Geometric Imprints Uncover Historical Global Events on Molecular Clouds

Abstract Many theories on the evolutionary process of molecular clouds have been proposed, but direct observational evidence remains scarce. The diverse cloud structures may preserve the history of cloud formation and evolution, making the search for critical evidence hidden in cloud structures essential to unraveling the history of clouds. To address this, we analyzed the geometric structure of molecular clouds by measuring the fractal dimension (DH) across intensity contours of individual clouds and examining its behavior within cloud samples. For a sample of 10,866 molecular clouds with both 12CO and 13CO emissions, DH was measured for 2,163 clouds. In general, the DH distribution peaks at 1.37, with a 95% Highest Density Interval (HDI) of [1.05, 1.54]. Individual molecular clouds exhibit two prominent features: (1) DH generally decreases linearly with increasing intensity, indicating that molecular clouds are multifractal; (2) DH undergoes one or more abrupt changes at specific intensity contours. The DH profiles can be classified into three distinct categories: no jumps (1,814 clouds, 84%), one jump (262 clouds, 12%), and two jumps (87 clouds, 4%). The intensity at which these jumps occur is strongly correlated with both the mean and peak temperatures of clouds. Strikingly, these jumps align closely with the breakpoints of recently discovered flux−intensity relations, while the DH slope exhibits a linear correlation with the exponential flux decay rate. The gradual and abrupt changes in DH and flux are indicative of historical global events, providing concrete observational evidence for cloud evolution.