Near-Infrared Extinction and Molecular Cloud Structure

A little more than a decade has passed since the advent of large-format infrared array cameras opened a new window on molecular cloud research. This powerful observational tool has enabled dust extinction and column density maps of molecular clouds to be constructed with unprecedented precision, depth, and angular resolution. Near-infrared extinction studies can achieve column density dynamic ranges of 0.3 < AV < 40 mag (6 × 1020 < N < 1023 cm-2), allowing with one simple tracer a nearly complete description of the density structure of a cloud free from the uncertainties that typically plague measurements derived from radio spectroscopy and dust emission. This has led in recent years to an empirical characterization of the evolutionary status of dense cores based on the shapes of their radial column density profiles and revealed the best examples in nature of Bonnor-Ebert spheres. Widefield infrared extinction mapping of large cloud complexes provides the most complete inventory of cloudy material that can be derived from observations. Such studies enable the measurement of the mass function of dense cores within a cloud, a critical piece of information for developing an understanding of the origin of the stellar IMF. Comparison with radio spectroscopic data has allowed detailed chemical structure studies of starless cores and provided some of the clearest evidence for differential depletion of molecular species in cold gaseous configurations. Recent studies have demonstrated the feasibility of infrared extinction mapping of galactic molecular clouds (GMCs) in external galaxies, enabling the fundamental measurement of the GMC mass function in these systems. In this contribution we review recent results arising from this powerful technique, ranging from studies of Bok globules to local GMCs, to GMCs in external galaxies.