FilDReaMS 2. Application to the analysis of the relative orientations between filaments and the magnetic field in four Herschel fields

Abstract

Both simulations and observations of the interstellar medium show that the study of the relative orientations between filamentary structures and the magnetic field can bring new insight into the role played by magnetic fields in the formation and evolution of filaments and in the process of star formation. We provide a first application of FilDReaMS, the new method to detect and analyze filaments. Our goal is to investigate the relative orientations between the detected filaments and the magnetic field. We apply FilDReaMS to a sample of four Herschel fields (G210, G300, G82, G202) characterized by different Galactic environments and evolutionary stages. First, we examine the networks formed by filaments of different bar widths as well as their hierarchical organization. Second, we compare the filament orientations to the magnetic field orientation inferred from Planck polarization data and, for the first time, we study the statistics of the relative orientation angle as functions of both spatial scale and ${\rm H_2}$ column density. We find preferential relative orientations in the four Herschel fields: small filaments with low column densities are slightly more parallel than perpendicular to the magnetic field, while large filaments (higher column densities) are oriented nearly perpendicular (or, in the case of G202, nearly parallel). In the two nearby fields (G210 and G300), we observe a transition from mostly parallel to mostly perpendicular at an $N_{\rm H_2} \simeq 1.1$ and $1.4\times10^{21}\,$cm$^{-2}$, respectively, consistent with the results of previous studies. Our results confirm the existence of a coupling between magnetic fields at cloud scales and filaments at smaller scale. They also illustrate the potential of combining Herschel and Planck observations, and they call for further statistical analyses with our dedicated method.