Turbulent magnetic field in the HII region Sh 2-27

Abstract

Magnetic fields in the turbulent interstellar medium (ISM) are a key element in understanding Galactic dynamics, but there are many observational challenges. One useful probe for studying the magnetic field component parallel to the line of sight (LoS) is Faraday rotation of linearly polarized radio synchrotron emission, combined with H$\alpha$ observations. HII regions are the perfect laboratories to probe such magnetic fields as they are localized in space, and are well-defined sources often with known distances and measurable electron densities. We chose the HII region Sharpless 2-27 (Sh 2-27). By using a map of the magnetic field strength along the LoS ($B_{\parallel }$) for the first time, we investigate the basic statistical properties of the turbulent magnetic field inside Sh 2-27. We study the scaling of the magnetic field fluctuations, compare it to the Kolmogorov scaling, and attempt to find an outer scale of the turbulent magnetic field fluctuations. We estimate the median value of $n_e$ as $7.3\pm 0.1$ cm${}^{-3}$, and the median value of $B_{\parallel }$ as $-4.5\pm 0.1$ $\mu$G, which is comparable to the magnetic field strength in diffuse ISM. The slope of the structure function of the estimated $B_{\parallel }$-map is found to be slightly steeper than Kolmogorov, consistent with our Gaussian-random-field $B_{\parallel }$ simulations revealing that an input Kolmogorov slope in the magnetic field results in a somewhat steeper slope in $B_{\parallel }$. These results suggest that the lower limit to the outer scale of turbulence is 10 pc in the HII region, which is comparable to the size of the computation domain. This may indicate that the turbulence probed here could actually be cascading from the larger scales in the ambient medium, associated with the interstellar turbulence in the general ISM, which is illuminated by the presence of Sh 2-27.

14 pages, 11 figures. Accepted by A&A

References in corpus (20)

• The NumPy array: a structure for efficient numerical computation
• The Gaia mission
• Theory of Star Formation
• Resolving the electron temperature discrepancies in HII Regions and Planetary Nebulae: kappa-distributed electrons
• Placing Confidence Limits on Polarization Measurements
• Simulations of Electron Magnetohydrodynamic Turbulence
• Polarised radio filaments outside the Galactic plane
• A Sino-German lambda6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region
• The role of magnetic fields in the formation of protostellar discs
• Electron Density Structure of the Local Galactic Disk
• Galactic interstellar turbulence in the southern sky seen through spatial gradients of the polarization vector
• The Global Magneto-Ionic Medium Survey: Polarimetry of the Southern Sky from 300 to 480 MHz
• Through thick or thin: Multiple components of the magneto-ionic medium towards the nearby $\mathrm{H}\mathrm{II}$ region Sharpless 2-27 revealed by Faraday tomography
• Turbulence in the Ionized Gas of the Orion Nebula
• Polarimetric studies of magnetic turbulence with interferometer
• Polarization shadows of extragalactic sources by the local magneto-ionic ISM
• Effects of Grain Alignment Efficiency on Synthetic Dust Polarization Observations of Molecular Clouds
• Confirmation of a Faraday Rotation Measure Anomaly in Cygnus
• Supersonic turbulence in Giant HII Regions: clues from 30 Doradus
• Depolarization canals and interstellar turbulence

Cited by in corpus (3)

• Rotation measure structure functions with higher-order stencils as a probe of small-scale magnetic fluctuations and its application to the Small and Large Magellanic Clouds
• Disentangling the Faraday rotation sky
• Faraday moments of the Southern Twenty-centimeter All-sky Polarization Survey (STAPS)