Study of Cosmic Magnetic Fields with Square Kilometer Array Keitaro Takahashi Nagoya University 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 of the greatest mystery in modern physics. Origin of Magnetic Fields We now know various astronomical objects have their own magnetic fields.

10 G 1mG 1 G 10 G km 10 km 1pc 1kpc 1Mpc size 6 neutron star white dwarf Earth active galactic nuclei SNR galaxy cluster of galaxies cosmological Sun ubiquitous magnetic fields 1nG -6

Extended Einstein’s Question 1. Earth => astronomical object - galaxies - clusters of galaxies 2. cosmological fields? - magnetic fields not associated with any objects - only an upper bound ~ 0.1nG Origin of Magnetic Fields (Extended)

Observation of Cosmic Magnetic Fields and Importance of Wideband Observation

Observation of Magnetic Fields 1. synchrotron radiation 2. Faraday rotation polarization angle rotation measure radio pulsar B Information on B can be obtained from the slope. integral from the source to the observer

Application We can probe target’s magnetic fields by observing a lot of sources behind the target. observer sources target Gaensler et al., sources behind LMC

Importance of Wideband So far, rotation measures have often been determined by very narrow band observations (1.365 – 1.435GHz). => very large error Wideband observation is important for precise determination of rotation measure. wavelength^2 (cm^2) polarization angle (degree)

target source Brentjens & de Bruyn, 2005 Importance of Wideband mixture of radio wave from source and target itself (and possible galactic foreground) polarization angle polarization degree The situation is more complicated in general. - emission of target itself - galactic foreground

Brentjens & de Bruyn, 2005 Importance of Wideband Wideband observation is crucial to probe magnetic fields. polarization angle polarization degree Faraday rotation measure synthesis, (Burn, 1966) polarized surface brightness F(φ): Faraday dispersion function Faraday depth (~ “distance”) F(φ) represents which part is emitting and where and how much magnetic fields exist. F(φ) can be obtained by inverse “Fourier transformation”.

Activity of Our Group

Galaxy M31 (Andromeda galaxy) ring & vertical fields (Berkhuijsen et al., 2003) global structure - ring - axisymmetric - bisymmetric - vertical How 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 galaxies large 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 associated with large scale structure rotation measure so small for the present telescope but 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 fields are generated behind the cloud. Upto 10 Gauss for reasonable situations. 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 Gpc1Mpc1kpc1pc comoving scale log B(G) at recombination horizon scale ～ 10 G -25 cutoff ～ 10 G -14 z = 10 B = 10 G can seed galactic fields. -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 END