Mapping Warm Molecular Hydrogen with the Spitzer Infrared Array Camera (IRAC)

Photometric maps, obtained with the Spitzer Infrared Array Camera (IRAC), can provide a valuable probe of warm molecular hydrogen within the interstellar medium. IRAC maps of the supernova remnant IC 443, extracted from the Spitzer archive, are strikingly similar to spectral line maps of the H₂ pure rotational transitions that we obtained with the Infrared Spectrograph (IRS) instrument on Spitzer. IRS spectroscopy indicates that IRAC bands 3 and 4 are indeed dominated by the H₂ v = 0-0 S(5) and S(7) transitions, respectively. Modeling of the 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 IC 443, obtained with IRAC, show band ratios that are inconsistent with the presence of gas at a single temperature. The relative strengths of IRAC bands 2, 3, and 4 are consistent with pure H₂ emission from shocked material with a power-law distribution of gas temperatures. CO vibrational emissions do not contribute significantly to the observed band 2 intensity. Assuming that the column density of H₂ at temperatures T to T + dT is proportional to T^−b for temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The power-law index, b, shows variations over the range ~3-6 within the set of different sight lines probed by the maps, with the majority of sight lines showing b in the range 4-5. The observed power-law index is consistent with the predictions of simple models for paraboloidal bow shocks.