Polarized radio filaments outside the Galactic plane

Abstract

We used data from the WMAP satellite at 23, 33 and 41 GHz to study the diffuse polarised emission over the entire sky. The emission originates mostly from filamentary structures with well-ordered magnetic fields. Some of these structures have been known for decades in radio continuum maps. Their origin is not clear and there are many filaments that are visible for the first time. We have identified and studied 11 filaments. The polarisation fraction of some of them can be as high as 40%, which is a signature of a well ordered magnetic field. The polarisation spectral indices, averaged over 18 regions in the sky is β = −3.06 ± 0.02, consistent with synchrotron radiation. There are significant variations in β over the sky (∆β ≈ 0.2).We explore the link between the large-scale filaments and the local ISM, using the model of an expanding shell in the solar vicinity. We compared observed polarisation angles with the predictions from the model and found good agreement. This strongly suggests that many large scale filaments and loops are nearby structures. This is important in the context of the Galactic magnetic field as these structures are normally included in global models, neglecting the fact that they might be local. We also studied the level of contamination added by the diffuse filaments to the CMB polarisation power spectra. We conclude that, even though these filaments present low radio brightness, a careful removal will be necessary for future all-sky CMB polarisation analysis.

Planck2015 results

“…22 and 23). Most of the high-latitude polarized emission is associated with distinct large-scale loops and spurs, and we rediscuss their structure following the earlier study of Vidal et al (2015) based on WMAP observations. We argue that nearly all the emission at −90 • < l < 40 • is part of the Loop I structure, and show that the emission extends much further into the southern Galactic hemisphere than previously recognized, giving Loop I an ovoid rather than circular outline.…”

“…Synchrotron polarization amplitude map, P = Q 2 + U 2 , at 30 GHz, smoothed to an angular resolution of 60 , produced by a weighted sum of Planck and WMAP data as described in Planck Collaboration XXV (2016). The traditional locii of radio loops I-IV are marked in black, a selection of the spurs identified by Vidal et al (2015) in blue, the outline of the Fermi bubbles in magenta, and features discussed for the first time in Planck Collaboration XXV (2016) in red. Our measured outline for Loop I departs substantially from the traditional small circle.…”

Planck2015 results

“…The synchrotron spectral index varies by only a few tenths across the sky, and to map it with useful precision we need a substantial gain in sensitivity. This has been achieved mainly by working at low resolution (Fuskeland et al 2014;Vidal et al 2015); in the current paper we use a fixed template for the synchrotron spectrum (Sect. 4.3 below).…”

“…The main limitation in obtaining accurate components maps is the precise quantification of the synchrotron component. Fuskeland et al (2014) and Vidal et al (2015) have shown that the polarized spectral index of synchrotron emission varies across the sky and these variations should be taken into account to obtain an accurate separation between the synchrotron and AME components; low frequency (5-20 GHz) data will be crucial for this.…”

“…While these "loops" and "spurs" have long been known in total intensity (Quigley & Haslam 1965;Berkhuijsen et al 1971;Berkhuijsen 1971), their pattern is clarified by the new polarization maps. Vidal et al (2015) present an analysis of the filaments using WMAP data. They have catalogued them, studied the polarization angle distribution and polarized spectral indices, and presented a model to explain the origin of some of them.…”