Magnetic Fields in Clusters of Galaxiessirius.unist.ac.kr/SRC-CHEA/presentations/presentation... · 2017. 11. 12. · Clusters of galaxies aggregates of galaxies, which are the largest

Magnetic Fields in Clusters of Galaxies

Dongsu Ryu (UNIST, Korea)

November 2, 2017 CHEA Collaboration Meeting UNIST, Korea

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Faraday rotation measure

Hydra North

(Vogt & Ensslin 2005)

(Ryu et al 2008)

The large-scale structure of the universe

observed and simulatedgalaxy distributions

simulated matter distributionclustersfilaments void regions

the cosmic web

where most matters resideNovember 2, 2017 CHEA Collaboration Meeting UNIST, Korea

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Clusters of galaxies

aggregates of galaxies, which are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation

Hubble space telescope image mostly star light

the intraclustermedium (ICM)

the superheated plasma with T ~ a few to several keV, presented in clusters of galaxies

optical (Hubble, white)

X-ray (Chandra, blue) hot gas

radio (VLA, red) cosmic rays

MACSJ0717MACSJ0717


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Perseus cluster: X-ray

Coma cluster: X-rayICMs are dynamical:- turbulent flow motions

- large-scale flow motions

- magnetic fields

- cosmic-rays

- shock waves


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Faraday rotation measure

Hydra North

(Vogt & Ensslin 2005)

CIZA J2242.8+5301 Sausage relic

(van Weeren et al 2010)

Coma clusterRelic halo

(Brown & Rudnick 2011)

Magnetic fields in galaxy clusters appears in observations

(Clarke et al 2004)


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(Govoni & Feretti 2004)

modeling of synchrotron of the Sausage relic B1 ~ 2 – 2.5 G modeling of synchrotron of the Toothbrush relic B1 ~ 1 – 1.5 G

(Kang & Ryu 2015)

(Kang, Ryu, & Jones 2017)


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Fluid quantities in ICM outskirts

baryon number density

gas temperature

magnetic fields

flow velocity

gas kinetic energy

gas thermal energy

cosmic-ray energy

magnetic energy

size of clusters

plasma beta is high with β ~ 50 - 100

magnetic

thermal

magnetic

thermal

3E

2E

P

Pβ

flows are subsonic (Ms ~ 0.5) but super-Alfvenic (MA > 1)

Lcluster ~ a few Mpc ~ 1025 cm

n ~ 10-3 cm

v ~ severalⅹ100 km/s

T ~ 108 K (8.6 keV) cs ~ 1,500 km/s

B ~ a fewⅹμG cA ~ 100 km/s

Ethermal ~ a fewⅹ10-11 erg/cm3

Ekinetic ~ a fewⅹ10-12 erg/cm3

Emagnetic ~ a fewⅹ10-13 erg/cm3

ECR <~ a fewⅹ10-13 erg/cm3 (Ethermal / 100)


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large-scale structure formation

shocks in the LSS of the Univ

gravitational collapse, mergers, & flow motions

stretching and compression

development into turbulence

amplification of magnetic fields

Turbulence dynamo model for magnetic fields in clusters of galaxies

generation of vorticity

small-scale or turbulent dynamo

(e.g., Ryu et al 2008)

shock dissipation

seed fields


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Observation of shocks in clusters: X-ray

The Bullet Cluster

Cluster A665

shock

cold front

6.03XM

(Dasadia et al. 2016)

(no associated radio relic)

5.2XM(Shimwell et al.

2015)

0.3XM

(Markevitch 2006)

(no associated radio relic)

cold front


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Observation of shocks in clusters: radio relics

PLCKG287.0+32.9

Sausage relic

‘Giant Gischt Relic’

Abell3411-3412

(van Weeren et al. + Ryu 2017)


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weak inreacluster shocks with M<4 (orange)

Shock waves in a simulated clusters of

galaxies

strong accretion shocks with M>10 (green)

Shock waves induced during the formation of the large-scale structure of the universe

(Vazza et al + Ryu)


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preshock density

postshock density

preshock flow speed

unit normal to shock surf.

curvature radius of surf.

Vorticity generated at cosmological shocks

R

nU

12

2

12cs

)(~

1

2

U

R

n

at shocks

- directly at interacting and curved shocks

- by the baroclinic term

p

2bc

1

constant ρ

constant p

baroclinity

due to entropy variation induced at shocks

Magnitude of vorticity or enstropy (e) enhanced by stretching and compression increases during the development of turbulence

div(v) ω

interacting shocks

(Porter, Jones, Ryu 2015)


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the spectrum of Kolmogorov turbulence

a schematic representation of the turbulent energy cascade from large to small scales

Kolmogorov's theory for incompressible hydrodynamic turbulence: it is based on the notion that that large eddies can feed energy to the smaller eddies and these in turn feed still smaller eddies, resulting in a cascade of energy from the largest eddies to the smallest ones.

on dimensional grounds, the only way of writing ε (energy transfer rate) in terms of V (velocity) and l(scale) is power spectrum of velocity!

Theory of turbulence

3/5

3/1

3

~

~

~

kP

lV

l

V

k

e ~ constant2

t

V~


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Turbulence in high resolution simulations of galaxy clusters

curl(v) in the plane through the cluster center(Miniati 2013)

in simulations, turbulence exists in all over the clusters!


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Turbulence energy of in the ICM

assuming that turbulence is contained in vorticalmotions

Mturb ~ 1(transonic turbulence)

in filaments

Mturb < ~1(subsonic turbulence)Eturb/Etherm ~ 0.1 – 0.3

inside and outskirtsof clusters

clusters

(Ryu et al 2008)November 2, 2017 CHEA Collaboration Meeting UNIST, Korea

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at the core of the Perseus cluster, the estimated

turbulence velocity ~ 165 km/s, (while the expected

value ~ 500 km/s)

(Hitomi Collaboration 2016)

X-ray spectrum of the core of the Perseus cluster

Evidences for turbulence in clusters

observations of turbulence in clusters are mostly around core regions!


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data n = 2 n = 11/3

- RM in Abell 119 for comparison with simulation

- turbulent cluster magnetic fields with power-low slope n=2 give a good fit (Kolmogorov n=11/3)

Murgia et al. (2004)

patchy Faraday rotation measure (RM) in clusters observed with radio galaxies

an evidence for turbulence in cluster outskirts?November 2, 2017 CHEA Collaboration Meeting UNIST, Korea

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Turbulence in astrophysical environments with magnetic fields

oB

BBpt

4

11

Fluid and magnetic fields interact with each otherfluid drags magnetic fieldmagnetic field exerts tension and pressure fluid and magnetic field moves together (“frozen”)

with large magnetic Reynolds number

B

t

B

Turbulence + magnetic field

Magnetohydrodynamic turbulence


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Origin of magnetic fields in clusters

- generation of seed fields

- amplification of seed fields by turbulent flow motions


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Suggestions include:

- generation in the early universe (e.g., see Widrow, Ryu et al 2012 for review)

e.g.) during the electroweak phase transition (t~10-12sec)

during the quark-hadron transition (t~10-5sec)

uncertain but maybe challenging (?)

- generation before the formation of the LSS of the universe

through plasma physical processes (e.g., see Ryu et al 2012 for review)

e.g.) Biermann battery or Weibel Instability at shocks(Kulsrud, Cen, Ostriker, & Ryu 1997) (Medvedev & Loeb 1999)

instabilities, thermal fluctuations, photo-ionization and etc …

weak (~ 10-20 G) and some at small scales, yet most promising(?)

- astrophysical processes

e.g.) magnetic fields from the first stars

maybe not the first magnetic field

Origin of seeds for comic magnetic fields is uncertain!

Seed magnetic fields in the large-scale structure (LSS) of the universe


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Small-scale dynamo of MHD turbulence with weak initial field

at saturationEmag/Ekin ~ ½ - 2/3

Ekin

Emaglinear

growth

saturation

growth of magnetic energy

exponentialgrowth

(Jo & Ryu, unpublished)


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exponential growth

linear growthinverse cascade

1

Evolution of magnetic field power spectrum at early stages(From J. Cho)


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Power spectrum at saturation

Ekin

Emag

(Jo & Ryu, unpublished)


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Bω

A model for magnetic fields in galaxy clusters

- vorticity generated at shocks and also due to baroclinity

- further enhanced by stretching and compression

- developed into turbulence- magnetic field produced by

turbulence dynamo

(Ryu et al 2008)

The model may reproduce the observed strength of magnetic field!


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CIZA J2242.8+5301 (sausage radio relic)

(van Weeren et al 2010)

Can Mpc-scale magnetic fields in outskirts , “if real”, be explained by turbulence dynamo?

Large-scale magnetic fields in cluster outskirts ?


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The growth time of magnetic fields in turbulence dynamo

Emag

Ekin

time evolution of energies(Porter, Jones, & Ryu, 2015)

10 t / teddy

teddy ≡ Linjection / vturb

In clusters, if turbulence is driven mainly by major mergers,

Linjection ~ 500 kpc (clump size)

vturb ~ 500 km/s

teddy ~ 1 Gyrs ~ 1/10 tage

or tage ~ 1/10 teddy

(tage the age of the universe)

There seems to be enough time for the growth of B!

It takes t ~ severalⅹteddy to reach

Emag ~ 1/10 Ekin


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Pmag(k)

Pkin(k)

power spectrum at saturation(Porter, Jones, & Ryu, 2015)

(Cho & Ryu 2009)

The scale of magnetic fields in turbulence dynamo

~3 t / teddy

At t ~ 10 teddy,the peak of Pmag(k) is ~ 1/3 Linjection

the peak of kPmag(k) is ~ 1/10 Linjection

If Linjection ~ 500 kpc,

the peak of “kPmag(k)” (the peak scale of magnetic energy) ~ 50 kpc, too small?


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in clusters- turbulence is driven “sporadically”- the medium is “stratified”

(Vazza et al + Ryu, 2016)

In clusters of galaxies, the turbulence is driven “sporadically”, mostly by “mergers”.

(Coma cluster)

Intracluster media are highly “stratified”, i.e., high density at cores and low density at outskirts.

turbulence energy dissipation rate in a cluster formed in a simulation for the formation of the large-scale structure of the universe

Turbulence dynamo in clusters


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Simulations of turbulence “sporadically” driven in “staratified” clusters

clusters of T = 108 K (cs = 1,500 km/s) described the beta model with β = 1 and rc = 300 kpc in a box of Lsize = 4 Mpc

4 Mpc

~tageturbulence of Mturb ~ ½ , driven for 4 times (that is, forcing on for Δt = 0.1 tage and off for Δt = 0.15 tage) during the age of the universe, tage, with the peak of forcing at Linjection = 500 kpc

(Roh, Ha & Ryu, preliminary)


Amplification of B up to Emag ~ Ekin /10 would be OK.

How about the scale of B?

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The power spectra of velocity and magnetic field at t = 5 (~ tage)

stratified medium, constant forcing

uniform medium, constant forcing uniform medium, sporadic forcing

stratified medium, sporadic forcing


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Power spectra of turbulence in stratified medium with sporadic forcing

- Pkin(k) shows large-scale (scales up to the box size) powers- Pmag(k) has a broad peak over ½ kinjection – 2 kinjection

- kPmag(k) has a peak at ~ 2kinjection

or if Linjection = 500 kpc, the peak scale of magnetic energy ~200 - 300 kpc

Pmag(k)

Pkin(k)

kinjection


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log(Emag)

|curl(v)| div(v)

- less concentration of Ekin and Emag than ρ

- higher flow activities at outskirts


can this reproduce the observed polarization properties of radio relics?32/36

log(Ekin)log(ρ)

Slice images through the cluster center at t = 4.54 Mpc


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Shocks and magnetic fields

B on the top of div(v)

(|B| on the log scale)

zoomed regions showing perpendicular shocks (B and shock normal are perpendicular)


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Summary

- Magnetic field is ubiquitous in the large-scale structure of the universe including in clusters of galaxies.

- Magnetic fields of ~ a few μG in cluster outskirts could be explained by turbulence dynamo.

- The existence Mpc-scale magnetic fields in cluster outskirts need to be established in observation.

- Explaining large-scale magnetic fields of ~ Mpc scale, if they exist, would remain challenging in the picture of turbulence dynamo.


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Thank you !


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Magnetic Fields in Clusters of Galaxiessirius.unist.ac.kr/SRC-CHEA/presentations/presentation... · 2017. 11. 12. · Clusters of galaxies aggregates of galaxies, which are the largest