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The bipolar outflow from the massive star forming cluster in DR21 is one ofthe most powerful known, and in IRAC images the outflow stands out by virtue ofits brightness at 4.5 um (Band 2). Indeed, IRAC images of many galactic andextragalactic star formation regions feature prominent Band 2 morphologies. Wehave analyzed archival ISOSWS spectra of the DR21 outflow, and compare them toupdated H2 shocked and UV-excitation models. We find that H2 line emissioncontributes about 50% of the flux of the IRAC bands at 3.6 um, 4.5 um , and 5.8um, and is a significant contributor to the 8.0 um band as well, and confirmthat the outflow contains multiple excitation mechanisms. Other potentiallystrong features, in particular Br alpha and CO emission, have been suggested ascontributing to IRAC fluxes in outflows, but they are weak or absent in DR21;surprisingly, there also is no evidence for strong PAH emission. The resultsimply that IRAC images can be a powerful detector of, and diagnostic for,outflows caused by massive star formation activity in our galaxy, and in othergalaxies as well. They also suggest that IRAC color-color diagnostic diagramsmay need to take into account the possible influence of these strong emissionlines. IRAC images of the general ISM in the region, away from the outflow, arein approximate but not precise agreement with theoretical models.Comment: Accepted for publication in the Astrophysical Journal; 32 pages; 7  figure

Outflows from Massive Young Stellar Objects as Seen with the Infrared Array Camera