Fig. 2a and b. Distributions of FIR emission along the major axis of M31 (crosses; bars with arrows for upper limits), plotted as a function of the spatial offset x (positive in the northeast) relative to the nucleus of the galaxy. The M31 data are quoted from Table 1. The horizontal bars indicate the size (FWHM) of the spatial sampling profile. The dotted curves simulate our Galaxy (Nakagawa et al. 1998; the ISSA), located at the distance of M31 and observed with the LWS. The flux and flux density of our Galaxy were multiplied by . Extended-source correction is not applied for the LWS nor IRAS data. a  [CII ] line flux (; this work). The upper dotted curve indicates the flux corrected for the possible offset of the Galactic observations. The lower dotted curve is for that without the correction. b  Continuum flux density at . The dashed curve indicates IRAS HiRes brightness in M31 multiplied by the solid angle of the LWS beam.