rm due to magnetic fields in the cosmic web and ska observations takuya akahori a krcf fellow @ kasi...

RM due to magnetic fields in the cosmic web and SKA observations

Takuya AkahoriA KRCF fellow @ KASI (-2012.9)A JSPS fellow @ Sydney U. (2012.10-)

2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

References:TA, Ryu (2010), ApJ, 723, 476TA, Ryu (2011), ApJ, 738, 134TA, et al. submitted x2

1/16

Collaborators: D. Ryu, J. Kim , B. G. Gaensler, K. Takahashi, K. KumazakiSpecial thanks: J. Stil, S. A. Mao, X. Sun, M. Machida

Magnetic Fields in the Cosmic Web(Inter-Galactic Magnetic Field, IGMF)


3oo Mpc

©4D2U, NAOJ

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GCs

sheet & void

Groups&

WHIM

Piro+ 07IG

MFFa

raday Rotation

Residual RM, RRM (observed RM-Galactic RM)7-15 [rad/m2] RRM deviation tends to be larger for higher redshift

extragalactic sources (e.g., Kronberg+ 08; Hammond 11)RMs through superclusters in nearby universe

9-60 [rad/m2] RM enhancements in Hercules and Perseus-Pisces (but very tentative, see Xu+ 06)

Cluster outskirts observations<50 [rad/m2] RM deviation at cluster outskirts (e.g., Govoni+ 10)

Observational Implications of RMIGMF


Govoni+ (10)

RMIGMF~O(1-10) [rad/m2]in filaments?3Mpc 4Mpc 5Mpc …

RM~O(100) [rad/m2] in GCs |B|~O(1) µG

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Estimations from LSS formation simulationsHD and MHD (e.g., Ryu+ 98; Dubois & Teyssier 08; Cho & Ryu 09;

Dolag & Stasyszyn 09; Stasyszyn+ 10)

IGMF based on MHD turbulence<(ρB)2>1/2/<ρ2>1/2 ∼ a few × 100 nG (note: <B> ∼ 10 nG) Coherent length, Lint/L0~1/15

Theoretical Estimations of RMIGMF


Cho+ (09) Cho & Ryu (09)

10-4μG |B| 10μG

100 h-1 Mpc

RMIGMF~O(1) [rad/m2] in a filament?

Ryu+ (08)

4/16

Questions:What’s the nature of RM due to turbulent-amplified IGMF?Can we discover (and test) them with latest/future observations such

as SKA and its pathfinders?To answer these questions, we calculate RM

maps using a model of the IGMF by Ryu+ (08)

Calculations of RMIGMF


ne [cm-3], B|| [μG], l [kpc]

RM Map

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2D RM map & 1D RM profile100 (GCs), ~10 (GGs), ~1 (filaments) [rad/m2]RM behaves like a random walk with the scale ~ several x 100 [kpc]

RM at the density peak mostly contributes to the accumulated RM

Result: Local Universe (1/2)2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

2.0

1.0

0.0

-

1.0

-2.0

Log10 |RM| [rad m-2]

-10 -5 0 5 10[Mpc/h]

TA, Ryu (2010), ApJ, 723, 476

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T. Akahori, IAU-GA2012-SpS4@Beijing

Statistics of RMs for LoSs with Tx = 105-7 KProbability Distribution Function (PDF)Log-normal Root mean square value <RM>rms~1.4 [rad/m2]PRM(k)~PB||,proj(k), LB~1 Mpc ~Lcrl/4 in the linear growth

stage

Result: Local Universe (2/2)2012.8.20-23

TA, Ryu (2010), ApJ, 723, 476

↓Lcrl

↓LB

48 runsaverage of thembest-fit lognormal

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Simple estimation of RMIGMF RM~1 [rad/m2] for a filament in a local universeRM ∝ √N since an accumulation of RM is a random walk processIf N from the path length N~100 [Mpc] / 10 [Mpc] ~10 1×√10 ~3.2 [rad/m2]If N from the column density N~2×10-3 [Mpc/cm3] / 5×10-5 [Mpc/cm3] ~40 1×√40 ~6.3 [rad/m2]IGMF in the WHIM range increases with z by a factor of ~2 (Ryu+

08)

Calculation 2: Cosmological Contribution


path length (dashed) and column density (solid) across the WHIM

RMIGMF~several-10 [rad/m2] through

filaments?

8/16

Result: Cosmological Contribution (1/2)


※Galaxy Cluster Subtraction CLS:ALL – grids (1 Mpc around Tx > 2 keV) TM7:ALL – grids (T > 107 K) TS8:ALL – pixels (Tx* > 107 K & Sx*>10-8 erg/s/cm2/sr) TS0:ALL – pixels (Tx* > 107 K & Sx*>10-10 erg/s/cm2/sr)

Integration of RMIGMFRM behaves like a random walk <RM>rms~7-10 [rad/m2] through

filaments

TA, Ryu (2011), ApJ, 738, 134

200 runaverage

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Statistics of RMs for LoSs through filamentsSF2 is flat at >0.2° with 100-200 [rad2/m4]

Result: Cosmological Contribution (2/2)


South Pole　　●：Mao+ (10) WSRT+ACTA　　ー： Stil+ (11) NVSS(VLA)

North Pole　　◯：Mao+ (10) WSRT+ACTA　　ー： Stil+ (11) NVSS(VLA)

Our Predictions　　 Color： Akahori, Ryu (2011)

TA, Ryu (2011), ApJ, 738, 134

200 runaverage

10/16

RMIGMF in filaments would be 1-10

[rad/m2] RMrms~1.4 [rad/m2] in the local universe

RMrms~7-10 [rad/m2] up to z=5

It would have characteristic structuresA peak scale of 1 Mpc for RMs in the local universeA flat SF at Θ>0.2º for RMs up to z=5

Next questions:How much is the Galactic Foreground?How can we find (and test) the RMIGMF?

Summary of the points so far2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing 11/16

Toward the poles, turbulent magnetic field is predominant. Its precise modeling is importantAmplitudes of regular and turbulent fields should be related each other,

depending on rms Mach number of turbulence and plasma β

Analytic + MHD turbulence model (Akahori+)SF is <100 [rad2/m4] at 10° scale toward the polesThe slope is much steeper that the observed ones at Θ<~1ºObserved RMs would contain significant contributions at small angular

scales from local structure, intrinsic RM, and/or the IGMF!

Furure (1/3): Galactic Foreground2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

TA, Ryu, Kim, Gaensler, submitted

12/16

The Square Kilometer ArrayUltra-wide band & ultra-dense RM grid“Cosmic Magnetism” is one of the five key

science projects

Furure (1/3): Dense RM Grid!!2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

Previous

Pathfinders SKA

Observational uncertainty [rad m-2]

10 1 0.1

Average separation of sources [deg]

1 0.1 0.01

0.1 1 10

Θ [degree]

S [

rad

2 m

-4]

10

100

ASKAP 30 deg2

0.1 1 10

Θ [degree]

SKA 30 deg2

TA+ in prep.

previous

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IGMF + GMF IGMF + GMF

Faraday Tomography (RM Synthesis)What is the best target, and what is the necessary

dataset for the IGMF study?

Furure (2/3): Faraday Tomography2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

0.01 0.1 1 10

λ2 [m2]

P[mJy]=Q+iU

Polarized intensity

IGMF

GMF

Intrinsic

F[mJy]

0 20 40

φ [rad/m2]

ModelFaraday Disp. Func.

TA, Kumazaki, Takahashi, Ryu, submitted

Reconstructed Faraday Disp. Func.

0 20 40

φ [rad/m2]

Faraday Tomography

F[mJy]LOFAR+GMRT+ASKAP

~F[mJy]SKA

~

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RRM or high-pass filter in Fourier-spaceHow can we treat unevenly sampled data?

What is the necessary dataset for IGMF studies?

Furure (3/3): Image Processing2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

TA+ in prep.

Fourier transformation of RM map

Chop the large-scale power in Fourier space

Inverse Fourier transformation

IGMF + GMF IGMF onlyHigh-pass filtered

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Summary2012.8.20-23T. Akahori, IAU-GA2012-SpS4@Beijing

RM due to turbulent-driven IGMF in filamentsLocal universeRMrms~1.4 [rad/m2], lognormal1 Mpc scale

Cosmological contributionRMrms~7-10 [rad/m2], lognormal0.1-0.2º scale & flat SF at

Θ>0.2º

FuturePrecise modeling of Milky WayRM Synthesis & Image processingTests with SKA & its pathfinders!

16/16

rm due to magnetic fields in the cosmic web and ska observations takuya akahori a krcf fellow @ kasi...

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