An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys

摘要

We describe algorithms that detect 21cm line HI self-absorption (HISA) inlarge data sets and extract it for analysis. Our search method identifies HISAas spatially and spectrally confined dark HI features that appear as negativeresiduals after removing larger-scale emission components with a modified CLEANalgorithm. Adjacent HISA volume-pixels (voxels) are grouped into features in(l,b,v) space, and the HI brightness of voxels outside the 3-D featureboundaries is smoothly interpolated to estimate the absorption amplitude andthe unabsorbed HI emission brightness. The reliability and completeness of ourHISA detection scheme have been tested extensively with model data. We detectmost features over a wide range of sizes, linewidths, amplitudes, andbackground levels, with poor detection only where the absorption brightnesstemperature amplitude is weak, the absorption scale approaches that of thecorrelated noise, or the background level is too faint for HISA to bedistinguished reliably from emission gaps. False detection rates are very lowin all parts of the parameter space except at sizes and amplitudes approachingthose of noise fluctuations. Absorption measurement biases introduced by themethod are generally small and appear to arise from cases of incomplete HISAdetection. This paper is the third in a series examining HISA at high angularresolution. A companion paper (Paper II) uses our HISA search and extractionmethod to investigate the cold atomic gas distribution in the Canadian GalacticPlane Survey.