Parsec-scale structure in the warm ISM from polarized galactic radio background observations

We present multi-frequency polarization observations of the diffuse radio synchrotron background modulated by Faraday rotation, in two directions of positive latitude. No extended total intensity I is observed, which implies that total intensity has no structure on scales smaller than approximately a degree. Polarized intensity and polarization angle, however, show abundant small-scale structure on scales from arcminutes to degrees. Rotation Measure (RM) maps show coherent structure over many synthesized beams, but also abrupt large changes over one beam. RM's from polarized extragalactic point sources are correlated over the field in each of the two fields, indicating a galactic component to the RM, but show no correlation with the RM map of the diffuse radiation. The upper limit in structure in I puts constraints on the random and regular components of the magnetic field in the galactic interstellar medium and halo. The emission is partly depolarized so that the observed polarization mostly originates from a nearby part of the medium. This explains the lack of correlation between RM from diffuse emission and from extragalactic point sources as the latter is built up over the entire path length through the medium.Comment: To appear in "Astrophysical Polarized Backgrounds", Conference Proceedings, eds S. Cecchini, S. Cortiglioni, R. Sault and C. Sbarra, in pres

Constraining regular and turbulent magnetic field strengths in M51 via Faraday depolarization

We employ an analytical model that incorporates both wavelength-dependent and wavelength-independent depolarization to describe radio polarimetric observations of polarization at $\lambda \lambda \lambda \, 3.5, 6.2, 20.5$ cm in M51 (NGC 5194). The aim is to constrain both the regular and turbulent magnetic field strengths in the disk and halo, modeled as a two- or three-layer magneto-ionic medium, via differential Faraday rotation and internal Faraday dispersion, along with wavelength-independent depolarization arising from turbulent magnetic fields. A reduced chi-squared analysis is used for the statistical comparison of predicted to observed polarization maps to determine the best-fit magnetic field configuration at each of four radial rings spanning $2.4 - 7.2$ kpc in $1.2$ kpc increments. We find that a two-layer modeling approach provides a better fit to the observations than a three-layer model, where the near and far sides of the halo are taken to be identical, although the resulting best-fit magnetic field strengths are comparable. This implies that all of the signal from the far halo is depolarized at these wavelengths. We find a total magnetic field in the disk of approximately $18~\mu$G and a total magnetic field strength in the halo of $\sim 4-6~\mu$G. Both turbulent and regular magnetic field strengths in the disk exceed those in the halo by a factor of a few. About half of the turbulent magnetic field in the disk is anisotropic, but in the halo all turbulence is only isotropic.Comment: Accepted for publication in Astronomy & Astrophysics, 10 pages, 5 figures, 5 table

Structure in the polarized Galactic synchrotron emission, in particular `depolarization canals'

The polarized component of the diffuse radio synchrotron emission of our Galaxy shows structure, which is apparently unrelated to the structure in total intensity, on many scales. The structure in the polarized emission can be due to several processes or mechanisms. Some of those are related to the observational setup, such as beam depolarization -- the vector combination and (partial) cancellation of polarization vectors within a synthesized beam --, or the insensitivity of a synthesis telescope to structure on large scales, also known as the 'missing short spacings problem'. Other causes for structure in the polarization maps are intrinsic to the radiative transfer of the emission in the warm ISM, which induces Faraday rotation and depolarization. We use data obtained with the Westerbork Synthesis Radio Telescope at 5 frequencies near 350 MHz to estimate the importance of the various mechanisms in producing structure in the linearly polarized emission. In the two regions studied here, which are both at positive latitudes in the second Galactic quadrant, the effect of 'missing short spacings' is not important. The properties of the narrow depolarization 'canals' that are observed in abundance lead us to conclude that they are mostly due to beam depolarization, and that they separate regions with different rotation measures. As beam depolarization only creates structure on the scale of the synthesized beam, most of the structure on larger scales must be due to depth depolarization. We do not discuss that aspect of the observations here, but in a companion paper we derive information about the properties of the ISM from the structure of the polarized emission.Comment: 12 pages, 10 figures, accepted for publication by A&

Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations

We discuss observations of the linearly polarized component of the diffuse galactic radio background. These observations, with an angular resolution of 4', were made with the Westerbork Synthesis Radio Telescope (WSRT) in 5 frequency bands in the range 341-375 MHz. The linearly polarized intensity P (with polarized brightness temperature going up to 10K) shows a `cloudy' structure, with characteristic scales of 15'-30', which contains relatively long, but very narrow `canals' (essentially unresolved) in which P is only a small fraction of that in the neighbouring beams. These `canals' are generally seen in more than one frequency band, although their appearance changes between bands. They are probably due to depolarization within the synthesized beam, because the change in polarization angle across the deepest `canals' is in general close to 90 degrees (or 270 etc.). These very abrupt changes in polarization angle, which are seen only across the `canals', seem to be accompanied by abrupt changes in the Rotation Measure (RM), which may have the right magnitude to create the difference of close to 90 degrees in polarization angle, and thereby the `canals'. The structure in the polarization maps is most likely due to Faraday rotation modulation of the probably smooth polarized radiation emitted in the halo of our Galaxy by the fairly local ISM (up to 500 pc). Therefore, the abrupt changes of RM across the `canals' provide evidence for very thin (about 1 pc), and relatively long transition regions in the ISM, across which the RM changes by as much as 100%. Such drastic RM changes may well be due primarily to abrupt changes in the magnetic field.Comment: 4 pages, 4 figures, to be published in A&A Letter

Depolarization of synchrotron radiation in a multilayer magneto-ionic medium

Depolarization of diffuse radio synchrotron emission is classified in terms of wavelength-independent and wavelength-dependent depolarization in the context of regular magnetic fields and of both isotropic and anisotropic turbulent magnetic fields. Previous analytical formulas for depolarization due to differential Faraday rotation are extended to include internal Faraday dispersion concomitantly, for a multilayer synchrotron emitting and Faraday rotating magneto-ionic medium. In particular, depolarization equations for a two- and three-layer system (disk-halo, halo-disk-halo) are explicitly derived. To both serve as a `user's guide' to the theoretical machinery and as an approach for disentangling line-of-sight depolarization contributions in face-on galaxies, the analytical framework is applied to data from a small region in the face-on grand-design spiral galaxy M51. The effectiveness of the multiwavelength observations in constraining the pool of physical depolarization scenarios is illustrated for a two- and three-layer model along with a Faraday screen system for an observationally motivated magnetic field configuration.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 4 figures, 2 table

Wavelet-based Faraday Rotation Measure Synthesis

Faraday Rotation Measure (RM) Synthesis, as a method for analyzing multi-channel observations of polarized radio emission to investigate galactic magnetic fields structures, requires the definition of complex polarized intensity in the range of the negative lambda square. We introduce a simple method for continuation of the observed complex polarized intensity into this domain using symmetry arguments. The method is suggested in context of magnetic field recognition in galactic disks where the magnetic field is supposed to have a maximum in the equatorial plane. The method is quite simple when applied to a single Faraday-rotating structure on the line of sight. Recognition of several structures on the same line of sight requires a more sophisticated technique. We also introduce a wavelet-based algorithm which allows us to consider a set of isolated structures. The method essentially improves the possibilities for reconstruction of complicated Faraday structures using the capabilities of modern radio telescopes.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter

HI anisotropies associated with radio-polarimetric filaments. Steep power spectra associated with cold gas

LOFAR detected toward 3C 196 linear polarization structures which were found subsequently to be closely correlated with cold filamentary HI structures. The derived direction-dependent HI power spectra revealed marked anisotropies for narrow ranges in velocity, sharing the orientation of the magnetic field as expected for magneto hydrodynamical turbulence. Using the Galactic portion of the Effelsberg-Bonn HI Survey we continue our study of such anisotropies in the HI distribution in direction of two WSRT fields, Horologium and Auriga; both are well known for their prominent radio-polarimetric depolarization canals. At 349 MHz the observed pattern in total intensity is insignificant but polarized intensity and polarization angle show prominent ubiquitous structures with so far unknown origin. Apodizing the HI survey data by applying a rotational symmetric 50 percent Tukey window, we derive average and position angle dependent power spectra. We fit power laws and characterize anisotropies in the power distribution. We use a Gaussian analysis to determine relative abundances for the cold and warm neutral medium. For the analyzed radio-polarimetric targets significant anisotropies are detected in the HI power spectra; their position angles are aligned to the prominent depolarization canals, initially detected by WSRT. HI anisotropies are associated with steep power spectra. Steep power spectra, associated with cold gas, are detected also in other fields. Radio-polarimetric depolarization canals are associated with filamentary HI structures that belong to the cold neutral medium (CNM). Anisotropies in the CNM are in this case linked to a steepening of the power-spectrum spectral index, indicating that phase transitions in a turbulent medium occur on all scales. Filamentary HI structures, driven by thermal instabilities, and radio-polarimetric filaments are associated with each other.Comment: Accepted for publication by A&A, 28 pages, 41 figures, minor updates in styl