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Exploring the Diffuse Ionized Gas
in Our Galaxy by means of Long
Wavelength Radio Observation(2016 Edition)
Takuya AKAHORI
Kagoshima University, Japan
SKA-Japan Project A/Prof. | SKA-Japan Vice Chair |SKA Cosmic Magnetism SWG

The 6th DTA Symposium 201616.11.24-25 @ NAOJ, Mitaka

SQUARE KILOMETRE ARRAYContentsv Introductionv Magnetized diffuse ionizedgas (DIG) in the Galaxy

SKA– Thermal electrons– Magnetic fields• Global magnetic fields• Mangetic turbulence– Cosmic-raysv Toward Square Kilometre
Array (SKA)– Specification– Possible application?
• Star-forming region• Jet terminal shock• Dense molecular gas
16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016

1. Introduction

Radio Milky Way

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composit

Total Intensity(Stokes I)
thermal dust

CO (1-0)

Polarized Intensity(|P|=|Q+iU|)

synchrotron

free-freehttps://www.nasa.gov/jpl/planck/pia18913
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The 6th DTA Symposium 2016

1. Introduction

Radio Milky Way

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composit
RM

Total Intensity(Stokes I)thermal dust

CO (1-0)

Polarized Intensity(|P|=|Q+iU|)

synchrotron

free-freehttps://www.nasa.gov/jpl/planck/pia18913
16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016

1. Introduction

SQUARE KILOMETRE ARRAYCapability of cm-m radio:M51
 
mJy/beam

1.5
1.0
0.5

1.5 GHz Stokes I

10-1-2

spec. index of I
200-201.5 GHz Pol.
intrinsic B vector rotation measure
16.11.24-25 @ NAOJ, Mitaka

Kolmogorov(outer: 1 kpc, σ=7 rad/m2)

large-scale halo
magnetic fieldv Long wavelength radio(centimeter – meter) isa very powerful toolfor magnetism.

Mao et al. 2015, ApJ, 800, 92

The 6th DTA Symposium 2016

2.1 Thermal electrons

SQUARE KILOMETRE ARRAYRadio pulse observation  v Pulsars and Fast Radio Bursts (FRBs)– Pulse dispersion à dispersion measure (DM)• pulse delay time Δt– Diffractive () / refractive (à scattering measure (SM)• pulse modulation time Δt

ne: free electron density) scintillations
CN: spectral coefficeint of thedensity power spectrum– <ne> ~ DM/L, or <ne2>~SM/L• L: distance (e.g. by annual parallax)
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The 6th DTA Symposium 2016

2.1 Thermal electrons

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Modified NE2001 model  v NE2001 modelv Scale height– Hn too small in NE2001– Hn = 0.97 kpc à 1.8 kpc– n0 à 0.014 cm-3
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Cordes, Lazio (2002;2003)
DM sin|b| [pc cm-3]
110– Analytical model of thethermal electron density
based on observations– Components: thick disk,thin disk, arm, local,clumps, voids

Gaensler et al. (2008)
10100 1000 10000
height |z| [pc]The 6th DTA Symposium 2016

2.1 Thermal electrons

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Clumpiness and fillingfactor v DM gives the average of thermal electron density– Electron distribution is clumpy– Need the filling factor to reproduce free-free emission

0.1
0.01
0.5
1.0
height |z| [kpc]Berkhuijsen et al. (2006)
16.11.24-25 @ NAOJ, Mitaka

fe=DM2/(EM*L)

1.00obs
fixed
non-fixed

dotted 22.8GHz free-free template data
dashed w/o filling factor, solid: w/ filling factor
Sun et al. (2008)The 6th DTA Symposium 2016

2.2 Magnetic fields

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Faraday rotation measure  v Faraday rotation measure ofbackground sources– Faraday rotationà rotation measure (RM)
• Rotation of polarized angle ΔΦ

B||: line-of-sight componentof magnetic field– Depolarization à dispersion of RM (σRM)• Degree of internal/external Faraday rotation
depolarization DPi/DPe– <B||> ~ RM/DM
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The 6th DTA Symposium 2016

2.2 Global magnetic fields

Disk field

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v Axi-Symmetric Spiral (ASS) vs. Bi-Symmetric Spiral (BSS)

RM [rad m-2]-500 0 500– ASS + reversal, B0~2-6 µG (Sun et al. 2008; Van Eck et al. 2011)

ASS+RING
900-90G. longitude l [deg]16.11.24-25 @ NAOJ, Mitaka

ASS+ARM

BSS

900-90900-90G. longitude l [deg]G. longitude l [deg]RM (CGPS+SGPS, |b|<5°) Lines model (Sun+ 2008)

RM data (NVSS+CGPS, |b|<3°)
Lines model (Van Eck et al. 2011)

The 6th DTA Symposium 2016

2.2 Global magnetic field

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Disk-halo transition   v ASS to BSS transition at mid galactic latitudes?
**M51
ASS in disk
BSS in halo

– ASS-RING in the galactic disk, BSS in halo**– Effect of vertical (X-shape) field?
pitch~0°l=180, outerObs

pitch~10°l=180, local

BSS Model
Van Eck et al. (2011)16.11.24-25 @ NAOJ, Mitaka

Pshirkov et al. (2011)
The 6th DTA Symposium 2016

2.2 Global magnetic field

Halo field

SQUARE KILOMETRE ARRAYv Toroidal field (~2 µG) + vertical field near the Sun– inner: galactic-center symmetry, outer: galactic-axis symmetry(Taylor et al. 2009; Oppermann et al. 2011)– vertical field toward only south galactic pole (Mao et al. 2010)− + − +− − + +RMs of 37543 extragalactic pol.
sources (NVSS) (Taylor et al. 2009)

RM ~ +0.0±0.5 rad/m2
B||~0.00±0.02 µG
Mao et al. (2010)
RM ~ +6.3±0.5 rad/m2
B||~0.31±0.02 µG
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The 6th DTA Symposium 2016

2.2 Magnetic turbulenceSQUARE KILOMETRE ARRAY

Properties of turbulence  v Power spectrum, P~k-β1

P– Kolmogorov (β=5/3)(Armstrong et al. 1995)– Outer scale of > ~1 kpc(β=0.37) (Han et al. 2004)– Amplitude: 3-5 µG at disk

10l [kpc]
0.1 0.01 0.001

Han+ 04– ~1 pc @ spiral-arms– ~100 pc @ inter-arms(Haverkorn et al. 2008)v Remarks– Ordered fields (Jaffe et al.
2010)
16.11.24-25 @ NAOJ, MitakaP

v Driving scaledotted 11/3
dashed 12/3
Armstrong+ 95
30

0.3 3e-3 3e-5 3e-7 3e-9l [pc]The 6th DTA Symposium 2016

2.2 Magnetic turbulence

SQUARE KILOMETRE ARRAYHigh galactic latitudes  v Mach number M, plasma β, and driving scale– EM + MHD à M~1.4-2.4, ldrive~ 500 pc (Hill et al. 2008)– DM + EM + <RM> + MHD à β~0.1-1, σRM ~ few rad/m2

thin: observation, thick: MHD model
Hill et al. (2008)
16.11.24-25 @ NAOJ, Mitaka

Akahori et al. (2013)
The 6th DTA Symposium 2016

2.2 Magnetic turbulence

SQUARE KILOMETRE ARRAYPlasma diagnostics 1  
v Polarization
gradient Punveils theMach number– Gaensler et al.
(2011)– prefer M<2

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The 6th DTA Symposium 2016

2.2 Magnetic turbulence

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Plasma diagnostics 2  v Alfven velocity (in SI):v Rotation measure:v Density-weighted Alfven velocity:v Dimensionless factor R–

R=1 uniform density, R=1.0755 gaussian

Hα at VLSR=-45 km/s,Extragalactic |RM| max 70 rad/m2
v Sound speed & plasma β:v β<~1 in the filaments– β<~0.1 at b~50°

16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016

2.3 Cosmic-ray electrons

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Energy distribution function  v Cosmic-ray electrons (CRe):– p~2.5-3 @ ~3-30 GeV CRe
AMS01, CAPRICE, HEAT, Fermi-LAT,PAMELA, ATIC-1-2, HESS

16.11.24-25 @ NAOJ, Mitaka

N0: CRe density normalization (cm-3)
γ: Lorentz factor
p: energy spectral index

Strong+11, A&A, 534, 54

The 6th DTA Symposium 2016

2.3 Cosmic-ray electrons

SQUARE KILOMETRE ARRAYSynchrotron radiation  v CRe + B = Synchrotron– Specific stokes I, Q, U

dashed I, P data (|b|<5°)
solid model (Sun et al. 2008)
I(408 MHz)

B||: LoS component of magnetic field [µG]ω: 2πν, χ: intrinsic polarization angle ( B )– Spectral index I να
• α=(1-p)/2P(22.8 GHz)v Spatial distribution of CRe– nc0 = 5x10-5 cm-3, Hc = 0.8 kpc
900-90galactic longitude l [deg]16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016

2.3 Cosmic-ray electrons

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Free-free absorption  

p 2.3 2.4 2.5 2.6Spectral index map
at 22 MHz
(Roger et al. 1999)v Shallower slope (α~-0.5) of Stokes I at 22-408 MHz– Free-free absorption is significant ( >1); absorption ofsynchrotron (I, Q, U)e(e.g. Waelkens et al. 2008)– Temperature: Hα observation[K] (Paladini+ 04)
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The 6th DTA Symposium 2016

3. Toward SKASpecification

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v Sensitivity– 9000PSR+1400MSP (SKA1)– 30000PSR+3000MSP (SKA2)– QSOs ~ 200,000,000 (SKA2)– dense RM grids / rare objects

v Angular resolution– ~1″ (baseline/60km)-1(frequency/1GHz)-1v Bandwidth– 0.1 - 25 GHz, 64k ch./band– spectral index / depolarization/
Faraday tomography

v FoV and Image quality– x 100 survey speed of JVLA(SKA1)– x 104-6 of JVLA (SKA1)– A+B+C+D+E+A+
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The 6th DTA Symposium 2016

3. Toward SKA

QUARE KILOMETRE ARRAYSCORPIO (ATCA 1.4 SGHz) 4 deg2, (344.25,0.66), 30 µJy, 14″ x 6.5″

IRAS16566-4204

IRAS16573-4214
16.11.24-25 @ NAOJ, Mitaka

Umana et al. 2015, MNRAS, 454, 902

The 6th DTA Symposium 2016

3. Toward SKA

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Synchrotron from clouds  v Dense molecular clouds are bright at centimeter– Gives field strength & orientation
primary nCResynchrotron I & P
S [Jy/bm]nCRe Strong+11, γ~3, α~-1Bcloud Crutcher+10, Bz nHκ, κ~0.6

O(0.1-1) mJy @ 1 GHz (5″~0.25pc @ 1kpc)16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016

3. Toward SKA

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Terminal shock and Jet  v “Manatee” Nebula W50
Stokes I, 2.1 GHz

Stokes I
Stokes QU
RM (Φ)
Hα EM
pulsar DM

X-ray + mangetic field vector
Sakemi et al. (in preparation)

Farnes, TA+ (2016) (1604.06552)
16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016

Summary

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v Diffuse Ionized Gas (DIG) in the Galaxy– Sophisticated model of thermal electron density (DM, EM, SM)– ASS/BSS disk + halo toroidal + vertical field (DM, EM, RM)– Magnetic turbulence (EM, RM, I, P)– Cosmic-ray electrons (I, P)

v Toward Square Kilometer Array (SKA)– High sensitivity, high resolution, wide band, wide field– Continuum mapping of star-forming regions & clouds– High resolution imaging of Jet terminal region (shocks, lobes)

16.11.24-25 @ NAOJ, Mitaka

The 6th DTA Symposium 2016