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 towards massive star-forming clumps in various evolutionary
stages has the potential to shed light on massive star formation (MSF).
  Aims. We conducted an outflow survey towards ATLASGAL clumps using SEDIGISM
data and aimed to obtain a large sample of clumps exhibiting outflows in
different evolutionary stages.
  Methods. We identify the high-velocity wings of the 13CO lines toward
ATLASGAL clumps by (1) extracting the simultaneously observed 13CO and C18O
spectra from SEDIGISM, and (2) subtracting Gaussian fits to the scaled C18O
from the 13CO, line after considering opacity broadening.
  Results. We have detected high-velocity gas towards 1192 clumps out of a
total sample of 2052, giving an overall detection rate of 58%. Outflow activity
has been detected in the earliest quiescent clumps (i.e., 70$\mu$m weak), to
the most evolved HII region stages i.e., 8$\mu$m bright with MSF tracers. The
detection rate increases as a function of evolution (quiescent=51%,
protostellar=47%, YSO=57%, UCHII regions=76%).
  Conclusion. Our sample is the largest outflow sample identified so far. The
high-detection rate from this large sample is consistent with previous results
and supports that outflows are a ubiquitous feature of MSF. The lower detection
rate in early evolutionary stages may be due to 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 70$\mu$m dark stage. This suggests that the absence of 70$\mu$m
emission is not a robust indicator of starless/pre-stellar cores.
