Statistical analysis of the relative orientations between filaments and magnetic fields using Herschel and Planck data in star-forming regions

Abstract

Observations and simulations of the interstellar medium both suggest that magnetic fields play a key role in the formation and evolution of filaments and in the process of star formation, yet their exact role is still poorly understood. Here, we aim to statistically examine the relative orientations between filaments and magnetic fields in various star-forming regions with different physical properties and Galactic environments. We used a dedicated method, FilDReaMS, to detect and extract filaments at multiple scales, and we applied it to the 116 fields of the Herschel "Galactic Cold Cores" key project (18"-36" resolution). We then compared the filament orientations to the orientation of the plane-of-sky (PoS) magnetic field (B_PoS), inferred from Planck observations (7' resolution) using histograms of relative orientations (HROs). We find that low-N_H2 filaments tend to be roughly parallel to B_PoS at all scales, while narrow high-N_H2 filaments do not have any preferred orientations and wide high-N_H2 filaments tend to be roughly perpendicular. This change in preferred orientations occurs at a transition column density typically in the range [0.8, 8] x 10^{21} cm^{-2}, a range consistent with results of previous Planck studies. We also analyzed the HROs for filaments with embedded cores and find them to be consistent with HROs for high-N_H2 filaments. However, several fields do not follow the general trends, with a variety of behaviors that can be due to factors such as projection effects, confusion along the line of sight (LoS), or magnetic field tangling. Our analysis of projection effects shows that, statistically, preferred orientations in the PoS are indicative of true preferred orientations in 3D. Our results suggest that higher polarization fractions, p, entail weaker projection effects, consistent with the presumed link between p and the magnetic field inclination to the LoS.