Self-enhanced Faraday rotation by extragalactic jets

Martin Krause (MPE Garching & LMU München)with: Martin Huarte-Espinosa, Paul Alexander, Julia Riley & Christian Kaiser

Annual meeting of the Astronomische Gesellschaft, September 2010, Bonn

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

Overview

• Introduction: Faraday rotation in clusters (& groups) of galaxies

• FR II jets in magnetised Intra-cluster gas:• enhance rotation measure• change structure function

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

Magnetic fields in clusters of galaxies: Upcoming surveys ➔ Square Kilometre Array

Semianalytic cosm. model.:

Pred. SKA mid freq. array:

[Krause et al. 2009, MNRAS 400, 646]

Detection statistics:

- pol. background sources measure mag. field in clusters- SKA will measure Faraday rotation in 1000s of gal. clusters w. redshift > 1- SKA: mainly weaker star forming galaxies- currently/ near future: mainly stronger jet sources

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

Faraday rotation in clusters (and groups) of galaxies: current observations

Intra cluster

(Radians m-2)

A2255 Govoni et al. 2006

3C449, 2231.2+3732 Guidetti et al.

2010

• Faraday rotation is measured against polarised radio emission of typically embedded radio jets.• What is the local effect of the jets on the measured RM?

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

displaced & compressed magnetised

ICM

Jet-effect on RM:sketch version

y

z

jet beamlobe / cocoon

shocked (compressed)ambient gas

y

x

Magnetic field vectors:1 - radial2 - non-radial

12

O1

O2

• Tangled field• Non-radial component amplified by compression, expansion energy →MF• If observer not ⊥ MF, RM enhanced• RM-Enhancement for observer O1 => edge effect

X-ray cavity1

2

O1 O2

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

• Flash v3, 3D, MHD, no AMR, constrained transport

• generate turbulent magnetic field cube by the Tribble/ Murgia et al method, Bk∝k-11/3

• multiply with King profile

• let cluster gas relax

• inject bipolar FR II jets in the middle

• vary density & velocity of the jets

• ∀ sim., do sim without jet

• compare RM + statistics: jet-sim. vs. no-jet sim.

Jet ⇔ cluster magnetic fields: simulations

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

Results [90 deg]: RM ≈ + 20%

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

Results [45 deg]: RM ≈ +70%

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

• reproduce observed flattening without intrinsic outer scale of MF

simulations flattening / lobe scale

TefxtfStructure functions

45 degree

Guidetti et al. 2010

Wednesday, September 15, 2010

Martin Krause - Self-enhanced Faraday rotation by extragalactic jets - AGM, Bonn 2010

Conclusions

• Simulated FR II jets in turbulent cluster mag. fields• RM enhanced @ edges, by about < x 2• ∝cocoon width / inclination• structure function reproduces obs.• FR Is also done, but so far HD, only• expect. similar, maybe weaker effect for FR Is

Wednesday, September 15, 2010