Perception of Interstellar Structure: Facing Complexity

Abstract

This paper challenges some orthodox notions concerning the structure and evolution of star-forming regions, proposing that they arise largely by a dual process in which conceptual models are fashioned after categories which are in great part reflections of observational limitations, and the models are projected onto interpretations of data, an example of hypostatization of categories. Several examples are discussed. The need for internal support of molecular clouds is questioned. It is suggested that the inverse density-size relation often claimed for clouds and accounted for by several theoretical models is an artifact caused by limited dynamic range in column density detectability, selection bias, distance uncertainties, and internal density gradients, and is contradicted by several unbiased surveys. Limited spatial dynamic range (ratio of image size to resolution) in maps of column density structure results in a “Mr. Magoo effect” which tends to accommodate quasi-static evolutionary concepts. Column density structures mapped with a large spatial and column density dynamic range are dominated by irregular, connected, and nested forms on all scales. Contour shapes of both atomic and molecular clouds exhibit self-similar irregularity with a common fractal dimension over a large range in scale. These features and a technique for the quantification of complex structure are illustrated with a densely-sampled column density image of the Taurus region constructed from IRAS data. A comparison with 553 high-accuracy polarization vectors in the region is also given.