THE DRAO PLANCK DEEP FIELDS: THE POLARIZATION PROPERTIES OF RADIO GALAXIES AT 1.4 GHz

Abstract

We present results of deep polarization imaging at 1.4 GHz with the Dominion Radio Astrophysical Observatory as part of the DRAO Planck Deep Fields project. This deep extragalactic field covers 15.16 deg 2 centered at α 2000 = 16 h 14 m and δ 2000 = 54 • 56 ′ , has an angular resolution of 42 ′′ × 62 ′′ at the field center, and reaches a sensitivity of 55 µJy beam −1 in Stokes I and 45 µJy beam −1 in Stokes Q and U. We detect 958 radio sources in Stokes I of which 136 are detected in polarization. We present the Euclidean-normalized polarized differential source counts down to 400 µJy. These counts indicate that sources have a higher degree of fractional polarization at fainter Stokes I flux density levels than for brighter sources, confirming an earlier result. We find that the majority of our polarized sources are steep-spectrum objects with a mean spectral index of −0.77, and there is no correlation between fractional polarization and spectral index. We also matched deep field sources to counterparts in the Faint Images of the Radio Sky at Twenty Centimeters catalogue. Of the polarized sources, 77% show structure at the arc-second scale whereas only 38% of the sources with no detectable polarization show such structure. The median fractional polarization is for resolved sources is 6.8%, while it is 4.4% for compact objects. The polarized radio sources in our deep field are predominantly those sources which are resolved and show the highest degrees of fractional polarization, indicating that the lobe dominated structure may be the source of the highly polarized sources. These resolved radio galaxies dominate the polarized source counts at P 0 = Q 2 + U 2 < 3 mJy.

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

“…The anti-correlation between total intensity and fractional polarization at 1.4 GHz has been extensively discussed (such as Mesa et al 2002;Tucci et al 2004;Taylor et al 2007;Grant et al 2010;Subrahmanyan et al 2010;Stil et al 2014). Recently, Banfield et al (2014) used WISE colors to suggest that the anti-correlation was due to the difference in environments between WISE-AGNs (IR colors dominated by AGN) and WISE-Ellipticals (IR colors dominated by starlight).…”

“…When many lines of sight each have independent single Faraday depths, this problem is approached statistically. Another long-standing puzzle is the anti-correlation between the total intensity of radio sources and their degree of polarization, as observed by many groups such as Mesa et al (2002), Tucci et al (2004), Sadler et al (2006), Taylor et al (2007), Grant et al (2010), Subrahmanyan et al (2010), and Stil et al (2014). The physical nature of this relation has been a mystery for almost a decade and is confused by its dependency on other source properties.…”

Deep GMRT 610 MHz observations of the ELAIS N1 field: catalogue and source counts

“…Sirothia et al (2009) observed it at 325 MHz using the Giant Metrewave Radio Telescope (GMRT), with the objective of identifying active galactic nuclei and starburst galaxies and examining their evolution with cosmic epoch. Grant et al (2010) observed 15 deg 2 with the Dominion Radio Astrophysical Observatory synthesis telescope at 1420 MHz to a minimum rms of 55 µJy beam −1 in Stokes I and 45 µJy beam −1 in polarisation. Banfield et al (2011) observed 10 deg 2 at 1.4 GHz with the JVLA in B configuration to a minimum rms of 87 µJy beam −1 in total intensity.…”

blobcat: software to catalogue flood-filled blobs in radio images of total intensity and linear polarization

“…Secondly, recent polarimetric studies have indicated an increase in the fractional polarization of faint extragalactic radio sources (e.g. Taylor et al ; Grant et al ; Shi et al ; Subrahmanyan et al ), which are difficult to reconcile with population modelling (O'Sullivan et al ). We seek here to subject the process of polarization measurement to close scrutiny, and to provide the community with a measurement tool that has been assessed within a controlled testing environment.…”