A Spitzer Space Telescope infrared survey of supernova remnants in the inner Galaxy

Using Infrared Array Camera ( IRAC) images at 3.6, 4.5, 5.8, and 8 mu m from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within Galactic longitudes 65 degrees > vertical bar l vertical bar > 10 degrees and latitudes vertical bar b vertical bar < 1 degrees. Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow the separation of bright H (II) regions and pervasive mid- infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock- heated dust, atomic fine- structure lines, and molecular lines. The detected remnants are G11.2 - 0.3, Kes 69, G22.7 - 0.2, 3C 391, W44, 3C 396, 3C 397, W49B, G54.4 - 0.3, Kes 17, Kes 20A, RCW 103, G344.7 - 0.1, G346.6 - 0.2, CTB 37A, G348.5 - 0.0, and G349.7+ 0.2. The infrared colors suggest emission from molecular lines ( nine remnants), fine- structure lines ( three remnants), polycyclic aromatic hydrocarbons ( four remnants), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid- infrared wavelengths. The IRAC- detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid- infrared.