Power-law tails in probability density functions of molecular cloud
  column density
  Power-law tails are often seen in probability density functions (PDFs) of
molecular cloud column densities, and have been attributed to the effect of
gravity. We show that extinction PDFs of a sample of five molecular clouds
obtained at a few tenths of a parsec resolution, probing extinctions up to
A$_{{\mathrm{V}}}$ $\sim$ 10 magnitudes, are very well described by lognormal
functions provided that the field selection is tightly constrained to the cold,
molecular zone and that noise and foreground contamination are appropriately
accounted for. In general, field selections that incorporate warm, diffuse
material in addition to the cold, molecular material will display apparent
core+tail PDFs. The apparent tail, however, is best understood as the high
extinction part of a lognormal PDF arising from the cold, molecular part of the
cloud. We also describe the effects of noise and foreground/background
contamination on the PDF structure, and show that these can, if not
appropriately accounted for, induce spurious tails or amplify any that are
truly present.
