The DRAO Planck Deep Fields: the polarization properties of radio galaxies at 1.4 GHz

TL;DR

This study presents deep polarization imaging at 1.4 GHz of the DRAO Planck Deep Fields, analyzing the polarization properties of 958 radio sources, revealing that fainter sources tend to have higher fractional polarization and that polarized sources are mostly resolved, lobe-dominated radio galaxies.

Contribution

First detailed polarization properties of radio galaxies at 1.4 GHz in a deep field, including source counts and polarization characteristics, with implications for radio galaxy structure.

Findings

Higher fractional polarization at fainter flux densities.

Majority of polarized sources are steep-spectrum and resolved.

Resolved sources have median fractional polarization of 6.8%.

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 square degrees centered at RA = 16h 14m and DEC = 54d 56', has an angular resolution of 42" x 62" at the field center, and reaches a sensitivity of 55 microJy/beam in Stokes I and 45 microJy/beam 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 microJy. 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 = sqrt(Q^2 + U^2) < 3 mJy.