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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 1016 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 30G 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 100G 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