Blue Asymmetries in MALT90 HCO+ Line Profiles of Molecular Clumps: Evidence for Global Gravitational Collapse

The Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90) detected dense molecular gas tracers toward thousands of high-mass molecular clumps. We have examined the optically thick HCO+ 1-0 line in 1,093 clumps for the ``blue asymmetry'' spectroscopic signature of infall. 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 total integrated intensity that lies to the blueshifted side of the systemic velocity, as determined from the optically thin tracer N2H+. For a sample of 1,093 clumps, the mean of A (0.083 +- 0.010) indicates a preponderance of infall motions over expansion motions at the 8 sigma level. Two other infall indicators, the slope of the optically thick line at the systemic velocity and the "delta v" parameter, also indicate infall motions, but with smaller statistical significance (3 and 4 sigma, respectively). This blue asymmetry indicates that these high-mass clumps are predominantly undergoing gravitational collapse. The blue asymmetry is larger (A ~ 0.12) for the earliest evolutionary stages (quiescent, protostellar and compact H II region) than for the later H II region (A ~ 0.06) and PDR (A ~ 0) classifications, indicating that global collapse is already underway in the very earliest stages of high-mass star formation, and that either the collapse halts, or else the collapse signal is erased by expansion motions, after the H II regions form. Global gravitational collapse is required by "competitive accretion" models of high-mass star formation. We also present data from a new infall indicator, the 63 um [O I] line, measured with SOFIA, that clearly indicates gravitational collapse toward an active star-forming region in the "Nessie Nebula."