Note: Due to a minor bug in the original code, the files on this website have been updated on February 16th, 2015. The numbers have changed only slightly. However, if you have retrieved the files before this date, please download them again.

Estimating extragalactic Faraday rotation

Here you can download the results presented in the paper "Estimating extragalactic Faraday rotation" (arXiv:1404.3701) by Niels Oppermann, Henrik Junklewitz, Maksim Greiner, Torsten A. Enßlin, Takuya Akahori, Ettore Carretti, Bryan M. Gaensler, Ariel Goobar, Lisa Harvey-Smith, Melanie Johnston-Hollitt, Luke Pratley, Dominic H.F.M. Schnitzeler, Jeroen M. Stil, and Valentina Vacca

The results are provided as hdf5 files. See here for information about the file format and here for the python interface.
Alternative fits files containing the same information are also provided.

All results provided make use of the 'polar caps' data split described in the paper.

Galactic foreground

In this hdf5 file, you find the reconstruction of the Galactic Faraday depth, stored in HEALPix format. The structure of the file is the following:

'angspec': The reconstructed angular power spectrum of the dimensionless auxiliary field as shown in Fig. 10 of the paper. The values correspond to multipoles from l=0 to l=383.
'profile': The Galactic variance profile that relates the dimensionless auxiliary field and the Galactic Faraday depth, as shown in Fig. 11, in units of rad/m2.
'maps/s': The reconstructed dimensionless auxiliary field, i.e., the Galactic Faraday depth divided by the Galactic variance profile.
'maps/phi': The reconstructed Galactic Faraday depth in units of rad/m2.
'uncertainty/s': The pixel-wise uncertainty of the reconstructed dimensionless auxiliary field.
'uncertainty/phi': The pixel-wise uncertainty of the reconstructed Galactic Faraday depth in units of rad/m2.

The last five results are stored as RING-ordered HEALPix maps at a resolution of Nside=128. The last four results correspond to the top four panels of Fig. 15.

Alternatively, you may use this fits file, which contains six binary table extensions corresponding to the six data sets described above. The names of the extensions are the same as the names of the data sets in the hdf5 file (i.e. ANGSPEC, PROFILE, MAPS/S, MAPS/PHI, UNCERTAINTY/S, UNCERTAINTY/PHI).

Extragalactic contributions

Here we provide samples drawn from the approximate posterior probability distribution for the extragalactic contributions to each data point. Each sample consists of 41632 numbers, the extragalactic contributions to each of the data points, in rad/m2. We provide 1000 such samples in total. Each one of these samples represents a possible configuration of extragalactic Faraday rotation contributions that is not ruled out by the data.

How to use the samples

A range of values that is less likely appears less often in the samples and vice versa. Thus, the frequency with which the sample values lie within a certain interval gives the posterior probability for the true extragalactic contribution to lie within that interval.
When calculating a quantity as a function of the extragalactic Faraday contribution for one or several sources, this function should be evaluated for each of the samples. This will yield 1000 different answers. The distribution of these answers corresponds roughly to the posterior distribution for the quantity of interest, given the data and assumptions used in our paper.
Obviously, using a smaller number of samples is possible, but the more samples are used, the more accurate the resulting distribution.
See Appendix D of the paper for a more detailed description of the suggested usage of these samples.

How to get the samples

We provide one file containing all 1000 samples and one containing a subset consisting of the first 100 samples. The latter is provided to limit the data volume to be downloaded for testing purposes or if a smaller number of samples is to be used.
The structue of the hdf5 files is the following. The file contains three groups, 'sourceinfo/', 'samples/', and 'summary/'. The contents of 'sourceinfo/' are:

'sourceinfo/catalog': The name of the catalogs from which the data points are taken, using the nomenclature of Table 1 of Oppermann et al. (2012) and "Mao 2012" for the newly added data points.
'sourceinfo/Gal_lon': Galactic longitude of the data points in degrees.
'sourceinfo/Gal_lat': Galactic latitude of the data points in degrees.
'sourceinfo/observed': The observed Faraday rotation value, for cases for which this is public, in rad/m2.
'sourceinfo/sigma_observed': The error bar of the observed value, for cases for which this is public, in rad/m2. For the data points of the `Taylor' catalog, the published error bars are multiplied by a factor 1.22.
'sourceinfo/foreground': The reconstructed posterior mean for the Galactic Faraday depth at the source's location, in rad/m2.
'sourceinfo/sigma_foreground': The estimated posterior uncertainty for the Galatctic Faraday depth at the source's location, in rad/m2.

The group 'samples/' contains the samples, named 'samples/sample000' to 'samples/sample999'. Each of these data sets also has an attribute 'randomseed' that coincides with the sample number (000 to 999).

The group 'summary/' summarizes the distribution of the sample values by providing the mean in a data set 'summary/mean' and the standard deviation in a data set 'summary/standarddev', both in rad/m2.

For easier identification, the data points of each individual catalog are used in the same order as they appear in the original catalogs, with the exception of the observations that appear in several catalogs. For these, both catalogs are mentioned in the 'sourceinfo/catalog' dataset; e.g., "Tabara / Broten".

samples.hdf5 (1000 samples; 162 MB)
samples_subset.hdf5 (100 samples; 19 MB)

We again provide fits files as an alternative. The file with the complete set of samples has four binary table extensions, the file containing the subset of samples has three binary table extension.
The first extension contains 41632 rows for the 41632 sources and seven columns for the catalog identifier, Galactic longitude in degrees, Galactic latitude in degrees, observed Faraday rotation value in rad/m2, its error bar in rad/m2, the reconstructed Galactic foreground in rad/m2, and its estimated uncertainty in rad/m2.
The second and third (in the case of the large file) extension tables have 41632 rows for the 41632 sources and 500 columns each for the different samples (or 100 columns in the case of the smaller file). The values in this table are in rad/m2.
The last extension contains 41632 rows for the 41632 sources and two columns for the summary of the distributions, given by their mean in rad/m2 and their standard deviation in rad/m2.

samples.fits (1000 samples; 162 MB)
samples_subset.fits (100 samples; 19 MB)

The following table gives an overview over the information stored in the files. These first ten rows correspond to the first ten sources in the catalog. The large number of digits for the sky position is due to a coordinate conversion.



Niels Oppermann

Canadian Institute for Theoretical Astrophysics
60 St. George Street
Toronto, ON, M5S 3H8



The authors would like to thank the anonymous referee for constructive criticism, as well as Shane P. O'Sullivan for providing his unpublished rotation measure synthesis data and S. Ann Mao for providing her newest data set in digital form. The results in this publication have been derived using the NIFTy package (Selig et al. 2013), as well as the HEALPix package (Gorski et al. (2005). This research has made use of NASA's Astrophysics Data System. It was supported by the DFG Forschergruppe 1254 "Magnetisation of Interstellar and Intergalactic Media: The Prospects of Low-Frequency Radio Observations". T.A. is supported by the Japan Society for the Promotion of Science (JSPS). B.M.G. acknowledges the support of the Australian Research Council through grant FL100100114. Some of the calculations were carried out on the Canada Foundation for Innovation funded CITA Sunnyvale cluster.

Page last updated on 2015-02-16.