  The bipolar outflow from the massive star forming cluster in DR21 is one of
the most powerful known, and in IRAC images the outflow stands out by virtue of
its brightness at 4.5 um (Band 2). Indeed, IRAC images of many galactic and
extragalactic star formation regions feature prominent Band 2 morphologies. We
have analyzed archival ISOSWS spectra of the DR21 outflow, and compare them to
updated H2 shocked and UV-excitation models. We find that H2 line emission
contributes about 50% of the flux of the IRAC bands at 3.6 um, 4.5 um , and 5.8
um, and is a significant contributor to the 8.0 um band as well, and confirm
that the outflow contains multiple excitation mechanisms. Other potentially
strong features, in particular Br alpha and CO emission, have been suggested as
contributing to IRAC fluxes in outflows, but they are weak or absent in DR21;
surprisingly, there also is no evidence for strong PAH emission. The results
imply that IRAC images can be a powerful detector of, and diagnostic for,
outflows caused by massive star formation activity in our galaxy, and in other
galaxies as well. They also suggest that IRAC color-color diagnostic diagrams
may need to take into account the possible influence of these strong emission
lines. IRAC images of the general ISM in the region, away from the outflow, are
in approximate but not precise agreement with theoretical models.
