L1157 southern blueshifted lobe in CO (1–0) (white contours; Gueth et al. 1996) and NH₂ CHO (4_1,4 –3_1,3) (cyan contours; Codella et al. 2017). The precessing jet ejected by the central object L1157-mm (outside the frame and toward the northwest) excavated several clumpy cavities, named B0, B1, and B2, respectively. The maps are centred at α(J 2000) = 20^h 39^m10^s.2, δ(J2000) = +68°01′10′′.5 (Δα = +25′′ and Δδ = –63′′.5 from L1157-mm). For the CO image, the first contour and step are 6σ (1σ = 0.5 Jy beam⁻¹ km s⁻¹) and 4σ, respectively. The first contour and step of the NH₂CHO map (cyan contours) correspond to 3σ (15 mJy beam⁻¹ km s⁻¹) and 1σ, respectively. The dashed circle shows the primary beam of the CH₃CHO images(64′′). Upper panels: map (in colour scale) of line emission (integrated over the whole velocity range) due to CH₃ CHO (5_0,4 –4_0,4) E, (5_0,4 –4_0,4) A, (5_1,4–4_1,3) E, and (5_1,4–4_1,3) A, respectively. The first contour and step are 3σ: 7 mJy beam⁻¹ km s⁻¹ (5_0,4 –4_0,4 E, 5_0,4 –4_0,4 A), and 8 mJy beam⁻¹ km s⁻¹ (5_1,4 –4_1,3 E, and 5_1,4–4_1,3 A). The cyan, white, and magenta ellipses depict the synthesised beams of the NH₂ CHO (5′′.79 × 4′′.81, PA = –94°), CO (3′′.65 × 2′′.96, PA = +88°), and CH₃CHO (2′′.97 × 2′′.26, PA = –155°) observations, respectively). Yellow labels are for the B0, B1, and B2 regions: the positions of the different clumps inside these regions are shown in Fig. 1 (see also text, and e.g. Codella et al. 2009, 2017). Lower panels: same as upper panels but for the CH₃ CHO (5_2,4 –4_2,3) A, (5_2,4–4_2,3) E, (5_2,3 –5_2,2) E, and (5_2,3–4_2,2) A. The first contour and step are 3σ: 5 mJy beam⁻¹ km s⁻¹ for 5_2,4 –4_2,3 A,E, and 4 mJy beam⁻¹ km s⁻¹ for 5_2,3 –5_2,2 A,E.