The “true” Column Density Distribution in Star-forming Molecular Clouds

We use the COMPLETE Survey’s observations of the Perseus star-forming region to assess and intercompare three methods for measuring column density in molecular clouds: near-infrared extinction mapping; thermal emission mapping in the far-IR; and mapping the intensity of CO isotopologues. Overall, the structures shown by all three tracers are morphologically similar, but important differences exist amongst the tracers. We find that the dust-based measures (near-IR extinction and thermal emission) give similar, log-normal, distributions for the full (∼ 20 pc-scale) Perseus region, once careful calibration corrections are made. We also compare dustand gas-based column density distributions for physically-meaningful sub-regions of Perseus, and we find significant variations in the distributions for those (smaller, ∼few pc-scale) regions. Even though we have used CO data to estimate excitation temperatures, and we have corrected for opacity, the CO maps seem unable to give column distributions that consistently resemble those from dust measures. We have edited out the effects of the shell around the B-star HD 278942 from the column-density distribution comparisons. In that shell’s interior and in the parts where it overlaps the molecular cloud, there appears to be a dearth of CO, which is likely due either to CO not yet having had time to form in this young structure, and/or destruction of CO in the molecular cloud by the HD 278942’s wind and/or radiation. We conclude that the use of either dust or gas measures of column density without extreme attention to calibration (e.g. of thermal emission zero-levels) and artifacts (e.g. the shell) is more perilous than even experts might normally admit. And, the use of CO data to trace total column density in detail, even after proper calibration, is unavoidably limited in utility due to threshold, depletion, and opacity effects. If one’s main aim is to map column density (rather than temperature or kinematics), then dust extinction seems the best probe, up to a limiting extinction caused by a dearth of sufficient background sources. Linear fits amongst all three tracers’ estimates of column density are given, allowing us to quantify the inherent uncertainties in using one tracer, in comparison with the others. Subject headings: dust, extinction — ISM:abundances — ISM:molecules — ISM:individual (Perseus molecular complex)