Mid-infrared surveys of the Galactic plane

Two complementary satellite-based mid-infrared surveys of the Galactic plane were conducted in the 1990s. The Midcourse Space eXperiment (MSX) surveyed the entire plane within \b\ <5degrees in four mid-infrared spectral hands between 8.3 and 21.3 mum at a resolution of 20". The camera on the Infrared Space Observatory (ISO) was used to survey similar to16 deg(2) of the inner Galaxy at 7 and/or 15 mum to at 6" resolution under the aegis of the ISOCAL experiment. The images and catalogs produced by these surveys are available to the general astronomical community and we summarize the results of a number of scientific papers based on the survey products. The surveys reveal the true complexity of the structure in the interstellar emission along the Galactic plane. Both MSX and ISO "discovered" a new component of the Galaxy, several thousand infrared dark clouds that are so cold (T<15K) that they are seen in absorption against the infrared background emission along the Galactic plane. When combined with velocity information from millimeter molecular line observations both the dark clouds and the mid-infrared identified young stellar objects are tracers of Galactic structure. The catalogs have been used to derive the mid-infrared luminosity function of the bulge asymptotic giant branch stars, and to identify and characterize OH and SiO masers.