The magnetic field of the Hii region NGC 6334A from Faraday rotation

Abstract

We have studied the polarization characteristics and Faraday rotation of the extragalactic radio source J17204-3554, which appears projected on the north lobe of the Galactic Hii region NGC 6334A. From observations made with the Very Large Array at 6.0 and 3.6cm in three different epochs (1994, 1997 and 2006), we estimate a rotation measure of +5100 ± 900radm -2 for the extragalactic source. This large rotation measure implies a line-of-sight average magnetic field of B ∥≃+ 36 ± 6μG, the largest obtained by this method for an Hii region. NGC 6334A is significantly denser than other Hii regions studied and this larger magnetic field is expected on the grounds of magnetic flux conservation. The ratio of thermal to magnetic pressure is ∼5 in the range of values determined for more diffuse Hii regions. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.