2022A&A...663A.170R

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 Hα 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) as it is located at intermediate latitudes (b ∼ 23°), meaning that it suffers 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.
Aims. By using a map of the magnetic field strength along the LoS (B_||)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 diffuse Galactic synchrotron polarization. We estimated the rotation measure (RM) caused by the HII region, using the synchrotron polarization angle. We used the Hα data from the Southern Hα Sky Survey Atlas to estimate the free electron density (n_e) in the HII 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 diffuse 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 uG, which is comparable to the magnetic field strength in diffuse 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 HII region, which is comparable to the size of the computation domain.
Conclusions. The structure functions of B_|| fluctuations in Sh 2-27 show that the magnetic field fluctuations in this HII region are consistent with a Kolmogorov-like turbulence. Comparing the observed and simulated B_|| structure functions results in the estimation of a lower 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.