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Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), canprovide a valuable probe of warm molecular hydrogen within the interstellarmedium. IRAC maps of the supernova remnant IC443, extracted from the Spitzerarchive, are strikingly similar to spectral line maps of the H2 pure rotationaltransitions that we obtained with the Infrared Spectrograph (IRS) instrument onSpitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeeddominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling ofthe H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5)and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC,show band ratios that are inconsistent with the presence of gas at a singletemperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistentwith pure H2 emission from shocked material with a power-law distribution ofgas temperatures. CO vibrational emissions do not contribute significantly tothe observed Band 2 intensity. Assuming that the column density of H2 attemperatures T to T+dT is proportional to T raised to the power -b fortemperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. Thepower-law index, b, shows variations over the range 3 to 6 within the set ofdifferent sight-lines probed by the maps, with the majority of sight-linesshowing b in the range 4 to 5. The observed power-law index is consistent withthe predictions of simple models for paraboloidal bow shocks.

Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)