Turbulent magnetic field in the HII region Sh 2-27

Context. 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 α observations. H ii 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 H ii region Sharpless 2 − 27 (Sh 2 − 27) as it is located at intermediate latitudes ( b ∼ 23 ◦ ), meaning that it su ff ers from little LoS confusion from other sources. In addition, it has a large angular diameter ( ∼ 10 ◦ ), enabling us to study the properties of its magnetic field over a wide range of angular scales. ) 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. Methods. We used the polarized radio synchrotron emission data from the S-band Polarization All-Sky Survey (S-PASS) at 2.3 GHz, which allowed us to test the impact of Sh 2 − 27 on di ff use Galactic synchrotron polarization. We estimated the rotation measure ( RM ) caused by the H ii region, using the synchrotron polarization angle. We used the H α data from the Southern H α Sky Survey Atlas to estimate the free electron density ( n e ) in the H ii region. Using an ellipsoid model for the shape of Sh 2 − 27, and with the observed RM and emission measure ( EM ), we estimated the LoS averaged B ∥ for each LoS within the ellipsoid. To characterize the turbulent magnetic field fluctuations, we computed a second-order structure function of B ∥ . We compared the structure function to Kolmogorov turbulence, and to simulations of Gaussian random fields processed in the same way as the observations. Results. We present the first continuous map of B ∥ computed using the di ff use polarized radio emission in Sh 2 − 27. We estimate the median value of n e as 7 . 3 ± 0 . 1 cm − 3 , and the median value of B ∥ as − 4 . 5 ± 0 . 1 µ G, which is comparable to the magnetic field strength in di ff use ISM. The slope of the structure function of the estimated B ∥ -map is found to be slightly steeper than Kolmogorov, consistent with our Gaussian-random-field B ∥ simulations revealing that an input Kolmogorov slope in the magnetic field results in a somewhat steeper slope in B ∥ . These results suggest that the lower limit to the outer scale of turbulence is 10 pc in the H ii region, which is comparable to the size of the computation domain. limit to the outer scale of the turbulent magnetic field fluctuations of 10 pc, which is limited by the size of the field of view under study. 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. National Nat- ural Science Foundation of China: 11988101. XS, JH and XG are funded by the CAS-NWO cooperation programme (Grant No. GJHZ1865). This work has made use of S-band polarization All-Sky Survey (S-PASS) data. We acknowl-edge the Southern H-Alpha Sky Survey Atlas (SHASSA), which is supported by the National Science Foundation (Gaustad et al. 2001). This work made use of Astropy 7 a community-developed core Python package for Astronomy (As- tropy Collaboration et al. 2018), NumPy 8 (Oliphant 2006; van der Walt et al. 2011), and Matplotlib 9 python module (Hunter 2007). We have made use of the “inferno” and “bone” colour-maps, and colour-blind-friendly figures.