Top panel, color and white contours: map of the 850 μm continuum emission (in arbitrary units) degraded to the beam size and orientation of the 1.3 mm continuum map with b = 0.5 weight (see Table 1). This was necessary to construct the spectral index map. The map is shown for the nuclear 1″ × 1″ region. We mark the approximate orientations of the ionization cone axis and the nuclear star forming disk. Bottom panel: map of the spectral index α as derived from the ALMA continuum observations in bands 6 and 7 using only pixels detected at > 2.5σ. The FoV is the same as in the top panel. The map has the same angular resolution as the band 6 map with the b = 0.5 weight (see Table 1). The color bar indicates the values of α (see text). The contours are the band 6 continuum emission at 1.3 mm as in the left panel of Fig. 2, and the star marks the AGN position.