Fig. 4. Distribution of the FIR line-to-continuum ratio along the major axis of M31 (crosses; bars with arrows for upper limits), plotted as a function of the spatial offset x relative to the nucleus of the galaxy. The M31 ratios with the uncertainties are 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 beam. The two dotted curves correspond to the cases with and without the correction for the possible [CII ] offset, as in Fig. 2a. For derivation of the Galactic ratios, the extended-source correction was applied to the emission.