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The Origin and Evolution of Cosmic Magnetism:Perspective from SKA
Luigina Feretti – IRA - Bologna MCCT-SKADS School, Medicina, 25–9-07
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!
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
– 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
Cosmic Magnetism
Proplyd in Orion MHD turbulence
SN 1006 Merger in gal. cluster
Magnetism is one of theMagnetism is one of theFundamental forces in Fundamental forces in nature. It is crucial in :nature. It is crucial in :
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: < 10-2 – 10-3 G
Large-scale fieldsChallenge to models
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
1 - Synchrotron emission
Total intensity : measures the total field strength
Polarization: gives the orientation and the degree of ordering of field
By writing the synchrotron luminosity as the observed source brightness I0 at the frequency 0, and thesource 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:
7/47/40
7/)412(7/40
7/412min )1()1(1023.1 dIzkxu
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
21
eq u7
24H
/
min
Equipartition magnetic field
BUT see Brunetti et al 1997, Beck and Krause 2005
The synchrotron radiation from a population of relativistic electrons in a uniform magnetic field is linearly polarized, withthe electric vector perpendicular to the magnetic field whichhas 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
PolarizationPolarization
8075073
33PInt ..
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) orwithin the source itself
(Sokoloff et al. 1998, 1999 : how fractional pol. is affected by magnetic field configurations)
Effelsberg 21cm (Reich et al 2003)
Synchrotron Emission from the Milky Way (Perseus - Auriga)
Polarized emissionl=166° l=150°
b=-4°
b=+4°
M51VLA +Effelsberg(Fletcher & Beck 2004)
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
COMA Cluster Beq 0.4 G
500 kpc
RA
DIO
: W
SR
T,
90 c
m (
Fere
tti et
al.
1998)
+Center
Cluster radio halos
Coma
A665
A2163
Cluster radio relics
A548b
0917+75
Abell 2256 I1.4 & B0
Clarke et al. (2004)
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
Results
Filament of galaxies ZwCl 2341.1+0000
(Bagchi et al. 2002)
z 0.3 Size 4 Mpc
320 MHz VLA
Intergalactic Fields:
GRB 000131 at z = 4.5(Bloom et al 2001)
Radio galaxy at z = 5.2 (van Breugel et al 1999)
Upper limits of intergalactic fields from existing studies: BIGM < 10-9…-8 G (model dependent)
Intergalactic Fields (cont.)
2 - Rotation measure
gives an indirect measurement of the strength and structure of the field along the line of sight
Faraday Rotationrotation 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
Kronberg 2002
0
Rotation Measure
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
Sources seen through a magnetized screen:
Infer B along the line of sight in the crossed medium by combining with info about ne from X-rays
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)
Numerical SimulationsPower 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)
Milky Way
Pulsar RMs + spiral arm field (Han et al 2002)
RMs of 21 polarized sources (Han et al 1998)
M 31
All-sky RM map (Johnston-Hollitt et al 2002RED = POSITIVE RM, BLU = NEGATIVE RM
RM approximate range: -300, +300
Faraday mapping• extended, polarized radio sources can be mapped at several frequencies to produce RM maps
Image courtesy of NRAO/AUI
Cygnus A
cD in a poor cooling-core cluster
A2255 Govoni et al. 2006
Magnetic fields at the G level are ubiquitousin clusters : - coherence scales of 10-100 kpc - large degree of ordering - structure
ORIGIN ?
When and how were the first
magnetic fields generated ?
z 10
z 5
z 0.5
z 0.1
MAGNETIC FIELD
Primordial
Early stars
Protogalaxies
GalaxiesAGN
RECOMBINATION
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
Primordial fields would affect the cosmogonic process
anisotropic expansion
effects on nucleosynthesis (larger He abundance)
regulate structure formation
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
Seed Fields
(Rees
20
04
)
Injection by galactic winds or active galaxies : Kronberg et al.1999, Völk & Atoyan 1999
Present-day fields of B ≥ 1 Present-day fields of B ≥ 1 μμG could have evolved G could have evolved from from B ~ 10B ~ 10-9-9–10–10-10-10 G G seed fields at z > 5 seed fields at z > 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 actiondynamo action compressioncompression cluster mergerscluster mergers
Square Kilometer ArraySquare 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 !
Adapted from
Gaensler et al. (2001) &
Hopkins et al. (2003)
• 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)
SKA Faraday Rotation SurveySKA Faraday Rotation Survey
Scientific breakthrough:
- magnetic field of the Galaxy
- magnetic field in nearby galaxies and clusters
- extended sources
Polarization from Fornax A (Fomalont et al 1989)
• Distant galaxies are too small Distant galaxies are too small to be probed by RM grid to be probed by RM grid
… … but can be probed by but can be probed by Faraday rotation and Faraday rotation and depolarization of depolarization of extendedextended background sourcesbackground sources
e.g. NGC 1310 against e.g. NGC 1310 against
Fornax A (Fomalont et al 1989)Fornax A (Fomalont et al 1989)
• Larger distances:Larger distances:
e.g. PKS 1229e.g. PKS 1229––021: absorber 021: absorber at at zz = 0.395 with B ~ 1= 0.395 with B ~ 1– – 4 4 μμGG (Kronberg et al 1992)(Kronberg et al 1992)
→ → powerful probe of powerful probe of evolution of galactic evolution of galactic magnetism as function magnetism as function of redshiftof redshift
Polarization SilhouettesPolarization Silhouettes
NGC 1310
Kronberg et al (1992)
• Large statistical samples can come from Large statistical samples can come from RMs and redshifts of quasarsRMs and redshifts of quasars
(e.g. Welter et al 1984; Oren & Wolfe (e.g. Welter et al 1984; Oren & Wolfe 1995)1995)
- trend of RM vs - trend of RM vs zz probes evolution probes evolution of of BB in Ly- in Ly-αα clouds clouds … … but Galactic contamination, but Galactic contamination, limited statisticslimited statistics
• Quasar RMs with SKA:Quasar RMs with SKA: - ~- ~101066 measurements measurements - identification & redshifts from - identification & redshifts from SDSS & successorsSDSS & successors - accurate foreground removal - accurate foreground removal using RM gridusing RM grid
Ly-Ly-αα Absorbers at Absorbers at zz ~ 1 – 3 ~ 1 – 3
→ magnetic field evolution in galaxies over cosmic time-scales
RRM ~ (1+z)-2
Residual RMs (Galaxy corrected) vs z of QSOs embedded in intervening clouds (Welter et al 1984) : marginal evidence of evolution !
Magnetic Fields in ProtogalaxiesMagnetic 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 > 4 (Driver et al 1998)
The Magnetized IGM: Cosmic WebThe Magnetized IGM: Cosmic Web
Existing limits (scale and model dependent): Existing limits (scale and model dependent):
|B|BIGMIGM| < 10| < 10-8-8-10-10-9-9 G G (e.g..Blasi et al 1999; Jedamzik et al 2000)(e.g..Blasi et al 1999; Jedamzik et al 2000)
RM pairs at separation needed to detect B = 1 nG at scale of 50 Mpc (Kolatt 1998)
z = 0.5
z = 1
z = 2
- Detection and polarimetry of very - Detection and polarimetry of very lowlowLevel synchrotron emissionLevel synchrotron emission
-RM measurements of extragalactic RM measurements of extragalactic sources are related to the amplitude sources are related to the amplitude and shape of the magnetic field and shape of the magnetic field power spectrum P(k) where k is the power spectrum P(k) where k is the wave number of the coherence scalewave number of the coherence scale
→ → SKA + z surveys can provide SKA + z surveys can provide magnetic power spectrummagnetic power spectrum of the of the UniverseUniverse
SKA SKA SpecificationsSpecifications for Polarimetry for Polarimetry
• Frequency: at least Frequency: at least 11–10 GHz–10 GHz, 0.3–20 GHz ideal, 0.3–20 GHz ideal• Large field of view: Large field of view: >1 deg>1 deg22 at a resolution of <1 at a resolution of <1"" • High sensitivity: High sensitivity: <0.1 mJy<0.1 mJy, confusion limited, confusion limited• Large bandwidth: Large bandwidth: >400 x 1>400 x 1 MHz MHz at 1.4 GHz at 1.4 GHz• Significant concentration ( Significant concentration ( > 50%> 50% ) of antennae in ) of antennae in
central corecentral core ( ~ ( ~ 5 km)5 km)
• High polarization purity ( High polarization purity ( ––40 dB40 dB at field center, at field center, ––30 dB30 dB at field edges) at field edges)
SKA pathfinders:
ATA (US) LOFAR (The Netherlands + Europe) LWA (US) KAT/MeerKAT (South Africa) MWA (Australia) MIRANDA (Australia + Canada) SKADS (Europe)
Low frequency
- Diffuse synchrotron emission of steep spectrum
- Polarized emission sources of low RM weak magnetic fields
2
= 10o
= 240 MHz, = 32 MHz
RM = 0.4 rad/m2
• Early primordial fields could have been generated by Early primordial fields could have been generated by battery effects, during inflation or phase transitions battery effects, during inflation or phase transitions
• A primordial intergalactic (IGM) field may have A primordial intergalactic (IGM) field may have regulated structure formation in the early Universeregulated structure formation in the early Universe
• ““Seed fields” at z > 5 may originate from primordial Seed fields” at z > 5 may originate from primordial fields or from post-recombination fieldsfields or from post-recombination fields
• Present-day large-scale fields of B ≥ 1 Present-day large-scale fields of B ≥ 1 μμGG could have could have evolved from evolved from BB00 ~ 10 ~ 10-9-9––1010-10-10 G G seed fields at z > 5 seed fields at z > 5
• Evolution from seed fields includes dynamo, Evolution from seed fields includes dynamo, compression, merger interactioncompression, merger interaction
Conclusions
THANK YOU
Biermann Battery effect
Electrostatic equilibrium
When gradients of electronthermodynamic quantities(e.g. density and temperature)are not parallel to thepressure gradient, theelectrostatic equilibrium is no longer possible. This leads to a current which generates A magnetic field restoringthe force balance.
Wid
row
20
02
First observed in the lab in1975 (Stamper & Ripin)
Zeeman effect
In a vacuum, the electronic energy levels of an atomare 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.
The Zeeman effect can be interpreted as due to the precessionof the orbital angular momentum vector in the magnetic field.The energy shift is proportional to the strength of the magneticfield.
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 (> mG) (sunspots + galactic objects)
Hydrogen
Bohr magneton