Magnetic field disorder and Faraday effects on the polarization of extragalactic radio sources

Abstract

We present a polarization catalog of 533 extragalactic radio sources with 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 selection effects and found that fractional polarization, $\pi$, of radio objects at both wavelengths depends on the spectral index, source magnetic field disorder, source size and depolarization. The relationship between depolarization, spectrum and size shows that depolarization occurs primarily in the source vicinity. The median ${\pi }_{2.3}$ of resolved objects in NVSS is approximately two times larger than that of unresolved sources. Sources with little depolarization are $\sim 2$ 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 $\sim$10% of all sources may be currently missing because of strong depolarization. Objects with ${\pi }_{1.4}\approx {\pi }_{2.3}\ge 4\%$ typically have simple Faraday structures, so are most useful for background samples. Almost half of flat spectrum ($\alpha \ge -0.5$) and $\sim$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.

Accepted by ApJ, 18 pages, 23 figures

References in corpus (16)

• Strong magnetic fields in normal galaxies at high redshifts
• MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets
• Giant Magnetized Outflows from the Centre of the Milky Way
• Estimating extragalactic Faraday rotation
• A Global Probe of Cosmic Magnetic Fields to High Redshifts
• The properties of extragalactic radio sources selected at 20 GHz
• Faraday Rotation from Magnesium II Absorbers towards Polarized Background Radio Sources
• Radio Polarimetry of the ELAIS N1 Field: Polarized Compact Sources
• The distribution of polarized radio sources $>$15$\mu$Jy in GOODS-N
• Degree of Polarization and Source Counts of Faint Radio Sources from Stacking Polarized Intensity
• Exploring the Intergalactic Magnetic Field by Means of Faraday Tomography
• Polarization shadows of extragalactic sources by the local magneto-ionic ISM
• Polarization signatures of unresolved radio sources
• The Magnetic Field and Polarization Properties of Radio Galaxies in Different Accretion States
• Radio galaxies and their magnetic fields out to z <= 3
• Stacking for Cosmic Magnetism with SKA Surveys

Cited by in corpus (15)

• Differences in Faraday Rotation Between Adjacent Extragalactic Radio Sources as a Probe of Cosmic Magnetic Fields
• Magnetic field strength in cosmic web filaments
• S-PASS/ATCA: a window on the magnetic universe in the southern hemisphere
• The detection of cluster magnetic fields via radio source depolarisation
• Downstream depolarization in the Sausage relic: a 1-4 GHz Very Large Array study
• The jet/wind outflow in Centaurus A: a local laboratory for AGN feedback
• A southern-sky total intensity source catalogue at 2.3 GHz from S-band Polarisation All-Sky Survey data
• On the nature of LOFAR RMs and new constraints on magnetic fields in cosmic filaments and on magnetogenesis scenarios
• Evidence for strong intracluster magnetic fields in the early Universe
• The Huge Magnetic Toroids in the Milky Way Halo
• Faint polarised sources in the Lockman Hole field at 1.4 GHz
• A MeerKAT Polarization Survey of Southern Calibration Sources
• The Galactic latitude dependency of Faraday complexity in the S-PASS/ATCA RM catalogue
• Properties of Compact Faint Radio Sources as a Function of Angular Size from Stacking
• The phase-space of tailed radio galaxies in massive clusters