Foreground: IRIS

IRIS Infrared Maps

Coordinate System:
Projection Type:
HEALPix, ring, res 10 (Nside=1024)
3.8 to 4.3 arcmin
Original Data Source:
Download Links:

From Miville-Deschênes and Lagache (2005):

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) suffers from calibration, zero level and striping problems that can preclude its use, especially at 12 and 25 micron. 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 microns). 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 micron the IRIS product is also a significant improvement from the Schlegel et al. (1998) maps. IRIS keeps the full ISSA resolution, it includes well calibrated point sources and the diffuse emission calibration at scales smaller than 1 degree was corrected for the variation of the IRAS detector responsivity with scale and brightness. The uncertainty on the IRIS calibration and zero level are dominated by the uncertainty on the DIRBE calibration and on the accuracy of the zodiacal light model.

The IRIS data products available here are HEALPix all-sky maps produced by Miville-Deschênes and Lagache, from the IRIS pages at the CITA website. The IRIS data were coadded over hours-confirmed sky coverages (HCON1, HCON2, and HCON3) and DIRBE data were used in unobserved regions (about 2% of the sky). Maps are available at Nside=1024 (3.4 arcminute pixels) and Nside=2048 (1.7 arcminute pixels). A few pixels in the Nside=2048 maps are undefined (value=-32768).

Main characteristics of the IRIS data

  12 micron 25 micron 60 micron 100 micron
Angular resolution (arcmin) 3.8 +- 0.2 3.8 +- 0.2 4.0 +- 0.2 4.3 +- 0.2
Noise level (MJy/sr) 0.04 +- 0.01 0.05 +- 0.02 0.03 +- 0.01 0.06 +- 0.02
Gain uncertainty (%) 5.1 15.1 10.4 13.5
Offset uncertainty (MJy/sr) 6.0E-5 8.3E-5 0.027 0.027

The offset uncertainties given in this table do not take into account the uncertainty on the zodiacal light removal (the zodiacal model used is that of Kelsall et al. 1998).


The IRIS data photometry is limited mainly by the zodiacal light subtraction of the DIRBE data, based on the model of Kelsall et al. 1998. A second order limitation is due to the fact that the ISSA plates were not constructed completely self-consistently. The overlapping regions between ISSA plates are not exactly the same. Usually this difference is negligible and ISSA/IRIS plates can be used to make mosaics. Nevertheless, in some cases, especially in bright regions, the difference can be of several percent. Both problems are illustrated in this image.

Further details and additional products can be found in:

"IRIS: A New Generation of IRAS Maps", Miville-Deschênes, M.-A., and Lagache, G. 2005, ApJS, 157, 302. ADS

IRIS pages at the CITA website

IRIS page at the Centre d'Analyse de Donnes Etendues

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