Broadband Radio Polarimetry and Faraday Rotation of 563 Extragalactic Radio Sources

Abstract

We present a broadband spectropolarimetric survey of 563 discrete, mostly unresolved radio sources between 1.3 & 2.0 GHz using data taken with the Australia Telescope Compact Array (ATCA). We have used rotation measure synthesis to identify Faraday complex polarized sources -i.e. objects whose frequency-dependent polarization behaviour indicates the presence of material possessing complicated magnetoionic structure along the line of sight (LOS). For sources classified as Faraday complex, we have analyzed a number of their radio and multiwavelength properties to determine whether they differ from Faraday simple polarized sources (i.e. sources for which LOS magnetoionic structures are comparatively simple) in these properties. We use this information to constrain the physical nature of the magnetoionic structures responsible for generating the observed complexity. We detect Faraday complexity in 12% of polarized sources at ∼ 1 resolution, but demonstrate that underlying signal-to-noise limitations mean the true percentage is likely to be significantly higher in the polarized radio source population. We find that the properties of Faraday complex objects are diverse, but that complexity is most often associated with depolarization of extended radio sources possessing a relatively steep total intensity spectrum. We find an association between Faraday complexity and LOS structure in the Galactic interstellar medium (ISM), and claim that a significant proportion of the Faraday complexity we observe may be generated at interfaces of the ISM associated with ionization fronts near neutral hydrogen structures. Galaxy clusters environments and internally generated Faraday complexity provide possible alternative explanations in some cases.

A STUDY OF BROADBAND FARADAY ROTATION AND POLARIZATION BEHAVIOR OVER 1.3–10 GHz IN 36 DISCRETE RADIO SOURCES

“…Finally, we note that Anderson et al (2015) implicated the Galactic interstellar medium in causing depolarization among sources which were partially resolved at an ∼arcminute resolution between 1.3 and 2 GHz. We reject a Galactic origin for the depolarization associated with the emission components discussed in this section.…”

“…For the combined total of 572 sources robustly detected in linear polarization in these data, Gaensler et al (2005) and Feain et al (2009) extracted Stokes I, Q, U, and V flux densities at 8 MHz intervals through the respective bands. We applied RM synthesis (Brentjens & de Bruyn 2005) and RMCLEAN (Heald et al 2009) to these data, identifying Faraday complex sources using the second moment (s F M ) of the RMCLEAN component distribution F M (Brown et al 2011, Anderson et al 2015. This provides a measure of the spread of RMCLEAN components in f space, and is given by: where n is the number of channels i possessing non-zero values in the RMCLEAN model and f i is the Faraday depth of channel i.…”

“…−2 as candidates for our Faraday complex sample (e.g., see Anderson et al 2015), and sources with s = 0 rad…”

“…In general, however, it remains unclear exactly which structures in radio sources are being studied, or indeed, whether other structures along the LOS also make a substantial contribution to the observed polarization behavior. For example, Anderson et al (2015) recently analyzed ∼160 polarized sources over 1.3-2 GHz and found evidence for a significant Galactic contribution to the Faraday complexity of background sources.…”

Faraday Rotation Measure Synthesis of Intermediate Redshift Quasars as a Probe of Intervening Matter

“…Recently, however, the fractional parameter q = Q/I and u = U/I has been utilized (Anderson et al (2015)). There are good reasons to do that, especially when one is dealing with steep spectral index sources.…”