An All-Sky Search for Molecular Cirrus Clouds

The infrared (IR) brightness of the sky, once the zodiacal light is removed, has been shown to be highly correlated with the total gaseous (H I and H_2_) content of the interstellar medium. We have used this correlation to deduce a map of infrared-excess gas by subtracting, in an appropriate way, the H I component from the infrared brightness. Having computed the correlation between the 100 micron IR and 21 cm H I emission data (with pixels of 20') for each small solid angle in the sky (~10 deg^2^), we deduced the residual for each pixel. The large IR-excess residuals that are spatially connected define IR-excess clouds (IRECs). The method is limited to part of the sky where the heating of the dust is relatively uniform and allows the identification of the nearby molecular clouds at high Galactic latitude (|b| >~ 5^deg^) and/or the clouds which have local heating sources or a larger dust-to-gas ratio than the average. An all-sky map and a catalog of 516 significant clouds are presented and statistically analyzed. About half the CO clouds at |b| >~ 25^deg^ observed by Magnani, Blitz, and Mundy coincide with entries in our catalog of IRECs, and are used as a comparison sample to assess the validity and the limits of our method. Further observations, particularly in CO, could help in understanding the nature of the objects in the present catalog. Spatial variations of the IR-to-gas ratio and locations of IR-deficient clouds also constitute results of the performed analysis.