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Planck intermediate results. XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds (1502.04123v4)

Published 13 Feb 2015 in astro-ph.GA

Abstract: Within ten nearby (d < 450 pc) Gould Belt molecular clouds we evaluate statistically the relative orientation between the magnetic field projected on the plane of sky, inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz, and the gas column density structures, quantified by the gradient of the column density, $N_H$. The selected regions, covering several degrees in size, are analyzed at an effective angular resolution of 10' FWHM, thus sampling physical scales from 0.4 to 40 pc in the nearest cloud. The column densities in the selected regions range from $N_H \approx 10{21}$ to $10{23}$ cm${-2}$, and hence they correspond to the bulk of the molecular clouds. The relative orientation is evaluated pixel by pixel and analyzed in bins of column density using the novel statistical tool called "Histogram of Relative Orientations". Throughout this study, we assume that the polarized emission observed by Planck at 353 GHz is representative of the projected morphology of the magnetic field in each region, i.e., we assume a constant dust grain alignment efficiency, independent of the local environment. Within most clouds we find that the relative orientation changes progressively with increasing $N_H$, from preferentially parallel or having no preferred orientation to preferentially perpendicular. In simulations of magnetohydrodynamic turbulence in molecular clouds this trend in relative orientation is a signature of Alfv\'enic or sub-Alfv\'enic turbulence, implying that the magnetic field is significant for the gas dynamics at the scales probed by Planck. We compare the deduced magnetic field strength with estimates we obtain from other methods and discuss the implications of the Planck observations for the general picture of molecular cloud formation and evolution.

Citations (256)

Summary

  • The paper demonstrates that magnetic fields transition from parallel to perpendicular alignment with gas structures as column density increases.
  • It uses the Histogram of Relative Orientations (HRO) method applied pixel-by-pixel to analyze alignment across different density bins.
  • Results imply magnetic fields govern cloud structure, directing material flow and setting initial conditions for star formation.

Analyzing the Role of Magnetic Fields in Molecular Cloud Structure Formation

The paper "Planck Intermediate Results. XXXV. Probing the Role of the Magnetic Field in the Formation of Structure in Molecular Clouds" investigates the influence of magnetic fields on the structure and evolution of molecular clouds (MCs) using data from the Planck satellite. High-resolution observations of polarized thermal emission from dust at 353 GHz allow for the detailed paper of the magnetic field's role in cloud dynamics by examining the alignment between magnetic fields and gas structures.

The paper employs a novel statistical technique, the Histogram of Relative Orientations (HRO), to analyze the alignment of magnetic fields with respect to the gradients of gas column densities in various molecular cloud regions. The HRO is applied pixel-by-pixel across different column density bins, enabling a comprehensive analysis of cloud structure at multiple scales. Throughout several Gould Belt molecular clouds, the results indicate a systematic change in the relative orientation of the magnetic field with increasing gas column density.

The key findings are as follows:

  • There is a transition from parallel to perpendicular alignment of the magnetic field with respect to column density structures as gas density increases. This trend is indicative of Alfvénic or sub-Alfvénic turbulence, emphasizing the dominance of magnetic fields over gas dynamics at these scales.
  • Magnetic fields can influence the shape and structure of MCs, potentially facilitating material accumulation along field lines leading to core and star formation.
  • The relative alignment observed suggests MCs may have configurations where gravitational collapse occurs preferentially along magnetic field lines, resulting in structures that are typically perpendicular to the magnetic field in denser regions.

The paper's application of the HRO method provides strong evidence that magnetic fields play a significant role in shaping molecular cloud structure, especially at higher gas densities. These findings align with and extend existing theoretical models and numerical simulations of magnetohydrodynamic (MHD) turbulence, which predict such alignments under sub-Alfvénic conditions.

The implications of this research are significant for our understanding of star formation processes. The alignment of magnetic fields could dictate the flow of material, impacting the initial conditions for star formation and potentially influencing the mass distributions and lifespans of molecular clouds. As a result, these insights can refine models of galaxy evolution by enhancing our understanding of star-forming regions.

Looking forward, the integration of higher resolution data and more sophisticated models of dust grain alignment could further elucidate the complex interactions between magnetic fields and cloud structure. Additionally, future studies might focus on examining how these influences play out across a wider variety of interstellar environments, potentially revealing universal patterns in the role of magnetic fields in star formation processes.