Right: positions (J2000) observed in CO in the central part of M 31. The circles indicate 1-mm (green) and 3-mm (red) beams of the 10 positions observed superimposed on the A_B extinction map obtained in Melchior et al. (2000). As discussed in this paper, the extinction with the central 2 arcsec is not well-defined on this map. The (blue) cross indicates the centre of M 31 (, , Crane et al. 1992). Left: velocity field measured in CO for the whole galaxy by Nieten et al. (2006). A small rectangle in the central part indicates the field studied in this paper. This global view recalls the large-scale configuration of this galaxy: a steeply inclined disc at 77 deg with a position angle of 35 deg.