IRAS Explanatory Supplement
VII. Analysis of Processing
G. Associations

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As described in Section V.H.9, the associations of cataloged sources with IRAS sources were done purely on a positional basis, except that no association was allowed between a star and an IRAS source detected only at 100 µm. For the catalogs with small position errors, the search box was 45" × 8" (half-width), and the probability of a chance association with an IRAS source was less than 0.1%. For those catalogs where the search radius was significantly larger, e.g., galaxy catalogs and catalogs of diffuse objects, the probability of random associations was significantly greater. For the majority of the galaxy catalogs, where the search radius was 90", the probability of chance associations with IRAS sources was about 0.3% at high galactic latitude ( |b| > 45° ) and increased with increasing IRAS source surface density. A particular concern is the association of galaxies with infrared cirrus. The far infrared properties of an external galaxy can be very similar to those of cirrus. The user is urged to examine the cirrus flags before attaching significance to such associations.

It must be emphasized that the associations are not identifications. To be certain of any specific identification of an IRAS source with a cataloged source, the user must investigate the IRAS source in some detail.

About 28% of IRAS sources were associated with cataloged objects. Approximately 46,000 IRAS sources were; associated with cataloged stars and approximately 10,000 IRAS sources were associated with cataloged galaxies. Other classes of objects accounted for 15,000 associations. The vast majority of these were galactic nebulae and galaxies in the ESO (B) atlas. A measure of the fraction of spurious associations can be found in the number of IRAS sources associated with both stellar and galaxy-type objects. At high galactic latitudes ~ 0.6% of the IRAS sources had both galaxy and stellar associations.

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