The SEDIGISM survey: A search for molecular outflows

Context. The formation processes of massive stars are still unclear, but a picture is emerging involving accretion disks and molecular outflows in what appears to be a scaled-up version of low-mass star formation. A census of outflow activity toward high-mass star-forming clumps in various evolutionary stages has the potential to shed light on high-mass star formation. Aims. We conducted an outflow survey toward ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) clumps using SEDIGISM (structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium) data and aimed to obtain a large sample of clumps exhibiting outflow activity in different evolutionary stages. Methods. We identify the high-velocity wings of the (CO)-C-13 lines, which indicate outflow activity, toward ATLASGAL clumps by (1) extracting the simultaneously observed (CO)-C-13 (2-1) and (CO)-O-18 (2-1) spectra from SEDIGISM, and (2) subtracting Gaussian fits to the scaled (CO)-O-18 (core emission) from the (CO)-C-13 line after considering opacity broadening. Results. We detected high-velocity gas toward 1192 clumps out of a total sample of 2052, corresponding to an overall detection rate of 58%. Outflow activity has been detected in the earliest (apparently) quiescent clumps (i.e., 70 mu m weak) to the most evolved H II region stages (i.e., 8 mu m bright with tracers of massive star formation). The detection rate increases as a function of evolution (quiescent = 51%, protostellar = 47%, YSO = 57%, UC H II regions = 76%). Conclusions. Our sample is the largest outflow sample identified so far. The high detection rate from this large sample is consistent with the results of similar studies reported in the literature and supports the scenario that outflows are a ubiquitous feature of high-mass star formation. The lower detection rate in early evolutionary stages may be due to the fact that outflows in the early stages are weak and difficult to detect. We obtain a statistically significant sample of outflow clumps for every evolutionary stage, especially for outflow clumps in the earliest stage (i.e., 70 mu m dark). The detections of outflows in the 70 mu m dark clumps suggest that the absence of 70 mu m emission is not a robust indicator of starless and/or pre-stellar cores.