On the multiplicity of ALMA Compact Array counterparts of far-infrared bright quasars

We present ALMA Atacama Compact Array (ACA) 870 mu m continuum maps of 28 infrared-bright SDSS quasars with Herschel/SPIRE detections at redshifts 2-4, the largest such sample ever observed with ALMA. The ACA detections are centred on the SDSS coordinates to within 1 '' for about 80 per cent of the sample. Larger offsets indicate that the far-infrared (FIR) emission detected by Herschel might come from a companion source. The majority of the objects (similar to 70 per cent) have unique ACA counterparts within the SPIRE beam down to 3 ''-4 '' resolution. Only 30 per cent of the sample shows clear evidence for multiple sources with secondary counterparts contributing to the total 870 mu m flux within the SPIRE beam to at least 25 per cent. We discuss the limitations of the data based on simulated pairs of point-like sources at the resolution of the ACA and present an extensive comparison of our findings with recent works on the multiplicities of sub-millimetre galaxies. We conclude that, despite the coarse resolution of the ACA, our data support the idea that, for a large fraction of FIR-bright quasars, the sub-mm emission comes from single sources. Our results suggest that, on average, optically bright quasars with strong FIR emission are not triggered by early-stage mergers but are, instead, together with their associated star formation rates, the outcome of either late-stage mergers or secular processes.