銀河磁場の活動性
Various Activities of the Galactic
Magnetic Field
MACHIDA Mami （Kyushu University）

Collaborators
MATSUMOTO Ryoji （Chiba Univ.）、NAKAMURA Kenji （Kyushu Sangyo Univ.）
AKAHORI Takuya, NAKANISHI Hiroyuki （Kagoshima Univ.）
FUKUI Yasuo （Nagoya Univ.）、TORII Kazufumi （NAOJ）
SUZUKI Takeru （Univ. of Tokyo）

Table of Contents
1. Introduction

Observational evidence of galactic magnetic fields
Formation Mechanism of the global structure

2. Instability
α-Ω dynamo model
Parker instability
MRI

3. Numerical Simulations
Global simulations of spiral galaxies

4. Magnetic field activities of galactic centerMolecular loop in CMZ
Formation mechanism of l-b diagram

Kakiuchi-sanMagnetic field structure in the Galaxy
Rotation Measure (RM) RM: = 0.81∫

L (pc)

0All sky distribution of the RMs
in Galactic coordinate

Taylor + (2009), Oppermann + (2012)n ⋅ B// dl , n[cm −3 ], B// [ µG]
Reversal of magnetic fields

Han +, ApJL, 570 (2002)

Complex distribution of RM
Mean line of sight magnetic fields: north-positive, south-negative
Direction of the fields change between stellar arms.

How to observe magnetic fields
1. Zeeman-effect --- difficult
2. Synchrotron radiation

3. Faraday rotationχ = χ 0 + RMλ2

χλ2

Global Magnetic Fields of extra galaxies
M31Gießűbel + (2012)

M51

Fletcher + (2011)

NGC891 Krause (2009)

Total Field : 10 – 100 μG
Ordered Field : a few- 10 μG
Regular Field : a few μG

Ordered field
Regular field
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Characteristic of Global Magnetic field structure

J. L. Han et al 2003(a) Ring-like(b) ASS-like[Axi-Symmetric Spiral](c) BSS-like[Bi-Symmetric Spiral]3 types of the global magnetic fields (Sofue 1987)

ASS (or Ring-like) Fields

Berkhuijsen et al. (2003)

RM distributions indicate
ASS-like field.
From λ2-χ relation:
inner region –ASS-likeouter region –BSS-likecenter- Vertical7

Global magnetic field structure (BSS)
M51 (Fletcher + 2011, Contour: CO line,Helfer + 2003)

Anraku + (2015)

M51 (cf, M83)
Strong density waves⇒ Enhanced ordered fields are formed at the inner edges of the spiral arms
Ordered Fields : 10-15μG,The origin of these magnetic fields are considered by α-Ω dynamo.
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BSS-like Fields without strong density wave
IC342

RM of north spiral arm shows periodicpattern.
⇒ The evidence of the magnetic loop.

Beck (2015) MPIfR Bonn

α-Ωynamo is not dominant mechanism.

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X-shape field (starburst edge-on galaxy)
NGC4631Mora & Krause (2013) MPIfR Bonn
Several edge-on galaxies reveal vertical field components in the halo an X-shapedpattern.
Radio scale height, estimated cosmic-ray lifetimes → outflow speed is 300km/s.
10Model of the Global Field structure

Sofue, Machida, Kudoh (2010) 11

Dynamo theory
V = V + v, B = B + b, X : ensemble average∂B= ∇ × (V × B ) + ∇ × (αB ), αB = v × b∂t∂B t= ∇ × (Vt × B p ) + ∇ × (αB p )V = Vt , B = Bt + B p → ∂t
∂Bp= +∇ × (αBt )∂t

α effect: Coriolis force
Ω effect: differential rotationBt

α effectBpΩ effect
Yokoyama (2008)

ΩSchematic drawing of Coliolis force

Since galactic gaseous disk is the differentially rotating disk,magneto-rotational instability also becomes important. Therefore,
we have to include the gas dynamics and back reaction frommagnetic fields.

Magnetic Flotation and the Parker
instability
if Buoyancy > Magnetic tension
B2(∆ρi − ∆ρ e )g >4πr
→ Magnetic fields rise by buoyancy.
Critical wave length for the instability

λ > λc = 4 2(1 + 1 / β )HParker (1966), Matsumoto + (1988)

The most unstable wavelength is about 10 times of the scaleheight.

Magneto-rotational instability
Magnetic stress is the origin of the viscosity(Velikhov 1959; Chandrasekhar 1960; Bulbus & Hawley 1991)
Magnetic field lines

Magnetic fields are tangled→Magnetic turbulence
Swallow into the center

perturbation

gravity
Centrifugal force
Tension force

Angularmomentum
gravity

Gravity – C.F> Tension

Centrifugal force

Simulation of cosmological magnetic fields
Adiabatic

Full physics

Marinacci et al. (2015)15

Milkey Way – 1, Pakmor (2015)

Cosmological simulation + moving mesh MHD code (AREPO)
z～127 , random seed field strength is about 10-5 μG.
Z～4、 turbulent fields are formed
z～2、 ordered fields are formed. The strength is about 6 μG.
z～0, Emag/Eth ~ 10, Emag/Ekin ~ 0.1
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Gressel + (2013) Hybrid dynamo model

Both α-Ω dynamo term and MRI are considered.
α-Ω dynamo is stronger fields than MRI case.
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Dobbs + (2016)

They considered the appearance of reversals of the fieldwith SPHMHD code.
Reversal points: 1. inner Lindblad resonance2. Corotation of the spiral arm
Only appears when the spiral potential becomes strong.
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MRI –driven model•Initial weak fields are amplified byMRI, and the disk becomes turbulent.
•Averaged plasma β is about 10
inside the disk.
•When the magnetic energy
becomes comparable to the gaspressure, magnetic pressure driven
outflows emerge from the central
region. This outflows creates a largescale poloidal magnetic field in the
inner most region.

Machdia + 2013

Vertical fields in the Galactic
center and toroidal disk fields
are natural structure.

RM and Synchrotron Intensity from N.S.

Initial condition – ring shapeEdge-on: ASS – like distribution
Face-on : ?? -- turbulent fields become
dominant.

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All sky maps of RM21Summary --- global magnetic fields
1. Three-types of global magnetic fields are observed
2. Beautiful spiral arms seem to be related to thespiral density waves.
3. X-shaped feature in edge-on galaxies are coursed
to the powerful winds which is made by SN.

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