Wavelet-based cross-correlation analysis of structure scaling in turbulent clouds

TL;DR

This paper introduces a wavelet-based cross-correlation method to analyze and compare the scale-dependent structure and correlations in molecular cloud maps, revealing chemical and physical transitions.

Contribution

The paper develops the WWCC method for scale-dependent analysis of turbulence in molecular clouds, calibrated with artificial maps and applied to real observations.

Findings

Confirmed chemical similarity of $^{13}$CO and C$^{18}$O across scales

Identified a chemical transition scale at ~7 pc in HCN maps

Detected systematic offsets indicating large-scale density gradients

Abstract

We propose a statistical tool to compare the scaling behaviour of turbulence in pairs of molecular cloud maps. Using artificial maps with well defined spatial properties, we calibrate the method and test its limitations to ultimately apply it to a set of observed maps. We develop the wavelet-based weighted cross-correlation (WWCC) method to study the relative contribution of structures of different sizes and their degree of correlation in two maps as a function of spatial scale, and the mutual displacement of structures in the molecular cloud maps. We test the WWCC for circular structures having a single prominent scale and fractal structures showing a self-similar behavior without prominent scales. Observational noise and a finite map size limit the scales where the cross-correlation coefficients and displacement vectors can be reliably measured. For fractal maps containing many structures on all scales, the limitation from the observational noise is negligible for signal-to-noise ratios >5. (abbrev). Application of the WWCC to the observed line maps of the giant molecular cloud G333 allows to add specific scale information to the results obtained earlier using the principle component analysis. It confirms the chemical and excitation similarity of $^{13}$CO and C$^{18}$O on all scales, but shows a deviation of HCN at scales of up to 7' (~7 pc). This can be interpreted as a chemical transition scale. The largest structures also show a systematic offset along the filament, probably due to a large-scale density gradient. The WWCC can compare correlated structures in different maps of molecular clouds identifying scales that represent structural changes such as chemical and phase transitions and prominent or enhanced dimensions.