Turbulent magnetic ﬁeld in the H II 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 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 (cid:107) ) 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 2.3GHz, 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 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 (cid:107) for each LoS within the ellipsoid. To characterize the turbulent magnetic ﬁeld ﬂuctuations, we computed a second-order structure function of B (cid:107) . 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 (cid:107) 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 (cid:107) as − 4 . 5 ± 0 . 1 µ G, which is comparable to the magnetic ﬁeld strength in diffuse ISM. The slope of the structure function of the estimated B (cid:107) -map is found to be slightly steeper than Kolmogorov, consistent with our Gaussian-random-ﬁeld B (cid:107) simulations revealing that an input Kolmogorov slope in the magnetic ﬁeld results in a somewhat steeper slope in B (cid:107) . 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. Conclusions. The structure functions of B (cid:107) ﬂuctuations in Sh 2–27 show that the magnetic ﬁeld ﬂuctuations in this H II region are consistent with a Kolmogorov-like turbulence. Comparing the observed and simulated B (cid:107) 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 the presence of Sh 2–27.