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Transcript
Magnetic fields in
galaxies
ΑΘΗΝΑ 2013
ΣΟΦΙΑΝΟΣ ΠΕΤΡΗΣ
Μ.Δ.Ε. ΑΣΤΡΟΦΥΣΙΚΗΣ
Α.Μ: 201233
Significance of galactic magnetic field
• Regulates star formation.
• Controls the distribution of cosmic rays.
Assuming
energy
equipartition
between
galactic field
and cosmic rays
• Complicates the propagation of UHECRs.
• Transports momentum and energy in galaxies.
• Collimates the dust grains in the interstellar medium.
• Reduces the gravitational effects into galaxies and stars.
Origin of galactic magnetic field
• Many theories try to explain the generation of large-scale
fields of that strength( G  mG ).
• The most prominent explanation has two steps:
* occurrence of a ‘primordial’ seed field.
* amplification of that field by a dynamo mechanism.
• The seed field can be generated:
* during a phase of transition in Early Universe (Caprin et
al. 2009).
* from the time of cosmological structure formation by
Wiebel instability (Lazar et al. 2009)
plasma instability
in nearly homogeneous plasma with anisotropy in
velocities which can lead to electromagnetic perturbations
and thus to a field.
Origin of galactic magnetic field
* by jets from black holes or injection from first stars
(Rees 2005).
* Biermann mechanism in the first supernova remnants
(Hanayama et al. 2005).
Because of turbulences the gradient of pressure cause
electron flow and hence an electric field.
Pe
E
ne  e
Pe
B
c
 c  (  E )    ( )
t
e
ne
Pe  ne  k B  Te
Origin of galactic magnetic field
c  kB
B

ne Te
t
ne  e
.
The gradients are not parallel, thus the equation leads to
1016 G strength.
the generation of magnetic field of
It’s the most significant theory for the seed field.
• Seed field sustained and amplified by a α-Ω dynamo
(Beck et al. 1996), based on:
* turbulence
* differential rotation
* α-effect(coriolis effect) powered by supernova explosion
(Gressel et al. 2008)
Origin of galactic magnetic field
A supernova explosion stretches the seed field create a
bubble and ,in a rotating regime, this provoke a counterrotational motion (coriolis effect).
8
• The ‘primordial’ field has been enhanced within 10 yr
(Schleicher et al. 2010) and according to simulations a
mean-field dynamo, as defined above, predict the
formation of large-scale regular fields by turbulent fields
9
10
yr (Arshakian et al. 2009), in spiral patterns.
within
Origin of galactic magnetic field
Figure 1:
Magnetic field probes
Note that each probe can reveal only one of the three
components of magnetic field (except Zeeman splitting).
• Faraday rotation of background radio sources and
pulsars.
• Zeeman splitting of radio spectral lines.
• Polarization of starlight.
• Polarization of infrared emission from dust grains and
molecular clouds.
• Synchrotron radiation intensity and polarization.
Faraday rotation
• Any linearly polarized wave can be considered as a
superposition of two counter-handed circularly polarized
waves of the same amplitude.
• These waves have slightly different velocities while
propagating through the medium (different refractive
index). Hence a difference in phase appears.
• Faraday rotation is the phenomenon that rotate the
orientation of wave’s linear polarization while propagating
in a medium with magnetic field.
• The rotation angle is directly proportional to the parallel
component of the field as well as to the square of
2
wavelength
  RM   (1)
Faraday rotation
where RM is the Rotation Measure, a parameter which
indicates the strength of the effect and depends on the
numerical density of electrons and the magnetic field.
3
e
l
(2)
RM 

n
(
s
)

B
(
s
)
dl
2 4 0 e
2  me  c

• The effect caused by free electrons: the electric field of a
circularly polarized wave cause circular motions of
electrons which in turn yield a new field parallel or antiparallel to the external field.
• Thus, by assuming the electron density we compute the
magnetic field strength in the line-of-sight  .
Faraday rotation
Figure 2: Faraday rotation in a magnetized gas.
• Vital probe from extragalactic sources and pulsars in our
galaxy, mainly for the magnetic field of our galaxy.
Faraday rotation
• Specifically as for the pulsars, which are widely spread in
our galaxy, we can use another tool which is the
Dispersion Measure DM:
l
DM  n dl (3)
0
e
• DM is a parameter which indicates the delay of arrival
pulses from a pulsar at a range of radio frequencies. Note
that each pulse is composed of a wide range of
frequencies but each one travels with different speed.
RM
• Thus, combining (2) and (3): B  1.232  DM (4)
This estimation is not dependent on electron density
model.
• Only regular fields give rise to Faraday Rotation while
random or anisotropic do not. Proof of large-scale pattern.
Faraday rotation
Figure 3: Pulsar and extragalactic sources distribution in our galaxy.
Zeeman splitting
• A probe which is widely used for measurements of the
parallel component of the field, in our galaxy, in starbursts

galaxies and few nearby galaxies
.
• It’s a way to determine field strength in gas clouds from
the emission line of 21 cm, or from maser emission from
dense core like galactic nuclei (Heiles & Robishaw 2009).
• The interacting energy U between the external magnetic
field and the magnetic dipole ‘nuclei-electron’ is:
U  m  B
L
m  B 
L  ml 
U  ml   B  B
Which is the energy
difference between the
two splitted lines.
Zeeman splitting
• Let f 0 be the frequency of the unshifted spectral line,
then the frequencies of the splitted lines will be:
e B
f  f0 
4  me  c
(5), hence measuring the
frequencies of the spectral lines, the parallel component
of the field is defined.
• From the change of the circular polarization we extract
the field direction.
Polarized emission at optical, infrared
and radio synchrotron emission
Starlight polarization:
• Optical linear polarization is the result of scattering from
elongated dust grains in the line-of-sight, which are
collimated in the interstellar magnetic field (DaviesGreenstein effect).
• Dust grains are not spherical, their long axis is
perpendicular to the field and they are spinning rapidly
with rotation axis along the magnetic field.
• E vector runs parallel to the field because grains tend to
absorb light polarized at the direction of the long axis,
thus we measure the vertical component B .
• Measurements from thousand of stars
• Reliable detector for distances <3kpc and mainly for
small-scale fields.
Polarized emission at optical, infrared
and radio synchrotron emission
Infrared polarized emission of clouds and dust:
• The same grains that polarize starlight also radiate in
the infrared. This thermal emission is polarized owing to
the shape of grains as presented above.
• Similarly we estimate the vertical component of the
magnetic field B .
Synchrotron emission:
• Accelerating electrons gyrating magnetic field lines
radiate radio synchrotron emission.
Polarized emission at optical, infrared
and radio synchrotron emission
• Significant tracer of magnetic field’s strength and
orientation, of external galaxies (Beck 2009) and our
Milky Way, by measuring the total radio intensity and
polarization respectively.
• Polarized emission traces ordered fields while
unpolarized synchrotron emission indicate turbulent
fields with random directions.
• We estimate the vertical component of the field B .
• The estimation is based on the distribution of relativistic
electrons in a range of energies: * widely assumed
power law distribution of electrons combined with the
equipartition of energy density between magnetic field
27
and cosmic rays lead to:
j B
syn

Polarized emission at optical, infrared
and radio synchrotron emission
Τable 1: Detectors of galactic magnetic fields.
Magnetic field structure of Milky Way
Best probes for a large-scale field in our galaxy: RM and
Zeeman splitting.
The other probes good at revealing field details.
Central region:
• A few hundred pc region.
• Toroidal field:
* field indicated by polarized emission from central
ring-like molecular cloud zone, Zeeman effect an OH
maser emission.
* field strength: 0.1mG.
* field orientation: parallel to the galactic plane.
• Poloidal field:
* field indicated by polarized radio filaments.
* field strength: few tens of μG.
Magnetic field structure of Milky Way
* field orientation: along the filaments perpendicular to
the galactic plane.
• Toroidal fields in the clouds are sheared from poloidal
fields.
• Smooth transition from toroidal to poloidal fields at
latitudes of b 0.4o .
• Both consistent to large-scale bi-symmetric field.
Figure 4: Toroidal and poloidal fields.
Magnetic field structure of Milky Way
Galactic disk:
• Ordered(regular or anisotropic) and turbulent field
components.
• Large-scale pattern in disk has a strong azimuthal
component.
• Small-scale structures also appear.
• Approximately the field follows the logarithmic spiral arms
having a pitch angle 10 . Parallel to the adjacent gas.
• Always clockwise in the arm region. Anti-clockwise in the
interarm regions displaying field reservals.
• Stronger field and polarized emission in interarm regions .
• Strength near the sun 6  G (Beck 2009).
• Norma arm 4  G.
Magnetic field structure of Milky Way
• Magnetic field in arms is passive to dynamics.
Figure 5: Field orientation in arm and interarm regions in Galaxy.
Magnetic field structure of Milky Way
Halo:
• Weaker fields in halo and less complex.
• Has a significant vertical component Bz 0.2 G .
• Best evidence in such a halo is total radio emission at
408 and 1420 Hz, diffuse polarized emission and RM
distribution from extragalactic sources.
• Reserval field below and above the disk which is also
consistent to the dynamo configuration.
Figure 6: Field configuration for A0
dynamo. Halo field shown.
Magnetic field in external galaxies
Spiral galaxies:
• Generally in spiral
galaxies, we observe
the same structure as in
our galaxy: 10  G .
• Gas-rich spiral galaxies
(M51, M83, NGC6946)
with high star formation
rate appear field
strength 20  30G
in spiral arms.
Figure 7: Total radio emission and B-vectors
of M51 combined from observations at 6
cm with VLA and Effelsberg telescope.
Magnetic field in external galaxies
Figure 8: Total radio emission and B-vectors of M83(left) and NGC6946
(right) combined from observations at 6 cm with VLA and Effelsberg
telescope.
Magnetic field in external galaxies
Barred and starburst
galaxies:
• In galaxies with massive
bars and intense SF
(M82), field lines follow
the gas flow.
• Gas rotates faster than
the bar pattern, thus a
shock occurs and gas is
compressed and sheared.
• Field strength 50  100G
Figure 9: Total radio emission and B-vectors of
IC1097 combined from observations at 6 cm
with VLA and Effelsberg telescope.
Magnetic field in external galaxies
Flocculent and irregulars:
• Flocculent (M33, NGC3521, NGC5055) have disk but not
prominent spiral arms.
• Nevertheless they dispose spiral magnetic patterns: meanfield dynamo acts independently of density waves.
• Ordered fields with strength similar to those of granddesign spiral galaxies.
• Irregulars (NGC4449) and especially the dwarf ones with
chaotic rotation (NGC1569) are radio-faint galaxies with
field strength 5  7 G.
• They have spiral pattern but not regular fields.
Magnetic field in external galaxies
Figure 10: Total radio emission and B-vectors of IC10 dwarf
irregular from observations at 6 cm with the VLA.
Magnetic field in external galaxies
Edge-on galaxies:
(NGC 891, NGC 253)
• Disk-parallel field near the
disk plane.
• Reveal vertical field
components in the halo
forming an X-shaped
pattern. Field propably
transported from the disk
to halo by a flow.
Figure 11: Total radio emission and B–vectors of the
edge-on spiral galaxy NGC 891, observed at 3.6 cm
with the Effelsberg 100m telescope.
Magnetic field in external galaxies
Interacting galaxies:
• Gravitational interaction between galaxies leads to
asymmetric gas flows, compression, shear, intense
turbulence and outflows which can lead to modification of
galactic and intergalactic magnetic fields.
• Interaction with dense intergalactic medium can also
imprint modifications in magnetic field and hence in radio
emission which exhibit asymmetries.
• Such galaxies are the pair NGC 4038/39, the ‘Antennae’,
NGC 4535 and NGC 4569.
Magnetic field in external galaxies
Figure 12: Polarized radio intensity(contours) and B-vectors of the Virgo
galaxies NGC 4535(left) and NGC 4536(right), observed at 6 cm with the
Effelsberg telescope.
Magnetic field in external galaxies
Early-type galaxies:
• Sa, S0 type spiral galaxies
and elliptical galaxies without
an active nucleus have very
little star formation and hence
do not produce cosmic rays
that could emit synchrotron
emission.
• Only the Sa type M104
revealed to have a weak
ordered magnetic field.
• Elliptical do not have much
gas and magnetic field appear
only in jets of central black
Figure 13: Sombrero galaxy M 104.
hole.
References
• Magnetic Fields in Galaxies, Rainer Beck 2011
• Magnetic fields in our Milky Way Galaxy and
nearby galaxies, Jin Lin Han 2012
• The Magnetic field of the Milky Way Galaxy,
J.C. Brown 2010
• Theoretical understanding of Galactic magnetic
fields, Katia Ferriere 2012