Abstract
Abstract Much of the information about the magnetic field in the Milky Way and other galaxies comes from measurements which are path integrals, such as Faraday rotation and the polarization of synchrotron radiation of cosmic ray electrons. The measurement made at the radio telescope results from contributions of volume elements along a long line of sight (LOS). A magnetic field measurement at a given spatial location is of much more physical significance. In this paper, I point out that H ii regions fortuitously offer such a “point” measurement, albeit of one component of the magnetic field, and averaged over the sightline through the H ii region. However, the LOS through an H ii region is much smaller (e.g., 30–50 pc) than one through the entire Galactic disk, and thus constitutes a “pseudo-local” measurement. I use published H ii region Faraday rotation measurements to provide a new constraint on the magnitude of magnetohydrodynamic turbulence in the Galaxy.
Highlights
It has been known for decades that the interstellar medium is threaded by a magnetic field
In the studies of Costa and coworkers, the mean line of sight (LOS) component of the field, < B >, was not explicitly measured; analyses of the data concentrated on fitting plasma shell models to the sets of rotation measure (RM) values for each HII region
In the simplified stellar bubble models presented in Savage et al (2013); Costa et al (2016); Costa and Spangler (2018) the expanding bubble modifies the Galactic field, but the RM enhancement due to the bubble is proportional to the Galactic field in the absence of the HII region
Summary
It has been known for decades that the interstellar medium is threaded by a magnetic field. One of the difficulties in determining the relative roles of a large scale and a turbulent component of the Galactic field is the fact that some of the main diagnostic techniques such as Faraday rotation, polarization of Galactic synchrotron radiation, and dust polarization consist of path integrals over long lines of sight through the Galaxy. What this means is that we get some sort of average of the Galactic field over a long line of sight (LOS) through the Galaxy, rather than the value at a set of points along that line of sight. We explore the consequences of this result for our inference about the Galactic magnetic field
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
