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 μm 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^deg>|l|>10^deg and latitudes |b|<1^deg. 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.