Compact Galactic Neutral Hydrogen Clouds in the GALFA-HI Survey
The more sensitive the observations, the more complex the gas in our Galaxy appears. Since the detection of neutral hydrogen in 1951, each new survey has revealed new structure. Using the GALFA-HI survey, we have discovered five populations of compact neutral hydrogen clouds. We began by developing a machine-vision algorithm to identify compact clouds in the GALFA-HI Data Release 1. Based on position, velocity, and linewidth we separated the 1964 identified clouds into five populations: galaxy candidates, high-velocity clouds, cold low-velocity clouds, warm low-velocity clouds, and warm positive low-velocity clouds in the third Galactic quadrant. We found that the dust properties of the compact clouds support our population definitions. Using both IRAS and the newly released Planck data, we found no dust detections in the high-velocity clouds, or the warm positive low-velocity clouds in the third Galactic quadrant. We claim that the third quadrant clouds are low-velocity halo clouds. The warm low-velocity clouds have a significantly greater dust-to-gas ratio than the cold low-velocity clouds. We interpret this as evidence that the warm clouds have an ionized component not present with the cold clouds, possibly because they are part of the Galactic fountain.
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