8-12 juin, 2014


Studying the ISM in the CGPS with polarization gradients

Joern Geisbuesch (NRC-DRAO)

Tom Landecker (DRAO) Roland Kothes (DRAO) Bryan Gaensler (University of Sydney)

The Inter-stellar Medium (ISM) is a complex system with multiple phases. Turbulence plays a major role in nearly all of them. So far still little is known about the exact nature of turbulence in the fully and partially magneto-ionized medium in the Galaxy. Here this issue is addressed by making use of the largest purpose-built available data set, the Canadian Galactic Plane Survey (CGPS). The CGPS is a major effort which aims at studying and understanding the ISM and Galactic Magnetic Field. The Galactic mid-plane is mapped in total and polarized radio continuum emission at 1420 MHz at arcminute resolution over an area of ~1300 square degrees. From the data we derive polarization gradient maps for the entire survey area to study their global and local characteristics. A comparison of polarization and Rotation Measure gradients is performed. It is shown that Faraday effects are the predominant cause of polarization gradient structures at 1420MHz. In a resolution study we demonstrate the dependence of the polarization gradient structure amplitudes apparent on a defined angular scale on the convolving beam and discuss how due to beam depolarization it is possible to probe ISM turbulence at different distances in a homogeneous ISM. On suitable patches and scales we utilize higher order moments of the polarization gradient amplitude probability distribution function to infer the state of turbulence of the magneto-ionic medium. From the CGPS polarization gradient panorama insights on large-scale properties of turbulence are obtained. Arm inter-arm variations in the polarization gradient map indicate changes of the spatial scale of turbulent fluctuations and a correlation with the line-of-sight polarization depth. In general we find the magneto-ionic medium in the disk plane to be in a sub- to trans-sonic state of turbulence (M_s<~2).
(doit être confirmé par le SOC)