cosmic magnetic fields elisabete m. de gouveia dal pino iag-usp ufrrj, october 2005
TRANSCRIPT
COSMIC MAGNETIC FIELDSCOSMIC MAGNETIC FIELDS
ElisaBete M. de Gouveia
Dal Pino
IAG-USP
UFRRJ, October 2005
PREAMBLE
Most of visible matter in the Universe is in plasma state:
composed of ionized or partially ionized gas permeated by magnetic
fields
Alfvén, Biermann, Chandrasekhar and Parker knew that decades ago !
WHY MAGNETIC FIELDS?
Charged Particle Fluid immersed in B
v x B J x B
WHY MAGNETIC FIELDS?
Charged Particle Fluid immersed in B
v x B J x B
TENSION PRESSURE
WHY MAGNETIC FIELDS?
Charged Particle Fluid immersed in B
v x B J x B
MAGNETIC FIELDS
Crucial in: star formation, solar and stellar activity, pulsars, accretion disks, formation and stability of jets, formation and propagation of cosmic rays, galaxy structure.
Probably crucial in: ISM, molecular clouds, supernova remnants, proto-planetary disks, and planetary nebulae, GRBs.
Importance not well understood in: stellar evolution, halos of galaxies, galaxy evolution, and structure formation in the early Universe.
Measuring magnetic fields
• Polarization: by aligned dust grains with B of ISM
Magalhães 2005
B
Measuring Magnetic fields• Zeeman effect (within galaxy):
= e B║/2 me
• Polarized synchrotron emission (Beck and Krause 2005):
I ∫ nCR B┴1+α dl
• Faraday rotation of the diffuse polarized emission:
RM ∫ ne B║ dl
Effelsberg
VLA
ATCA
Some Radio Telescopes
Outline
I. Magnetic Fields in Stars and Compact Objects
II. Magnetic Fields in the ISM & Star Formation
III. Magnetic Fields in the Milky Way
IV. Magnetic Fields in Galaxies, Clusters and IGM
V. Primordial Magnetic Fields
VI. Future Needs and Perspectives
PART I:
Magnetic Fields in Stars and Compact Objetcs
Solar Magnetic Fields
In corona (2 x 106 K):
Magnetic arcs (30-100 x 104 km)
Sunspots (B = 100-2000 G)
Solar Magnetic Fields
Magnetic arcs: rise by buoyancy due to convective motions (Parker-Rayleigh-Taylor instability)
Silva, 2005
sunspot
Solar Flares
Sudden release of 1030-1032 erg (seconds to hours)
Silva 2005
Solar Flares
Flares energized by magnetic reconnection
EB released: heating, particle acceleration, coronal mass
ejections (CMEs)
Shibata et al.
CMEs
Solar Flares
Flares energized by magnetic reconnection
EB released: heating, particle acceleration, coronal mass
ejections (CMEs)
Shibata et al.
CMEs
Solar Magnetic Fields
Dynamo action:
Conductive ionized flow
Convective and turbulent motions
Differential rotation
What is the origin of solar magnetic activity?
Conversion of kinetic energy of
these motions into EB
Magnetic Field Evolution
Fluid-B freezing diffusion
diff = 4 L2 >> 1 BA = constant
Dynamo MechanismResponsible for conversion of Ec EB
-effect:
turbulent/convection motions (BT Bp)
-effect: vo=r differential rotation (Bp BT)
= 1/4: magnetic diffusivity
: field dissipation due to turbulent motion
Dynamo Mechanism
-effect
-effect
Bp
BT
Dynamo Mechanism
-effect -effect
Magnetic Fields in other stars
Similar magnetic processes: in many stars (cool stars)
Shibata 2005
STARS
ASTROPHYSICAL JETS
EXTRAGALACTIC: 106 l.y., velocities c, source mass 108 M, L ~ 1043 - 1048 erg/s
Magnetic Fields in Jets
Jet from Active Galactic Nuclei (AGN)
EXTRAGALACTIC: 106 l.y., velocities c, source mass 108 M, L ~ 1043 - 1048 erg/s
Magnetic Fields in Jets
Jet from Active Galactic Nuclei (AGN)
Magnetic Fields in Jets
GALACTIC:
~ 1 l.y., velocities c, source mass 10 M, L ~ 1039 erg/s
Jets: What are they?
EXTRAGALACTIC
GALACTIC
Supersonic collimated outflows carry:
mass, momentum, energy and magnetic flux from stellar, galactic and extragalactic objects to the outer medium
WHAT IS THE JET ORIGIN ?
Magneto-centrifugal acceleration out off accretion disk around the source (Blandford & Payne)
Accretion Disks
Magnetic Fields in Accretion Disks
Similar magnetic processes in stars: in accretion disks, galactic disks
Shibata 2005STARS DISKS
Wind/Jet
Magnetic Fields in Accretion Disks
X-ray and radio flares: ejections accelerated during violent magnetic reconnection (de Gouveia Dal Pino & Lazarian 2001, 2005; de Gouveia Dal Pino 2006)
PART II:
Magnetic Fields in the ISM & Star Formation
ISM: Interstellar Medium
Effelsberg 21cm (Reich et al 2003)
ISM 21cm Emission from Perseus - Auriga
Polarized emissionl=166° l=150°
b=-4°
b=+4°
Effelsberg 21cm (Reich et al 2003)
ISM 21cm Emission from Perseus - Auriga
Polarized emission
• ISM diffuse polarized emission: traces B structures of pc and sub-pc sizes
• Carries information about the turbulent ISM
l=166° l=150°
b=-4°
b=+4°
Magnetic fields in the ISM
• MHD turbulence distributes energy from SN explosions, jets and winds within the ISM
• Magnetic fields control density and distribution of cosmic rays in the ISM and halo
• EB Eturb ECR
Magnetic Fields in Molecular Clouds
n(H2) 5 105 cm-3, N(H2) 4 1022, 13, Bpos 140 G (Crutcher et al. 2004)
Regular B and disk-like morphology
L1544 Core
Magnetic Fields in Molecular Clouds
L183 coreDR21OH core
n(H2) 2 106, N(H2) 3 1023, Bpos 0.7 mG (Lai et al. 2001)
Blos = 0.4, 0.7 mG
Bpos 0.7 mG
n(H2) 3 105, N(H2) 3 1022, 13, Bpos80 µG (Crutcher et al. 2004)
Magnetic Fields in Diffuse and Molecular Clouds
H I Clouds Molecular Clouds
Btotal (G) 6.01.8 10 –
3,000
M/(BA) <0.25 ~1
[B ] ~0 ~1/2
Pthermal/PB 0.29 0.04
Pturbulent/PB 1.3 0.7
Crutcher 2005
Magnetic Fields in Diffuse and Molecular Clouds
• Diffuse ISM and HI clouds: dominated by turbulence
Molecular clouds: formed by HI clouds accumulation along field lines
Parker 1972
Crutcher 2005
Star Formation in Molecular Clouds
• Observations consistent with approximate magnetic support in molecular cores
(gPB)
• Ambipolar diffusion driving star formation on a fast (~few free-fall times) timescale
Li & Shu (1996)
Magnetic fields also essential for removal of angular momentum from protostellar cloud (magnetic braking!)
Phases of Star Formation
(d) Star and protoplanetary disk with lifetime:
Δt = 1 – 5 Myr
(a) Formation of cores in giant molecular clouds by ambipolar diffusion and decay of turbulence:
Δt = 1 – 3 Myr
(b) Rotating, magnetized gravitational collapse:
Δt = ?
(c) Strong jets & bipolar outflows; reversal of gravitational infall:
Δt = 0.1 – 0.4 Myr
Shu, Adams, & Lizano 1987
PART III:
Magnetic Fields in the Milky Way
The Milky Way
Magnetic fields in our Galaxy
Han et al. 2001
?
Equipartition fields in the Galaxy (Berkhuijsen, priv. comm.)
Cosmic-ray energy density + radio synchrotron:
<B> 6 G
and in inner region:
<B> 10 G
Magnetic fields in our Galaxy
PART IV:
Magnetic Fields in Galaxies, Clusters and
IGM
Magnetic fields in Galaxies
B2 = Bt2 + Br
2
Polarized synchrotron: measures Br
M51
Spiral patterns of regular B:
observed in grand-design, flocculent and even in some irregular galaxies.
Magnetic fields in Galaxies
Spiral patterns of regular B observed in
Grand-design galaxies
Magnetic fields in Galaxies
M51
Flocculent galaxies:spiral field without
spiral arms !
NGC4414NGC4414(Soida et al. 2002)
Large
Irregulars:
some
traces of
spiral field
NGC4449NGC4449(Chyzy et al. 2000)
Barred Spiral galaxies:
Regular fields follow the shearing gas flowaround massive bars
NGC1097(Beck et al. 2004)
Organized B inside and outside of the circumnuclear “ring” !
Magnetic fields in Galaxies
M31
Magnetic fields in Galaxies
Turbulent fields are strongest in spiral arms (20 G): due to intense star formation, SN shocks.
Regular fields are strongest in interarm regions (15 G)
M51
Magnetic fields in Galaxies• Survey of 74 S galaxies (Niklas):
<Btot> = 9 μG
• Starburst galaxies:
B ≥ 30 - 50μG
• Nuclear starburst regions:
B ≥ 100μG!
NGC1067: nuclear SB
region Correlation B and SF!
Magnetic fields in Galaxies
consistent with dynamo!EB ≈ Eturb
Outer regions:
EB > Etherm :
B affects gas rotation curve !?
(Battaner & Florido 2000)
M31: very regular (coherent) field revealed by Faraday rotation
The coherent
magnetic field
in M31 is the
best evidence
so far for
dynamo action !
Fletcher et al. 2004
Magnetic fields in Galaxy Halos
B in Halos of galaxies with high SFR:correlated to diffuse ionized gas and X-rays (up to z=5 kpc)
Several halos:
v/z < 0
contribute to excitation of dynamo!
NGC5775 (Dettmar 2005)
Magnetic fields in Galaxy Halos
B filaments and loops coupled with charged dust in halo!
NGC891 (Rossa et al. 2005)
Do dynamos work in galaxies ?
YES:+ Spiral fields occur almost everywhere, even in
irregular galaxies and central rings
++ Magnetic arms occur between gas arms
++ Large-scale coherent fields exist
++ There is at least one case of a dominating axisymmetric mode (M31)
Do dynamos work in galaxies ?
NO:
- Single dominating modes are rare (nonlinear multiple dynamos? – Subramanian 1988)
- Coherent fields surprisingly weak in galaxies with strong density waves (M51) (strong compression and/or shear?)
- Spiral fields extend well into the centers - Fields are still strong in outer regions of galaxies
(magneto-rotational instability?)
Dynamo in Galaxies
Beck 2005
Magnetic fields in Clusters
A 2029 Coma
Rotation Measures in Clusters
RMs of polarized synchrotron radiation from background or embedded radio galaxies
Rotation Measures in Regular Clusters
-12000 Rad/m/m +5000 Rad/m/m
Hydra A Hydra A Taylor & Perley (1993)Taylor & Perley (1993)
Lane et al. (2004)Lane et al. (2004)
Rotation Measures in Irregular Clusters A400 (3C75) A400 (3C75) Eilek & Owen (2002)Eilek & Owen (2002)
-170 Rad/m/m +170 Rad/m/m
Magnetic fields in Clusters
• X-rays observations Abel clusters (e.g., Grasso & Rubinstein 2001):
• Coma cluster: B 8.3 G ! (tangled in L ~1 kpc)
• Clusters central regions with radio sources (Govoni et al. 2005):
B 5-30 G PB > Ptherm!
Origin of B in Clusters ?
B fields powered by jets from radio sources (Colgate & Li 2003, Kato et al. 2005).
But: pre-existing B may be required!
Numerical simulation (Kato et al 2005)
Hydra
Magnetic fields in IGM
• B probably pervades entire Universe
• IGM: rarefied ionized gas and coherence L of B poorly known
• Faraday rotation of polarized emission from distant quasars (up to z=2.5):
BIGM 10-9 G, for L 1 Mpc
PART V:
Primordial Magnetic Fields
Magnetic fields in Early Universe
Universe History
Magnetic fields in Early Universe
RMs of distant quasars (z>1) B in the past (Kronberg et al. 1992)
• 3C191 (z=1.945):
B 0.4 - 4 G in L 15 kpc (galaxy size!)
B 1 - 4 G
• Young spiral galaxy (z=0.395):
What is the origin of these B fields in early Universe ?
Bprim,o 10-9 G !
Primordial Magnetic fields?
Strong B in galaxy clusters and in galaxies at high redshifts:
Are the magnetic fields primordial ?
Pros:
Large conductivity of plasma in Universe:
diff = 4 L2 >> to BA = constant
Alternative to dynamo: If B is primordial Bgal 10-6 G results from compression of primordial B:
Primordial Magnetic fields?
Constraints:
• CMBR: Primordial B would influence CMBR via:
breaking spatial isotropy
MHD effects temperature and polarization fluctuations CMBR
CMB spectrum Bo(50 Mpc) < 10-8 –10-9 G
• BB Nucleosynthesis: Primordial B could change expansion rate of the Universe and 4He abundance
BBN B(100pc, T=109K) < 1011 G
Bo(1Mpc)<10-10 G
Primordial Magnetic fields?
CMBR and BBB constraints:
Imply B strengths the required by IGM today
diff >> to: diffusion length lo < 109 cm:
Small scale fields produced in early Universe survived and left no significant imprints on BBN or CMB (perhaps!)
Models for Primordial Magnetic fields
• Inflation (breaking conformal invariance of
electromagnetic field): Bo(1Mpc) 10-62 G! Too small to seed galactic dynamo
• QCD Phase transition (quarks combine to form
hadrons, T= 1.5 1012 K): Bo(100kpc) 10-9 G (under extreme conditions! Sigl et al.)
• Biermann Batery (pxn≠0): Bo 10-21 G (pre-galactic seed field is exponentially amplified by dynamo)
• Harrison effect: Bo 10-19 G (pre-galactic
seed field is amplified by dynamo)
• SN-driven turbulence (may amplify seed B-fields in 10 Myr only; Kim 2005)
Future Needs & Perspectives
• Higher radio polarization sensitivity
• Higher angular resolution (to map wealth of magnetic structures in galaxies)
Beck 2005
Square Kilometer Array (SKA)
• Total effective collecting area: 1 Km2 (100 MHz to 25 GHz)
• Stations of ~100 m diameter 150 stations – accounting for half the SKA area – will be distributed across continental distances (~3000 km).
• Remaining area will be concentrated within a central region of 5 km diameter (2020).
Square Kilometer Array (SKA) Map nearby galaxies 10x better angular resolution of present radio telescopes
10x more distant galaxies with similar spatial resolution as today
detect synchrotron emission from galaxies and structures in the earliest stage of evolution
search for the earliest magnetic fields and their origin
proto-planets
black-holes
pulsars (>10000)
Cordes 2001
Square Kilometer Array (SQA)
Magnetic Fields in the Universe: from Laboratory and Stars to
primordial Structures
American Inst. Phys., Conf. Procs., AIP, vol. 784
Square Kilometer Array (SQA)
Thank you !
Origin of B in Clusters ?
B fields powered by jets from radio sources (Colgate & Li 2003, Kato et al. 2005). But pre-existing B may be required!
Kato et al 2005
Dynamo Mechanism
-effect
-effect
Bp
BT
Magnetic fields at Early Universe
RMs of distant quasars (z>1)
B in the past (Kronberg et al. 1992)
• 3C191 (z=1.945):
B 0.4 - 4 G in L 15 kpc (galaxy size!)
B 1 - 4 G
• Young spiral galaxy (z=0.395):
What is the origin of these B fields in early Universe ?
Athreya et al. 1998
•VLA observations of 15 radio galaxies with z>2•Four gals show intrinsic RMs in excess of 1000 rad m-2
•The environs of the gals at z>2 have B-fields with micro-G strength• Kim 2005.