Abstract
A neutral hydrogen survey of the southern region of M 31 and selected regions in the north including the major axis has been made with a beamwidth of 15′ and a bandwidth of 200 kc/s (42 km/s). Neutral hydrogen was found out to 150′ from the centre along the major axis. A major concentration in the neutral hydrogen distribution was observed in a broad ring of radius 9 kpc. Within 120′ from the centre the neutral hydrogen concentrations coincide with the major H II features. The overall neutral hydrogen mass was estimated to be 2.4 × 10 9M⊙ at a distance of 630 kpc while the ionized hydrogen mass was calculated to be less than 1.5 × 10 7M⊙ . A new rotation curve has been obtained for M 31. It shows apparently significant differences between the NE and SW sections. The mean rotation curve indicates a mass of 2.4 × 10 11M⊙ out to 150′ and an estimated total mass of 4.8 x× 10 11M⊙ . The mass distribution determined from the rotation curve shows a less strong central concentration than Schmidt obtained from the van de Hulst, Raimond & van Woerden data. The M / LB distribution uncorrected for obscuration shows a constant value of ∼20 between 30′ and 75′ from the centre. The mean value of M / LB taken out to 120′ is ∼16 which on correction for obscuration is ∼9.
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