Infrared Excess and Molecular Clouds: A Comparison of New Surveys of Far‐Infrared and Hi21 Centimeter Emission at High Galactic Latitudes

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Abstract
We have created a map of the large-scale infrared surface brightness in excess of that associated with the atomic interstellar medium, using region-by-region correlations between the far-infrared and 21 cm line surface brightness. Our study updates and extends a previous attempt with the Infrared Astronomical Satellite and Berkeley/Parkes H I surveys; in this study we used far-infrared (60-240 μm) data from the Cosmic Background Explorer Diffuse Infrared Background Experiment and 21 cm data from the combined Leiden-Dwingeloo and Parkes 21 cm line surveys. Using the maps of excess infrared emission at 100, 140, and 240 μm, we created an atlas and identified the coherent structures. These infrared excess clouds can be caused both by dust that is warmer than average or by dust associated with gas other than the atomic interstellar medium. We find very few warm clouds—which are relatively bright at 60 μm—such as the H II region around the high-latitude B-type star α Vir and a new cloud of unknown origin that we name DIR 015+54. Using the ratio of 100 to 240 μm brightness, we find that infrared excess clouds are cold. The dust temperature in atomic gas is 19 ± 2 K, while the dust temperature in known high-latitude molecular clouds (all of which have infrared excess) is 15.5 ± 1 K. The dust temperature in those infrared excess clouds that are not known to be associated with molecular clouds (generally because they have never been observed) is 17 ± 2 K, suggesting they are similar to high-latitude molecular clouds. Infrared excess clouds are peaks of column density rather than dust temperature, and their excess infrared emission is likely due to dust associated with molecular gas. For a large region in Ursa Major-Ursa Minor-Camelopardalis, where the CO(1 → 0) line has been surveyed, we correlated the infrared excess CO line integral, allowing us to measure X = N(H2)/W(CO) = (1.3 ± 0.2) × 1020 cm2 (K km s-1)-1 for high-latitude molecular clouds. Our measurement of X takes into account the low dust temperature in molecular gas; this correction amounts to a factor of 3.8 increase in the X-value that would naively be determined using only 100 μm, CO, and H I data. Our value of X is consistent with a recent γ-ray determination for the same region, while it is a factor of about 2 lower than the value determined for the inner galactic plane. The surface mass density of infrared excess clouds is 0.3 M☉ pc-2. The atlas of infrared excess clouds may be useful as a guide to regions of relatively high interstellar column density, which might extinct light from extragalactic objects at optical to ultraviolet wavelengths and confuse structures in the cosmic background at infrared to microwave wavelengths.