Asymmetric Line Profiles in Dense Molecular Clumps Observed in MALT90:
  Evidence for Global Collapse
  Using molecular line data from the Millimetre Astronomy Legacy Team 90 GHz
Survey (MALT90), we have searched the optically thick \hcop\, line for the
"blue asymmetry" spectroscopic signature of infall motion in a large sample of
high-mass, dense molecular clumps observed to be at different evolutionary
stages of star cluster formation according to their mid-infrared appearance. To
quantify the degree of the line asymmetry, we measure the asymmetry parameter
$A = {{I_{blue}-I_{red}}\over{I_{blue}+I_{red}}}$, the fraction of the
integrated intensity that lies to the blueshifted side of the systemic velocity
determined from the optically thin tracer \nthp. For a sample of 1,093 sources,
both the mean and median of $A$ are positive ($A = 0.083\pm0.010$ and
$0.065\pm0.009$, respectively) with high statistical significance, and a
majority of sources (a fraction of $0.607 \pm 0.015$ of the sample) show
positive values of A, indicating a preponderance of blue-asymmetric profiles
over red-asymmetric profiles. Two other measures, the local slope of the line
at the systemic velocity and the $\delta v$ parameter of \citet{Mardones1997},
also show an overall blue asymmetry for the sample, but with smaller
statistical significance. This blue asymmetry indicates that these high-mass
clumps are predominantly undergoing gravitational collapse. The blue asymmetry
is larger ($A \sim 0.12$) for the earliest evolutionary stages (quiescent,
protostellar and compact H II region) than for the later H II region ($A \sim
0.06$) and PDR ($A \sim 0$) classifications.
