Estimating extragalactic Faraday rotation
Abstract
Observations of Faraday rotation for extragalactic sources probe magnetic fields both inside and outside the Milky Way. Building on our earlier estimate of the Galactic contribution, we set out to estimate the extragalactic contributions. We discuss the problems involved; in particular, we point out that taking the difference between the observed values and the Galactic foreground reconstruction is not a good estimate for the extragalactic contributions. We point out a degeneracy between the contributions to the observed values due to extragalactic magnetic fields and observational noise and comment on the dangers of overinterpreting an estimate without taking into account its uncertainty information. To overcome these difficulties, we develop an extended reconstruction algorithm based on the assumption that the observational uncertainties are accurately described for a subset of the data, which can overcome the degeneracy with the extragalactic contributions. We present a probabilistic derivation of the algorithm and demonstrate its performance using a simulation, yielding a high quality reconstruction of the Galactic Faraday rotation foreground, a precise estimate of the typical extragalactic contribution, and a welldefined probabilistic description of the extragalactic contribution for each data point. We then apply this reconstruction technique to a catalog of Faraday rotation observations for extragalactic sources. The analysis is done for several different scenarios, for which we consider the error bars of different subsets of the data to accurately describe the observational uncertainties. By comparing the results, we argue that a split that singles out only data near the Galactic poles is the most robust approach. We find that the dispersion of extragalactic contributions to observed Faraday depths is most likely lower than 7 rad/m^{2}, in agreement with earlier results, and that the extragalactic contribution to an individual data point is poorly constrained by the data in most cases.
Posterior samples for the extragalactic contribution to all data points and all results of our fiducial model are provided at http://www.mpagarching.mpg.de/ift/faraday/
 Publication:

Astronomy and Astrophysics
 Pub Date:
 March 2015
 DOI:
 10.1051/00046361/201423995
 arXiv:
 arXiv:1404.3701
 Bibcode:
 2015A&A...575A.118O
 Keywords:

 magnetic fields;
 methods: data analysis;
 ISM: magnetic fields;
 Astrophysics  Instrumentation and Methods for Astrophysics;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 20 + 6 pages, 19 figures