Magnetic Field Disorder and Faraday Effects on the Polarization of Extragalactic Radio Sources

Abstract

We present a polarization catalog of 533 extragalactic radio sources that have a 2.3 GHz total intensity above 420 mJy from the S-band Polarization All Sky Survey, S-PASS, with corresponding 1.4 GHz polarization information from the NRAO VLA Sky Survey, NVSS. We studied the selection effects and found that fractional polarization, π, of radio objects at both wavelengths depends on the spectral index, the source magnetic field disorder, the source size, and depolarization. The relationship between depolarization, spectrum, and size shows that depolarization occurs primarily in the source vicinity. The median p 2.3 of resolved objects in NVSS is approximately two times larger than that of unresolved sources. Sources with little depolarization are ∼2 times more polarized than both highly depolarized and re-polarized sources. This indicates that intrinsic magnetic field disorder is the dominant mechanism responsible for the observed low fractional polarization of radio sources at high frequencies. We predict that number counts from polarization surveys will be similar at 1.4 GHz and at 2.3 GHz, for fixed sensitivity, although ∼10% of all sources may currently be missing because of strong depolarization. Objects withtypically have simple Faraday structures, so they are most useful for background samples. Almost half of flat-spectrum (  a -0.5) and ∼25% of steep-spectrum objects are repolarized. Steep-spectrum, depolarized sources show a weak negative correlation of depolarization with redshift in the range 0 < z < 2.3. Previous non-detections of redshift evolution are likely due the inclusion of re-polarized sources as well.

The bispectrum of polarized galactic foregrounds

“…This apodization is performed with 3 degree Gaussian (as is used for the Galactic mask) and if many point sources were masked this would result a significant fraction of the map being masked. Thus we can only mask the brightest sources found in the Lamee et al (2016) polarization catalog and map. Even so with the apodized masking of the three extended sources and the bright sources we have a sizeable mask and potentially suppress non-Gaussianity.…”

Forecasting the Contribution of Polarized Extragalactic Radio Sources in CMB Observations

“…The S-band Polarization All Sky Survey (S-PASS) survey observed the Southern sky with declination δ < −1 • at 30, 44, 32.4 , 27.1 , 117,229, 427,692, 1560,934, 70, 100, Full sky 13.3 , 9.7 , 225, 106, 501,269, 1296,1742, 143, 217 7.3 , 5.0 75,81 mJy 177,152 mJy 2160,2135 2.3 GHz with full width at half maximum (FWHM) of 8.9 arcmin both in total intensity and polarization using the 64 m Parkes Radio Telescope. Lamee et al (2016) crossmatched it with the NRAO/VLA Sky Survey, Condon et al (NVSS 1998), at 1.4GHz (45 arcsec (FWHM) and rms total brightness fluctuations of ∼ 0.29 mJy beam −1 ). Lamee et al (2016) aimed at generating a novel and independent polarization catalogue 2 enclosing 533 bright ERS at 2.3 GHz with polarized flux-density stronger than 420 mJy.…”

“…Lamee et al (2016) crossmatched it with the NRAO/VLA Sky Survey, Condon et al (NVSS 1998), at 1.4GHz (45 arcsec (FWHM) and rms total brightness fluctuations of ∼ 0.29 mJy beam −1 ). Lamee et al (2016) aimed at generating a novel and independent polarization catalogue 2 enclosing 533 bright ERS at 2.3 GHz with polarized flux-density stronger than 420 mJy.…”

Sardinia Radio Telescope observations of Abell 194

“…We note that, indeed, re-polarisation of the signal is not unphysical and perfectly possible (e.g. Farnes et al 2014, Lamee et al 2016. Example plots of a as a function of λ 4 are shown in the bottom panels of Fig.…”