Study ofCosmic Magnetic Fields

with Square Kilometer Array

Keitaro TakahashiNagoya University

4/Nov./2010 @Japan-SKA workshop 2010

sub Science Working Group on “Cosmic Magnetic Fields” 11 researchers from 9 institutes Keitaro Takahashi (Nagoya): chair Takuya Akahori (Chungnam National University) Hidekazu Hanayama (NAOJ), Kiyotomo Ichiki (Nagoya) Makoto Inoue (ASIAA), Susumu Inoue (Kyoto) Takahiro Kudoh (NAOJ), Mami Machida (Kyusyu) Hiroyuki Nakanishi (Kagoshima) Yoshiaki Sofue (Meisei), Hajime Susa (Konan)

Method: theory, numerical simulation Target: galaxy, cluster of galaxies, cosmology

“Cosmic Magnetic Fields”

Introduction:Origin of Cosmic Magnetic Fields

Origin of magnetic fields

of the earth is one ofthe greatest mysteryin modern physics.

Origin of Magnetic Fields

We now know various astronomical objects havetheir own magnetic fields.

10 G

10 G

1mG

1 G

10 G

10 G

10 G

12

9

6

3

1km 10 km 1pc 1kpc 1Mpc size6

neutronstar

whitedwarf

Earth

activegalacticnuclei

SNR

galaxy

cluster ofgalaxies

cosmological

Sun

ubiquitous magnetic fields

1nG

-6

Extended Einstein’s Question1. Earth => astronomical object - galaxies - clusters of galaxies2. cosmological fields? - magnetic fields not associated with any objects - only an upper bound ~ 0.1nG

Origin of Magnetic Fields (Extended)

Observation of Cosmic Magnetic Fieldsand

Importance of Wideband Observation

Observation of Magnetic Fields

1. synchrotron radiation2. Faraday rotation polarization angle

　 rotation measureradio pulsar B1154-62

Information on Bcan be obtainedfrom the slope.

integralfrom the sourceto the observer

Application

We can probe target’s magneticfields by observing a lot of sourcesbehind the target.

observer

sources

target

Gaensler et al., 2005

292 sources behind LMC

Importance of Wideband

So far, rotation measures have often been determined byvery narrow band observations (1.365 – 1.435GHz).=> very large error

Wideband observation isimportant for precisedetermination ofrotation measure.

wavelength^2 (cm^2)

pola

riza

tion

ang

le (

degr

ee)

target

source

Brentjens & de Bruyn, 2005

Importance of Wideband

mixture of radiowave from sourceand target itself(and possiblegalactic foreground)

polarization angle

polarization degree

The situation is more complicatedin general.- emission of target itself- galactic foreground

Brentjens & de Bruyn, 2005

Importance of Wideband

Wideband observation iscrucial to probe magnetic fields.

polarization angle

polarization degree

Faraday rotation measure synthesis, (Burn, 1966)

polarized surface brightnessF(φ): Faraday dispersion functionFaraday depth (~ “distance”)

F(φ) represents which part isemitting and where andhow much magnetic fieldsexist. F(φ) can be obtained byinverse “Fourier transformation”.

Activity of Our Group

Galaxy

M31 (Andromeda galaxy) ring & vertical fields (Berkhuijsen et al., 2003)

global structure- ring- axisymmetric- bisymmetric- verticalHow were these formed?

Galaxy

primordial origin (Sofue, Machida & Kudoh, 2010)

example: bisymmetric field-uniform field at galaxy formation-winding up with rotating disk

confirmation by numerical simulations

Poster by Sofue-san

Galaxy Cluster and Large Scale Structure

cluster of galaxies: ~10 G collection of 10-1000 galaxieslarge scale structure: ??? filament, void

SDSS

-6

Galaxy Cluster and Large Scale Structure

Cosmological simulation of large scale structure(Ryu et al., 2008, Akahori & Ryu, 2010)Primordial magnetic fields evolve through structure formation.

magnetic fields associatedwith large scale structure

rotation measureso small for the present telescopebut observable by SKA

Talk by Akahori-san

Reionization

magnetogenesis at reionization(Ando, Susa et al., 2010)- UV and X-ray from first stars- momentum transfer from radiation to electrons- inhomogeneous ISM leads to rotational electric current => magnetic field generation

Reionization

Magnetic fieldsare generatedbehind the cloud.

Upto 10 Gaussfor reasonablesituations.

B(G)

-20

Cosmological

generation of cosmological magnetic fields in the early universe(KT, Ichiki et al., 2005, 2006, 2007, 2008)

tiny density fluctuations in the early universe=> electrons are stripped from protons by radiation pressure=> electric current, electric fields=> magnetic field generation

Cosmological

Spectrum at recombination

-38

-34

-30

-26

-22

-18

-14

1Gpc 1Mpc 1kpc 1pccomoving scale

log B

(G)

at

reco

mbin

ati

on

horizon scale ～ 10 G-25

cutoff ～ 10 G-14

z = 10B = 10 Gcan seedgalacticfields.

-18

Grand Scenario

primordial magnetic fields- density fluctuations

- reionization

large-scale structure- cluster of galaxies

- filament

galactic fields

wideband observation- Faraday rotation

- synchrotron

Review Paper

“Unsolved Problems on Cosmic Magnetic Fields” 1. Introduction (K. Takahashi) 2. Observation of Magnetic Fields (K. Takahashi) 3. Cosmology (K. Takahashi, K. Ichiki) 4. Cluster of Galaxies and Large Scale Structure (T. Akahori) 5. Galaxy (T. Kudoh) 6. AGN and Jet (M. Machida) 7. Conclusion (K. Takahashi)Currently Japanese (54pages)English version will appear soon.

Summary

Origin of cosmic magnetic fields -mystery in modern astrophysics -stars, galaxies, clusters of galaxies, universe itself -wideband is crucial for measurement through Faraday rotation -SKA-Japan consortium Science Working Group “Cosmic Magnetic Fields” -theory, numerical simulation -galaxy, cluster of galaxies, cosmology

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