TABLI I (continued) REFERENCES (TABLE 1) 1. GARDNER, F. F. and WHITEOAK, J. B. [1976]. Monthly Notes of the Royal Astronomical Society, 176, 57. 2. KROTO, H. W., KIRBY, C., WALTON, D. R. M., AVERY, L. W., BRUTEN, N. W., OKA, T. and MACLEOD, J. M. [1977], Bulletin of the American Astronomical Society, 9, 303. 3. SWEITZER, J. S., PALMER, P., MORRIS, M„ TURNER, B E. and ZUCKERMAN, B [1977] Bulletin of the American Astronomical Society, 9, 373. 4. BLACKMAN, G. L„ BROWN, R D., GODFREY, P. D., BASSEZ, M. P„ OTTREY, A. L„ WRINKLER, D. and ROBINSON, B. J. [1977], Monthly Notes of the Royal Astronomical Society, 180, 1. 5. TURNER, B. E„ KISLYAKOV, A. G„ LISZT, H. S. and KAIFU, N., [1975] Astrophys. Journ. (Letters), 201, L 149. 6. BLAKE, B. H. and PALMER, P [1977] Bulletin of the American Astronomical Society, 9, 429. 7. SNYDER, L. E., HOLLIS, J. M. and ULICH, B. L. [1976] Astrophys. Journ. (Letters), 208, L 91. 8. WATERS, J. W„ GUSTINCIC, J. J., KAKAR, R. K„ KUIPER, T. B. H„ SWANSON, P. N., KERR, A. R. and THADDEUS, P. [1976] Bulletin of the American Astronomical Society, 8, 564. 9. HUGGINS, P.J., PHILLIPS, T. G., NEUGEBAUER, G., WERNER, M. V., WANNIER, P. G. and ENNIS, D. [1977] Bulletin of the American Astronomical Society, 9, 353. 10. LOVAS, F. J., JOHNSON, D. R. and SNYDER, L. E. [1978], Astrophys. Journ. Suppl. (in preparation). ANNEX I THE NEUTRAL HYDROGEN LINE AT 1420.406 MHz Emission from atomic hydrogen occurs from a hyperfine spin-flip transition at the ground level of the atom. This emission was first observed in 1951 [Ewen and Purcell, 1951]. The study of the neutral hydrogen component of the interstellar medium is particularly suitable for a number of fundamental astronomical problems. Hydrogen, the most abundant element in the Universe, is also the main observed constituent of the interstellar medium. The motions and distribution of hydrogen are closely related to those of other galactic constituents, both stellar and interstellar. The interstellar medium is transparent enough to radio emission at 1420 MHz, so that, with the exception of a few directions along the galactic equator, it is possible to investigate the entire Milky Way galaxy and other distant galaxies. This transparency allows investigation of regions of the Milky Way which are too distant, or are too obscured by dust in the interstellar medium, to be studied optically. Interstellar neutral hydrogen is so abundant and is distributed in such a general fashion throughout the galaxy, that the emission line has been detected at every direction in the sky at which a suitably equipped radio telescope has been pointed. No time variation of this line has been found. Many 21-cm surveys of galactic hydrogen have been published. These are listed by [Kerr, 1969] and [Burton, 1974] and [Heiles and Wrixon, 1976], What is detected is a line profile giving intensity, usually expressed as a brightness temperature, as a function of frequency. The frequency observed for the line may indicate a Doppler shift from the natural rest frequency of 1420.406 MHz. The Doppler-shift information inherent in observations of hydrogen within.a galaxy is very important since it allows the kinematic distribution of the gas to be studied, in addition to the spatial distribution of intensities. In practice, the measured frequency shifts are converted to radial velocities. At 1420 MHz, a velocity of recession of I km/s corresponds to a reduction in frequency of 4.74 kHz. The natural width of the neutral hydrogen line is 5 x 10-16 Hz and is thus infinitesimally small compared to that which can be measured by radioastronomical methods. However, profiles observed near the Milky Way typically extend over 500 kHz. This broadening occurs throughout several mechanisms. The broadening corresponding to the thermal velocities of atoms within a single concentration of gas is typically 5 or 10 kHz. Turbulent motions within a concentration of hydrogen gas will also produce profile broadening of this order of magnitude. Large-scale streaming motions with Doppler-shift amplitudes of the order of 30 kHz have been observed in our galaxy and in others, and these motions influence the measured width of features in a profile. However, most of the total line broadening in galaxies comes from the differential rotation of the galaxy as a whole. For many galaxies this rotation broadens the profiles by 2500 kHz. Large-scale expansion motions in the still poorly understood central region of the Milky Way result in profiles broadened by a similar amount.
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