High resolution observations of neutral hydrogen in M31

Observations of the galaxy M31 at 21 cm wavelength have been made with the Cambridge Half-Mile telescope, enabling the neutral hydrogen to be mapped with an angular resolution of 1′.5 × 2′.2 and a velocity resolution of 39 km s−1. The central deficiency of H I is found to extend to 4 kpc from the nucleus but beyond that distance the H I emission is seen to be strongly associated with dust lanes. The surface densities of H II regions and the youngest OB associations are found to vary approximately as the square of the H I surface density. H I emission along the major axis has been detected to 162′(32.4 kpc) SW from the nucleus, and to 120′(24 kpc) NE.