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, we have searched the optically thick HCO+ (1-0) 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/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 N2H+ (1-0). For a sample of 1093 sources, both the mean and median of A are positive (A = 0.083 +/- 0.010 and 0.065 +/- 0.009, respectively) with high statistical significance, and a majority of sources (a fraction of 0.607 +/- 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 Mardones et al. (1997), 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 similar to 0.12) for the earliest evolutionary stages (quiescent, protostellar, and compact H II region) than for the later H II region (A similar to 0.06) and photodissociation region (A similar to 0) classifications.