Systematic underestimation of polarisation angle dispersion and its consequences for magnetic field strength estimates in star-forming regions

TL;DR

This study reveals that common methods underestimate the true polarisation angle dispersion in star-forming regions, which affects magnetic field strength estimates, and proposes a correction method based on scale-dependent analysis.

Contribution

The paper demonstrates the systematic underestimation of angular dispersion and introduces a correction technique applicable to observational data.

Findings

Measured angular dispersion is underestimated by factors of 1-10.

Correction factors depend on pixel size, beam convolution, and underlying structure.

Application to JCMT Orion A OMC-1 shows a correction factor of about 1.6.

Abstract

Polarised dust emission observations are a valuable tool to infer the structure of the magnetic field and the dispersion of polarisation position angles may be used to estimate magnetic field strengths. A natural consequence of magneto-dynamic turbulence is for the angular dispersion to have a length-scale dependence, making the measurement of angular dispersion non-trivial. In this paper, we present a study of parametrised, scale dependent maps, focusing on the effect of pixel size and beam convolution on the measured angular dispersion when using the commonly employed unsharp-masking and structure function methods. We find that in all cases the measured angular dispersion is underestimated compared to the true value. The degree to which the measured angular dispersion is underestimated varies by factors of 1-10 when measured on scales of 1-3x the beam size, and depends on the underlying structure of the polarisation angle field. This suggests that currently derived magnetic field strengths using angular dispersions are chronically overestimated, potentially leading to an overly magnetically-dominated view of star formation. We present a method to estimate a correction factor to account for this and apply it to JCMT Orion A OMC-1 observations. We find that the magnetic field in OMC-1 is predominately found to vary on scales much larger than the JCMT's 14'' beam and has a rather low degree of unresolved dispersion, leading to a correction factor of only $\sim$1.6 for angular dispersion measured at a scale of 14''/0.028 pc.