Exposing the plural nature of molecular clouds

Extracting filaments and the cosmic infrared background against the true scale-free interstellar medium

¹ CNRS, IPAG, Université Grenoble Alpes, 38000 Grenoble, France
e-mail: jean-francois.robitaille@univ-grenoble-alpes.fr
² I. Physikalisches Institut, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany
³ Istituto di Astrofisica e Planetologia Spaziali, INAF, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ CNRS, OASU/LAB University of Bordeaux, UMR5804, 33615 Pessac, France

Received: 26 March 2019
Accepted: 29 May 2019

Abstract

We present the Multiscale non-Gaussian Segmentation (MnGSeg) analysis technique. This wavelet-based method combines the analysis of the probability distribution function (PDF) of map fluctuations as a function of spatial scales and the power spectrum analysis of a map. This technique allows us to extract the non-Gaussianities identified in the multiscaled PDFs usually associated with turbulence intermittency and to spatially reconstruct the Gaussian and the non-Gaussian components of the map. This new technique can be applied on any data set. In the present paper, it is applied on a Herschel column density map of the Polaris flare cloud. The first component has by construction a self-similar fractal geometry similar to that produced by fractional Brownian motion (fBm) simulations. The second component is called the coherent component, as opposed to fractal, and includes a network of filamentary structures that demonstrates a spatial hierarchical scaling (i.e. filaments inside filaments). The power spectrum analysis of the two components proves that the Fourier power spectrum of the initial map is dominated by the power of the coherent filamentary structures across almost all spatial scales. The coherent structures contribute increasingly from larger to smaller scales, without producing any break in the inertial range. We suggest that this behaviour is induced, at least partly, by inertial-range intermittency, a well-known phenomenon for turbulent flows. We also demonstrate that the MnGSeg technique is itself a very sensitive signal analysis technique that allows the extraction of the cosmic infrared background (CIB) signal present in the Polaris flare submillimetre observations and the detection of a characteristic scale for 0.1 ≲ l ≲ 0.3 pc. The origin of this characteristic scale could partly be the transition of regimes dominated by incompressible turbulence versus compressible modes and other physical processes, such as gravity.