Infrared Extinction and the Structure of the IC 5146 Dark Cloud

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We report deep near-infrared (HK) imaging observations of the dark cloud associated with IC 5146. With an order of magnitude greater sensitivity, we have imaged roughly half the region of the cloud originally surveyed by Lada and coworkers. Using measurements of ~2000 stars, we have employed techniques previously developed by Lada and coworkers to construct ordered, uniformly sampled maps of the extinction through this cloud. With the improved sensitivity, we detected approximately 5 times as many heavily extincted stars (i.e., A~20-50 mag) as found in the earlier survey of this same cloud area. Moreover, we were able to produce a Gaussian-smoothed extinction map of the cloud with an angular resolution (30'') somewhat more than a factor of 2 higher than achieved in the earlier study. With the increased sensitivity and angular resolution we were also able to measure the average radial column density profile orthogonal to the major axis of this filamentary cloud. Assuming cylindrical symmetry, we modeled this column density gradient and determined that the corresponding volume density profile of the cloud must smoothly fall off as r-2. To investigate the structure of the cloud on size scales smaller than the effective resolution of our maps, we constructed plots of the relation between the derived mean extinction and its measured dispersion for all the pixels in a series of maps made with varying angular resolution. We find, similar to Lada et al., that the dispersion increases linearly with mean AV, independent of the angular resolution of our maps. However, although we quantitatively reproduce the earlier results at the same angular resolution (90''), we find the interesting result that the slope of the σ-A relation decreases in a systematic fashion with increasing angular resolution. We construct synthetic models of the cloud density distribution and use Monte Carlo techniques to produce artificial extinction maps and investigate the origin of the σ-A relations. These models show that both the observed form of the σ-A relation and its variation with angular resolution are the natural consequences of a smooth, radially decreasing volume density gradient in a cylindrically symmetric cloud. For a volume density gradient falling off as r-2, the quantitative agreements between the model predictions and data are excellent. Apparently, these relations can be understood without the need for random fluctuations in the structure of the cloud on small spatial scales.