Why are the radio sources missing in the Fornax cluster?

Alvina Yee Lian On^1,2*, Jennifer Yik Ham Chan^3,4,5, Paul Chong Wa Lai^2,6, Kinwah Wu^2,7, Jia Rou Liou⁸, Hsiang-Yi Karen Yang^1,9

¹Physics Division, National Center for Theoretical Sciences, Taipei, Taiwan
²Mullard Space Science Laboratory, University College London, Surrey, UK
³Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, Canada
⁴Faculty of Arts and Science, University of Toronto, Toronto, Canada
⁵Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Canada
⁶Department of Physics, University of Hong Kong, Hong Kong
⁷Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Chiba, Japan
⁸Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁹Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Alvina Yee Lian On, email:alvina.on@phys.ncts.ntu.edu.tw

Recently, the Australian Square Kilometer Array Pathfinder revealed an apparent lack of radio source counts in the Fornax galaxy cluster field. The sources in this patch of sky also appeared to be less polarized. These observations are peculiar and could be important signatures of depolarization on galaxy cluster scales. In this work, we quantified the effects on polarization of radio point sources behind a large-scale intracluster shock. Our ray-tracing calculations revealed that, generally, bright sources do not experience any significant changes in polarization, whereas faint sources either get severely depolarized or enhanced as their radiation propagates through the intracluster medium. Notably, most of the dim sources behind the shock gain linear polarization. This is because the gas and magnetic fields are highly compressed within the shock, causing it to be more linearly polarized than the surrounding medium. We also carried out a magnetohydrodynamic simulation of an in-falling subcluster into a main cluster and found a substantial increase in the local Faraday rotation measures behind the shock front. These effects would impact the interpretations of radio observations, particularly with BURSTT and SKA-Low, where the polarization bias in faint sources must be corrected for in the statistical analyses of rotation measure grids.

Keywords: polarization, radiative transfer, magnetic fields, galaxies: clusters: intracluster medium, radiation mechanism: non-thermal