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Study ofCosmic Magnetic Fields
with Square Kilometer Array
Keitaro TakahashiNagoya University
4/Nov./2010 @Japan-SKA workshop 2010
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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”
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Introduction:Origin of Cosmic Magnetic Fields
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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.
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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
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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)
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Observation of Cosmic Magnetic Fieldsand
Importance of Wideband Observation
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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
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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
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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
rizat
ion
angl
e (d
egre
e)
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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
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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 brightness
F(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”.
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Activity of Our Group
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Galaxy
M31 (Andromeda galaxy) ring & vertical fields (Berkhuijsen et al., 2003)
global structure- ring- axisymmetric- bisymmetric- verticalHow were these formed?
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Galaxyprimordial 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
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Galaxy Cluster and Large Scale Structurecluster of galaxies: ~10 G collection of 10-1000 galaxieslarge scale structure: ??? filament, void
SDSS
-6
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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
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Reionizationmagnetogenesis 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
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Reionization
Magnetic fieldsare generatedbehind the cloud.
Upto 10 Gaussfor reasonablesituations.
B(G)
-20
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Cosmologicalgeneration 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
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CosmologicalSpectrum at recombination
-38
-34
-30
-26
-22
-18
-14
1Gpc 1Mpc 1kpc 1pccomoving scale
log
B(G)
at r
ecom
bina
tion
horizon scale ~ 10 G-25
cutoff ~ 10 G-14
z = 10B = 10 Gcan seedgalacticfields.
-18
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Grand Scenario
primordial magnetic fields- density fluctuations
- reionization
large-scale structure- cluster of galaxies
- filament
galactic fields
wideband observation- Faraday rotation
- synchrotron
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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.
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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
END