Neutral Hydrogen in the Galactic Region--II LOCATION OF THE EMISSION FEATURES

An interpretation and discussion is given of Cohen's observations of 21-cm hydrogen line emission from the galactic centre region. Only the high-velocity emission distinct from the main maximum at zero velocity is considered. The distances of many of the individual emission features have been estimated from the variation of velocity with longitude, and these distances used to calculate hydrogen masses and the z-distances of the gas above and below the plane. The distribution of gas within 2·5 kpc of the centre is found to be tilted with respect to the plane, with its pole in the direction $$l\,=\,124\pm {28}^{^{\circ} },\,b=81\pm {4}^{^{\circ} }.$$ Expansional components of motion of up to 175 km s^–1 away from the centre are found, with a tendency for higher expansion velocities to occur nearer the centre. The rotational velocities of features lying out of the plane are found to decrease markedly towards the centre. These rotational velocities are considerably less than the circular velocities given by the Rougoor–Oort rotation curve. On the other hand the rotational velocities of the features in the disk follow this curve quite well for distances between 4 and 2 kpc from the centre, and again within about 800 pc of the centre. These results are discussed in terms of two current theories on the origin of non-circular motions in the central region of the Galaxy.