Estimating the Column Density in Molecular Clouds with Far-Infrared and Submillimeter Emission Maps

We have used a numerical simulation of a turbulent cloud to synthesize mapsof the thermal emission from dust at a variety of far-IR and sub-mmwavelengths. The average column density and external radiation field in thesimulation is well matched to clouds such as Perseus and Ophiuchus. We usepairs of single-wavelength emission maps to derive the dust color temperatureand column density, and we compare the derived column densities with the truecolumn density. We demonstrate that longer wavelength emission maps yield lessbiased estimates of column density than maps made towards the peak of the dustemission spectrum. We compare the scatter in the derived column density withthe observed scatter in Perseus and Ophiuchus. We find that while in Perseusall of the observed scatter in the emission-derived versus theextinction-derived column density can be attributed to the flawed assumption ofisothermal dust along each line of sight, in Ophiuchus there is additionalscatter above what can be explained by the isothermal assumption. Our resultsimply that variations in dust emission properties within a molecular cloud arenot necessarily a major source of uncertainty in column density measurements.Comment: Accepted to ApJ Letter