component [Davies et a!., 1966]. A number of the sources of OH emission have now been studied with interferometers, to get more precise positions of the sources [Rogers et a!., 1967] and estimates of the angular sizes of the sources [Davies et a!., 1967]. The new technique of “very long base-line” or VLB interferometry, with base-lines of length comparable with the diameter of the Earth, have been used to study some OH sources [Moran et al., 1967] and, also apparent angular sizes of less than 0.005 seconds of arc have been measured. This last technique, by an equipment improvement which is within the state of our present knowledge,
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