Band 7 continuum contours for the three ACA pointings in this study on top of their respective HST color images. In all three panels, green solid (dashed) lines indicate positive (negative) contours of 870 μm imaging at the ±2, 3, 4, 5, 6σ levels. The yellow circle indicates the 20% sensitivity cut-off of the antenna primary beam. The white crossed ellipses at the lower left corners show the size of the synthesized beams. Upper panel: no significant detections are obtained within ACA data. Middle panel: two blobs of unambiguous emission are detected toward the center of the ACA beam, roughly cospatial with G1 and B1. Lower panel: four bright sources are detected at λ = 870 μm. Three of them are associated with A.1, A.2 and G1 respectively whereas the fourth source to the east is a previously unreported source (SMG). The inset shows a close-up view of a possible counterpart of the SMG in the WFC3-IR-F140W image (see Sect. 2.2.3).