Profiling filaments: comparing near-infrared extinction and submillimetre data in TMC-1

Context. Interstellar filaments are an important part of the star formation process. In order to understand the structure and formation of filaments, the filament cross-section profiles are often fitted with the so-called Plummer profile function. Currently this profiling is often approached with submillimetre studies, especially with Herschel. If these data are not available, it would be more convenient if filament properties could be studied using groundbased near-infrared (NIR) observations. Aims. We compare the filament profiles obtained by NIR extinction and submillimetre observations to find out if reliable profiles can be derived using NIR observations. Methods. We use J-, H-, and K-band data of a filament north of TMC-1 to derive an extinction map from colour excesses of background stars. We also use 2MASS data of this and another filament in TMC-1. We compare the Plummer profiles obtained from these extinction maps with Herschel dust emission maps. We present two new methods to estimate profiles from NIR data: Plummer profile fits to median A(V) of stars within certain offset or directly to the A(V) of individual stars. We compare these methods by simulations. Results. In simulations the extinction maps and the new methods give correct results to within similar to 10-20% for modest densities (rho(c) = 10(4)-10(5) cm(-3)). The direct fit to data on individual stars usually gives more accurate results than the extinction map, and can work in higher density. In the profile fits to real observations, the values of Plummer parameters are generally similar to within a factor of similar to 2 (up to a factor of similar to 5). Although the parameter values can vary significantly, the estimates of filament mass usually remain accurate to within some tens of per cent. Our results for TMC-1 are in good agreement with earlier results obtained with SCUBA and ISO. High resolution NIR data give more details, but 2MASS data can be used to estimate approximate profiles. Conclusions. NIR extinction maps can be used as an alternative to submm observations to profile filaments. Direct fits of stars can also be a valuable tool in profiling. However, the Plummer profile parameters are not always well constrained, and caution should be taken when making the fits and interpreting the results. In the evaluation of the Plummer parameters, one can also make use of the independence of the dust emission and NIR data and the difference in the shapes of the associated confidence regions.