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A study of the distribution of atomic hydrogen in the Andromeda nebula by means of an interference spectrometer Argyle, Percy Edward

Abstract

The increasing importance of narrow-band radiations is traced in the recent history of radio astronomy. The need for multi-channel spectrometers to observe these radiations is stressed. The theoretical basis of spectral analysis is given with particular emphasis on autocorrelation or interference methods. A twenty-channel radio-frequency spectrometer designed on interference principles has been constructed and found to perform in accordance with its theory of operation. When used in conjunction with a 25-metre telescope and a hydrogen receiver the spectrometer is capable of producing low-noise wideband spectra at high speed. The spectrometer output was recorded on punched cards, and subsequent data processing was by digital methods. A large area including the position of the spiral galaxy M31 was surveyed with the help of the spectrometer. One hundred and forty-three independent spectra of the 21-cm radiation of atomic hydrogen were obtained and are analyzed in terms of area, velocity, and shape. The major axis of the nebula is found to extend about 2.5° either side of the centre, in agreement with the work of van de Hulst, Raimond, and van Woerden (1957). The length of the minor axis, after correction for the effect of the antenna beamwidth, is only 40', a result which indicates a reduction in the inclination (of the plane of the galaxy to the line of sight) from 14.5° to 8.2°. This lower value leads to an upward revision of the optical axial ratio (of galactic thickness to galactic diameter) to 0.2, while maintaining a low ratio (0.07, Schmidt, 1957) for the distribution of atomic hydrogen. A high axial ratio would clear the way for a reinterpretation of the optical velocities of emission nebulae in M31 (Mayall, 1950), which have so far appeared to be in violent contradiction to the radio velocities (van de Hulst et al, 1957). It is suggested that many of these emission objects may lie some distance from the plane of the galaxy. The position angle of M31, as revealed by the spectrometer observations is distinctly less than the 38° established optically. A new value of 33° is proposed and it is suggested that the former value can be accounted for in terms of an increased axial ratio and the observed asymmetries in the light distribution. The velocity of the centre of gravity of M31 has been obtained by summation of all 143 spectra. The result, -295.6±0.4 km/sec with respect to the local standard of rest, is in complete agreement with that found by van de Hulst (ibid). Radial motions of a few km/sec are possible in the outer parts of M31 but their presence has not been established. Many of the spectra have multiple peaks, which may be interpreted in terms of spiral structure. The velocities of certain concentrations of atomic hydrogen are measurable but their positions are not resolved by the antenna beam.

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