Spectral index maps after image alignment, convolved with a beam of a radius equal to the geometric mean of the lower-frequency map beam (see Table A.1). Left panel: 15 GHz image in contours and the 15−43 GHz spectral index in colours. The contours start at 4.41 mJy beam⁻¹ and increase with a factor of two. The spectral index map is convolved with a circular beam of 0.55 mas. Right panel: we zoomed in on the nuclear region of the 43−86 GHz spectral index image, shown in colour to better illustrate the spectral index gradient. The contours start at 4.38 mJy beam⁻¹, corresponding to the 43 GHz image, and increase in steps of two. The spectral index map is convolved with a circular beam of 0.27 mas. We apply the same cutoffs as in Fig. 1.