Left panel: stacked spectrum of 170 non-detections. Middle panel: stacked spectrum of 72 non-detections with compact radio emission or interacting sources (red) and of 80 non-detections with extended radio emission (blue). Right panel: stacked spectrum of 71 non-detections classified as 12μm bright or 4.6μm bright galaxies (orange), and of 97 non-detections classified as dust-poor (green), according to the WISE colours.