Texas in Florence

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The Origin and Evolution of Cosmic
Magnetism Perspective from SKA
Luigina Feretti IRA - Bologna
MCCT-SKADS School, Medicina, 259-07
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This topic is one of the 5 Key Science Projects
of SKA, selected by the Science Working Group
Motivations

• 1. Can address unanswered questions in
  fundamental (astro)physics
• 2. Is science which is unique to the radio band
  and to the SKA
• 3. Excites the broader community, is of
  interest to funding agencies
• and from a phase-space perspective, will almost
  certainly yield new and unanticipated results!

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• Outline
• Importance of the study of cosmic magnetism
• Observation of large-scale magnetic fields
• Current ideas on the origin of cosmic magnetic
  fields
• Studies with SKA and SKA pathfinders

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Cosmic Magnetism
Magnetism is one of the Fundamental forces in
nature. It is crucial in

• cloud collapse / star formation
• stellar activity / stellar outflows
• ISM turbulence / gas motions
• supernova remnants
• stability of galactic disks
• acceleration / propagation /
• confinement of cosmic rays
• heating in galaxy clusters
• AGNs / Jets

MHD turbulence
Proplyd in Orion
SN 1006
Merger in gal. cluster
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Most bodies in the Universe are magnetized on all
scales Earth ? 0.5 G Interplanetary Space ?
50 ?G Sun ? 10 G (poles) ? 1000 G
(sunspots) Protostars ? 1 mG White dwarfs ?
106 G Neutron stars ? 1012 G Milky Way ? 5
?G (widespread) ? 1 mG (nucleus) Spiral
galaxies ? 10 ?G (average) ? 30 ?G (massive
arms) Starburst galaxies ? 50 ?G Radio
galaxies ? ?G Clusters of galaxies ? 0.1-1
?G Intergalactic space lt 10-2 10-3 ?G

• Large-scale fields
• Challenge to models

?
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Magnetism and Radio Astronomy

• Most of what we know about cosmic magnetism
  derives from radio observations
• 1 - Synchrotron emission
• total intensity ? field
  strength
• polarization ?
  orientation/degree of ordering
• 2 - Faraday rotation

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1 - Synchrotron emission Total intensity
measures the total field
strength Polarization gives
the orientation and the degree
of ordering of field
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Equipartition magnetic field
By writing the synchrotron luminosity as the
observed source brightness I0 at the frequency
?0, and the source depth d (to be inferred),
applying the K-correction, assuming ? 1 (same
volume in particles and magnetic field), and
expressing the parameters in commonly used units

umin in erg/cm3
?0 in MHz

I0 in mJy/arcsec2

d in kpc Constant
computed for ? 0.7, ?1 10 MHz, ?2 100 GHz
Usually k 0 or k 1 assumed for clusters
BUT see Brunetti et al 1997, Beck and Krause 2005
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Polarization
The synchrotron radiation from a population of
relativistic electrons in a uniform magnetic
field is linearly polarized, with the electric
vector perpendicular to the magnetic field
which has generated the synchrotron emission. In
the optically thin case, for isotropic electron
distribution, and electron power-law energy
spectrum the degree of intrinsic linear
polarization is
N(E)dE N0E-? dE
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The above value is reduced in the more realistic
cases where - the magnetic field is not
uniform, since regions where the magnetic field
has different orientations give radiation with
different polarization angle orientations, which
tend to average (or cancel) each other. - there
is Faraday rotation effect arising both from
instrumental limitations (beamwidth bandwidth)
or within the source itself
(Sokoloff et al. 1998, 1999
how
fractional pol. is affected by
magnetic field
configurations)
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Synchrotron Emission from the Milky Way (Perseus
- Auriga)
b4
b-4
l166
l150
Polarized emission
Effelsberg 21cm (Reich et al 2003)
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M51 VLA Effelsberg (Fletcher Beck 2004)
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Clusters of galaxies being the largest
systems in the Universe, they represent an
ideal laboratory to test theories for the
origin of extragalactic magnetic
fields Reviews by
Carilli Taylor 2002,Govoni Feretti 2004
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COMA Cluster Beq ? 0.4 ?G
500 kpc

Center
RADIO WSRT, 90 cm (Feretti et al.1998)
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Cluster radio halos
A665
Coma
A2163
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Cluster radio relics
091775
A548b
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Abell 2256 I1.4 B0
Projected magnetic field direction Polarization
degree large scale order and generally follow
the bright filaments large regions (500 kpc) of
fairly uniform magnetic field direction
Clarke et al. (2004)
Results
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Intergalactic Fields
Filament of galaxies ZwCl 2341.10000
Size ? 4 Mpc
z ? 0.3
(Bagchi et al. 2002)
320 MHz VLA
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Intergalactic Fields (cont.)
Upper limits of intergalactic fields from
existing studies BIGM lt 10-9-8 G (model
dependent)
GRB 000131 at z 4.5 (Bloom et al 2001)
Radio galaxy at z 5.2 (van Breugel et al 1999)
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2 - Rotation measure gives an
indirect measurement of the strength
and structure of the field along the
line of sight
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Faraday Rotation
rotation of the plane of polarization of linearly
polarized emission as it passes through a
magneto-ionic plasma -- due to the different
phase velocities of the orthogonal circular modes
? ?2
?0
?
Kronberg 2002
? Rotation Measure
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Sources seen through a magnetized screen
ne is the electron density in cm-3 L is the path
length in kpc B is the line of sight component
of the field in ?G
Infer B along the line of sight in the crossed
medium by combining with info about ne from
X-rays
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Values derived for B are model dependent -
analytical solution only for simplest models
of the Faraday screen Otherwise -
numerical techniques (Murgia, Govoni, 2004 -
2005) - semianalytical approach (Ensslin,
Vogt 2004-2005)
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Numerical Simulations
Power spectrum analysis (Ensslin and Vogt
2003 Murgia et al. 2004) simulate a box with 3D
multi-scale fields which have a radial decrease
in field strength resolution 3 kpc, magnetic
structures from 6 to 770 kpc find n 1 2
provide the best fit to the data most of the
magnetic field energy resides in the small
scales field strength using this approach are a
factor 2 lower than the analytical approach
assuming smallest RM scale for coherence length
Murgia et al. (2004)
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Milky Way
Pulsar RMs spiral arm field (Han et al 2002)
All-sky RM map (Johnston-Hollitt et al 2002 RED
POSITIVE RM, BLU NEGATIVE RM RM approximate
range -300, 300
M 31
RMs of 21 polarized sources (Han et al 1998)
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Faraday mapping

• extended, polarized radio sources can be mapped
  at several frequencies to produce RM maps

cD in a poor cooling-core cluster
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A2255
Govoni et al. 2006
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Magnetic fields at the ?G level are
ubiquitous in clusters - coherence
scales of 10-100 kpc - large degree of
ordering - structure
? ORIGIN ?
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When and how were the first magnetic fields
generated ?
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MAGNETIC FIELD Primordial
Early stars Protogalaxies Galaxies AGN
RECOMBINATION
z ? 10 z ? 5 z ? 0.5 z ? 0.1
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Primordial Fields (Olinto 1998, Grasso
Rubinstein 2001) Created in the exotic
ultra-dense stages of the Big Bang ?
physics poorly known, cannot exclude the
creation of a magnetic field of the order
10-30 10-25 G
Remember present large scale fields ?10-6 G
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Primordial fields would affect the cosmogonic
process ? anisotropic expansion
? effects on nucleosynthesis
(larger He abundance) ? regulate
structure formation
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Post-recombination Fields 1
Early Stars (z ? 20) 2
First AGN (z ? 5 ?) 3
Protogalaxies and structure
formation (z ? 5)
(Kulsrud et al 1997, Kang et al. 1997)
? Seed fields
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Seed Fields
(Rees 2004)
Injection by galactic winds or active galaxies
Kronberg et al.1999,
Völk
Atoyan 1999
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Present-day fields of B 1 µG could have evolved
from B 10-910-10 G seed fields at z gt 5
Large-scale fields represent a problem
because the dynamo amplification time can be
large so not many e-foldings at the present
epoch Amplification ?
dynamo action ?
compression ? cluster
mergers
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Square Kilometer Array

• Very powerful in the detection of total intensity
  and
• polarized emission and in RM
  measurements
• SKA instant RMs and position angles
• ? 1.4 GHz, ?? 400 MHz
• - for ?t 1 hour, 1 ?
  0.1 µJy
• - for P 1 µJy RM ? 5
  rad/m-2, ? ? 10o !

?
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SKA Faraday Rotation Survey

• Five min observation
• with SKA at 1.4 GHz
• RMs down to
• P 3 ?Jy
• (Stot 0.1 mJy)
• Approx 500 RMs
• per deg2 (average
• separation 2-3)
• ? 107 sources over
• the entire sky,
• spaced by ?90
• (? 20000 pulsars)

Adapted from Gaensler et al. (2001) Hopkins et
al. (2003)
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Scientific breakthrough - magnetic field of
the Galaxy - magnetic field in nearby galaxies
and clusters - extended sources
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Polarization Silhouettes

• Distant galaxies are too small to be probed by RM
  grid
• but can be probed by Faraday rotation and
  depolarization of extended background sources
• e.g. NGC 1310 against
• Fornax A (Fomalont et al 1989)
• Larger distances
• e.g. PKS 1229021 absorber at z 0.395
  with B 1 4 µG (Kronberg et al 1992)
• ? powerful probe of
• evolution of galactic
• magnetism as function
• of redshift

NGC 1310
Polarization from Fornax A (Fomalont et al 1989)
Kronberg et al (1992)
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Ly-a Absorbers at z 1 3

• Large statistical samples can come from RMs and
  redshifts of quasars
• (e.g. Welter et al 1984 Oren Wolfe 1995)
• - trend of RM vs z probes evolution
• of B in Ly-a clouds
• but Galactic contamination,
• limited statistics
• Quasar RMs with SKA
• - 106 measurements
• - identification redshifts from
• SDSS successors
• - accurate foreground removal
• using RM grid

RRM ? (1z)b-2
Residual RMs (Galaxy corrected) vs z of QSOs
embedded in intervening clouds (Welter et al
1984) marginal evidence of evolution !
? magnetic field evolution in galaxies
over cosmic time-scales
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Magnetic Fields in Protogalaxies

• thousands of normal spiral galaxies at z 3
  detectable with the SKA
• (1.4 GHz size 1 - 3 , flux 0.2 µJy )
• their radio flux strongly depends on field
  strength and on star formation rate (and may be
  polarized)

HDF galaxies with z gt 4 (Driver et al 1998)
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The Magnetized IGM Cosmic Web

• Existing limits (scale and model dependent)
• BIGM lt 10-8-10-9 G
  (e.g..Blasi et al 1999 Jedamzik et al 2000)

• - Detection and polarimetry of very low
• Level synchrotron emission
• RM measurements of extragalactic sources are
  related to the amplitude and shape of the
  magnetic field power spectrum P(k) where k is the
  wave number of the coherence scale
• ? SKA z surveys can provide magnetic
  power spectrum of the Universe

z 0.5
z 1
z 2
RM pairs at separation ? needed to detect B 1
nG at scale of 50 Mpc (Kolatt 1998)
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SKA Specifications for Polarimetry

• Frequency at least 110 GHz, 0.320 GHz ideal
• Large field of view gt1 deg2 at a resolution of
  lt1"
• High sensitivity lt0.1 mJy, confusion limited
• Large bandwidth gt400 x 1 MHz at 1.4 GHz
• Significant concentration ( gt 50 ) of antennae
  in
  central core ( 5 km)
• High polarization purity ( 40 dB at field
  center,
• 30 dB
  at field edges)

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SKA pathfinders ATA (US) LOFAR (The
Netherlands Europe) LWA (US)
KAT/MeerKAT (South Africa) MWA (Australia)
MIRANDA (Australia Canada) SKADS (Europe)
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Low frequency ? - Diffuse
synchrotron emission of steep
spectrum - Polarized emission
sources of low RM
? weak magnetic fields
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?? 10o ? 240 MHz, ?? 32 MHz RM 0.4
rad/m2
?2
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Conclusions

• Early primordial fields could have been generated
  by battery effects, during inflation or phase
  transitions
• A primordial intergalactic (IGM) field may have
  regulated structure formation in the early
  Universe
• Seed fields at z gt 5 may originate from
  primordial fields or from post-recombination
  fields
• Present-day large-scale fields of B 1 µG could
  have evolved from B0 10-910-10 G seed fields
  at z gt 5
• Evolution from seed fields includes dynamo,
  compression, merger interaction

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THANK YOU
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Biermann Battery effect
Electrostatic equilibrium When gradients of
electron thermodynamic quantities (e.g. density
and temperature) are not parallel to the pressure
gradient, the electrostatic equilibrium is no
longer possible. This leads to a current which
generates A magnetic field restoring the force
balance.
Widrow 2002
First observed in the lab in 1975 (Stamper
Ripin)
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Zeeman effect
In a vacuum, the electronic energy levels of an
atom are independent of the direction of its
angular momentum. In the presence of magnetic
fields, the atomic energy levels are split into
a larger number of levels and the spectral lines
are also split.
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Hydrogen
Bohr magneton
The Zeeman effect can be interpreted as due to
the precession of the orbital angular momentum
vector in the magnetic field. The energy shift is
proportional to the strength of the
magnetic field. Zeeman splitting in Hydrogen
(1.4 GHz) 2.8 Hz ?G-1 Zeeman splitting in the
H2O molecule (22 GHz) ? 10-3 Hz ?G-1 Lines are
polarized, favouring their detection ? present
detection only for strong magnetic fields (gt mG)
(sunspots galactic objects)
?