MAGNETIC FIELD DISORDER and FARADAY EFFECTS on the POLARIZATION of EXTRAGALACTIC RADIO SOURCES

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 π_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 with π_1.4 ≈ π_2.3 ≥ 4% typically have simple Faraday structures, so they are most useful for background samples. Almost half of flat-spectrum (α ≥ -0.5) and ∼25% of steep-spectrum objects are re-polarized. 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.