Known heating sources. On the left side, we superimpose on the Hα+[NII] map from Ciardullo et al. (1988): the X-ray sources detected by Hofmann et al. (2013) with blue dots; the 1 Gyr young stellar cluster detected by Kang et al. (2012) with the green triangle; and the X-ray sources detected by Stiele et al. (2011) and Barnard et al. (2014) with white and purple dots. The blue dots with a yellow circle correspond to the first (and more central) ultra-luminous X-ray transient (ULX) detected in M31 (e.g. Kaur et al. 2012). The black contours correspond to the apparent A_B extinction map discussed in Melchior et al. (2000). The light blue contours correspond to the 6 cm polarised intensity detected by Gießübel & Beck (2014). The large red circles correspond to our detections. Light black circles provide typical projected radial distance from the centre (x pc and y pc). Thick ellipses show the approximate position of the inner ring. On the right side, we indicate, in the top panel, the number of X-ray sources (Hofmann et al. 2013) detected at each radius; in the bottom panel, the X-ray luminosity at each radius is shown as a function of the radius. The mean and median values of the X-ray luminosity (averaged in 100 pc radial bin) is also provided.