A deviation-defect method for the detection of optically thick neutral hydrogen

Quantitative measurements of the spatial fluctuations of neutral hydrogen emission can be used to show the presence of optically thick gas. Optically thick gas layers do not appear to produce the spatial fluctuations of emission characteristic of optically thin gas; moreover, if emission from optically thin gas passes through optically thick gas, its spatial fluctuations are diminished. Based on this effect, a detection technique called the deviation-defect method has been developed and tested against interferometric measurements of absorption spectra. All optically thick gas features detected by the deviation-defect method are confirmed by the interferometer results. The new method has been applied to a large sample of the sky, and a very extensive optically thick hydrogen layer in the galactic anticenter has been mapped. Not all optically thick gas layers can be detected by this method. The layers must be many degrees in size to be observable in present hydrogen line data. Given this restriction, it is astrophysically significant that detections have been achieved. The results may indicate that some triggering mechanism initiates the formation of the cloud medium. Subject headings: interstellar matter - 21-cm radiation