Neutral hydrogen in M31 - I. The distribution of neutral hydrogen.

This paper describes an investigation of the neutral hydrogen distribution in M 31 based on a survey of the entire object with a frequency resolution of 40 kHz ( km s-') and a beamwidth of 14' X 18' arc. The integrated neutral hydrogen map shows a neutral hydrogen distribution similar to the Milky Way with a deficiency in the central regions and with elongated spiral features. This distribution has been compared with the distribution of H ii regions, OB associations, blue stars and the radio continuum emission. It is found that there is a close correlation between the main neutral hydrogen and H ii features. An examination of the data suggests that the rate of star formation is greatest in areas of greatest hydrogen density. Individual neutral hydrogen features can be recognized on the narrow bandwidth contour maps. The velocities and locations of these features are listed in Table II.