IRIS: A New Generation of IRAS Maps

The Infrared Astronomical Satellite (IRAS) had a tremendous impact on many areas of modern astrophysics. In particular, it revealed the ubiquity of infrared cirrus that are a spectacular manifestation of the interstellar medium complexity but also an important foreground for observational cosmology. With the forthcoming Planck satellite there is a need for all-sky complementary data sets with arcminute resolution that can bring informations on specific foreground emissions that contaminate the cosmic microwave background radiation. With its ~4' resolution matching perfectly the high-frequency bands of Planck, IRAS is a natural data set to study the variations of dust properties at all scales. But the latest version of the images delivered by the IRAS team (the ISSA plates) suffer from calibration, zero level, and striping problems that can preclude its use, especially at 12 and 25 μm. In this paper we present how we proceeded to solve each of these problems and enhance significantly the general quality of the ISSA plates in the four bands (12, 25, 60, and 100 μm). This new generation of IRAS images, called IRIS, benefits from a better zodiacal light subtraction, from a calibration and zero level compatible with DIRBE, and from a better destriping. At 100 μm the IRIS product is also a significant improvement from the Schlegel et al. maps. IRIS keeps the full ISSA resolution, it includes well-calibrated point sources, and the diffuse emission calibration at scales smaller than 1° was corrected for the variation of the IRAS detector responsivity with scale and brightness. The uncertainty on the IRIS calibration and zero level is dominated by the uncertainty on the DIRBE calibration and on the accuracy of the zodiacal light model.