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Cosmic web with SKA

SKA conference at Giardini Naxos, June 2014
by

Franco Vazza

on 10 March 2016

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Transcript of Cosmic web with SKA



Filaments of the radio cosmic web:
opportunities and challenges for the SKA
How about CR-protons?
Pfrommer+07,+08; Ensslin+07; Jubelgas+08; Pinzke+10
2-fluid model in SPH (Gadget2 code)
CRp injected at shocks
CRp frozen in gas
pressure feedback
momentum spectrum for each SPH particle
many physical processes (losses; SNe; reacceleration...)

Intermission: radio halos and the gamma-ray test
10% inj.efficiency
efficiency(Mach)
Impact of baryon physics in Pcr/Pgas ratio
Pcr/Pgas<0.1-10%
~
Giovannini+93
COMA
Ackermann+10
Fermi+13
Fermi+13
Secondary models
for COMA
Reacceleration models
for COMA
To produce the full extent of the observed halos in radio
we require too many CR protons -> we overshoot the gamma limits
A model using turbulent reacceleration can do the job with less
CR-protons -> however, less constraints on the seed electrons
Brunetti + 12
Brunetti + 12
In many clusters, the "secondary" origin of radio halos can now be rouled out by FERMI
Fermi+13
(see also Zandanel & Ando 2013)
Faraday Rotation from polarised radio sources in the field of COMA (Bonafede+10; Bonafede,FV+13)
This finding is not trivially recovered by simulations!
Model: amplification of "primordial" weak fields
Model: amplification of fields
injected by AGN
Central fields are OK, predictions close
to virial radius are too low
The presence of
a filament might
explain the excess
Amplification from structure formation

However, MHD simulations predict
straight fields,
while the COMA observations shows
tangled fields
<50kpc scales

Also, turbulence from structure formation would only predict
a few nG
in filaments(Ryu + 08)
LOFAR LBA (60MHz)

giant filaments are detectable if B > 10% Beq (>0.1-0.2 muG)

but...

so far NO claim of detection

-> B < 10% of Beq (< 0.1 muG)
beam 22" , FOV 49 deg^2
12mJy/beam (8.7 muJ/arcsec^2)
What can different SKA instruments detect
(during Phase1) ?
SKA SUR/MID
: nearly
NO chance
of detection due to missing baseline. Only 1 giant filament every SKA-SUR FOV & 1000hr, z>0.4, only brightest parts.
SKA LOW:
significant
CHANCE of detection
for z<0.1-0.2 for efficient dynamo & shock acceleration.
Phase 1:
only brightest parts ar detected.
Phase 2:
a 10% fraction of the volume of filaments can be detected
EnzoMHD on the GPU by Wang & Abel (2010)
we are running a "CHRONOS" project (32 million CPUh) with it
log(T) log(B)
the maximum B-field is
0.001-0.01 muG
(<1 % of equipartition)
Similar setup, but for
a galaxy cluster
the maximum B-field is
0.1 muG (5-10% of equipartition)
Why is the simulated dynamo-amplification so inefficient in filaments?
SCHEKOCHIHIN et al. 2004
MHD turbulence, Pr=1
Mag.energy
resolution
resolution
Studies of the Magnetic field in COMA
very significant excess of B--field along this direction
20Mpc
log(T) log(B)
resolution






(see also Schober+12, Latif+13)

-> we need Reynolds number > 900, i.e. L/dx > 300
-> clusters: achieved for resolution = 10-20kpc
-> filaments: achived for resolution = 1-2 kpc(?)
in addition: dependence on the mode of forcing:
solenoidal is much more efficient than compressive

F. Vazza - Uni. Hamburg
C. Ferrari - Obs. Nice
A. Bonafede - Uni.Hamburg
M.Brüggen - Uni.Hamburg
C.Gheller - CSCS Lugano
P. Wang - Nvidia, Santa Clara
S. Brown - Uni. Iowa
& SKA-LOW "Tiger Team"

SKA Conference
Giardini Naxos, 12.06.2014
X-ray -> dense gas WHIM (>90% of baryons)

















Simulation: ENZO run of (300Mpc)^3 with 2048^ cells - FV, Gheller & Bruggen 2014 MNRAS
3D rendering : SPLOTCH - C. Gheller

what fundamental physics can we learn from the WHIM?
can the SKA detect the cosmic Web?
Synchrotron emission from the diffuse gas in filaments
"primary"
emission from
shock
accelerated electrons (as in relics, e.g. Hoeft & Bruggen07)
"secondary"
emission from hadronic collisions
(e.g. Dolag & Ensslin 00)
Unknowns: 1)CRe acceleration :
0.01%
shock efficiency (? from SNe)
2) magnetic fields:
100-1 nG
(? depends on seeding/dynamo)
ASSUMPTIONS:

-3sigma detection
- 2yr survey
or 1000hr exp.
-baseline filtering
- redshift 0.01<z<0.5
- Bfield: 1-100nG
- x_i=0.0-0.001%
- FG are removed
- no point sourc.
- LOFAR as a prior

-> to more realistic
radio pipeline
(e.g. Ferrari's talk)
The WHIM and fundamental physics:
(1) origin & amplification of cosmic magnetism
MAGNETIC FIELDS IN FILAMENT: UNKNOWN
seed fields?
1e-16-1e-9 G
(e.g. Neronov & Vovk 2011)
compression:
B > 0.001% Beq

small-scale dynamo
B ~ 0.1-10% Beq
(Ryu+08)
Faraday Rotation in COMA SE
B~1muG
(Bonafede+13)
Dolag+08
Small-scale dynamo:
only if
Re>900
(Schechochihin+04,Cho+09)
we need
dx < 1-2kpc

....numerically challenging
Big themes:(1) origin & amplification of cosmic magnetism
NO firm expectation from theory/sims:
B < 0.001 from compression
B 0.1 for fast dynamo
~
Dolag+06
~

compression

small-scale dynamo
probing high Reynolds number
in filaments is challenging
MAGNETIC FIELDS IN A COSMOLOGICAL VOLUME
just compression compression+dynamo













2400^3 ENZO_MHD_GPU@Lugano (FV, Gheller, Wang,Brüggen)
MACH NUMBER of SHOCKS
Sim: ENZO+AMR FV et al.
Rendering by T.Jones
Radio emission from the WHIM can teach us about:
plasma collisional scales in rarefied gas
diffusive shock acceleration in low B-field
CRs at the periphery of structures
seeding & amplification of magnetism
deflection of Ultra High Energy CRs
Radio emission from the ICM is teaching
us about:

magnetic fields
cosmic rays energy budget
diffusive shock acceleration
turbulent acceleration
effective plasma collisionallity


The WHIM and fundamental physics: (2) plasma physics & particle acceleration
B-field decreases
redshift increases
What can different SKA instruments detect? (Phase 1 vs Phase 2)
bright knots
are detected
a big portion of
the filament is detected
detected surface/flux fraction
Phase 1 Phase 2
Conclusions:
- If particle acceleration follows extrapolation from the ICM, the
COSMIC WEB
should shine in
synchrotron
at the ~0.01-0.1muJ/arcsec^2 level
-
SKA-LOW-1
-> "tip of the iceberg"
-
SKA-LOW-2
-> significant fraction
-
SKA-MID/SUR
-> nearly no chance
- Detections (and even non-detections) will open unexplored territory concerning
cosmic magnetism
,
particle acceleration
and
plasma conditions
in the WHIM.
THANKS
observed simulated







(Bonafede+12)
PRIMARY SECONDARY TOTAL
log10[muG]
(Based on SKA1 IMAGING SCIENCE PERFORMANCE by R.Braun)
Full transcript