Continuum maps at 3 mm and 1 mm. Contour levels start at a significance of 5σ with respect to the rms measured in both images (rms = 0.07 mJy channel⁻¹ and 0.6 mJy channel⁻¹ for the 3 mm and 1 mm maps respectively). The contour steps are 1 and 3 mJy km s⁻¹ beam⁻¹. The crosses at the centres mark the continuum peaks, which we take as the location of the northern nuclear source (see Sect. 1). The synthesised beams are shown in the bottom-left corner. The colour flux scales are in Jy km s⁻¹ beam⁻¹.