Magnetic fields in galaxies

Transcription

1 Magnetic fields in galaxies Selected items (high-speed version) Rainer Beck, MPIfR Bonn

2 Fundamental questions on galactic magnetic fields When and how were the first magnetic fields generated? How and how fast are magnetic fields amplified in galaxies? How do magnetic fields affect the dynamics of the ISM?

3 Radio waves are ideal tools to study magnetic fields

4 NGC cm VLA Total intensity (Beck 2007) Radio synchrotron disk: Extent is limited by the energy losses of the cosmicray electrons (few GeV)

5 Magnetic field strengths in spiral galaxies (from synchrotron intensity, assuming equipartition between energy densities of magnetic fields and cosmic rays) Total (mostly turbulent) field in spiral arms: Regular field in interarm regions: μg 5-15 μg Total field in circum-nuclear rings: Total field in galactic center filaments: μg 1 mg

6 Magnetic energy density (assuming equipartition with cosmic rays) Beck 2007 Magnetic energy density is similar to that of turbulence ISM is a low-beta plasma No modelling without magnetic fields!

7 Synchrotron polarization Beck & Hoernes 1996 NGC 6946 Total and polarized intensity at 6cm

8 NGC cm VLA+Effelsberg Polarized intensity + B-vectors (Beck & Hoernes 1996) "Magnetic arms": Ordered fields concentrated in interarm regions Sterne und Weltraum

9 Origin of "magnetic arms" High mode of the mean-field dynamo? (Rohde et al. 1999) Slow MHD waves? (Lou & Fan 1998, Lou & Bai 2006) Drifting of magnetic arms away from the gas arms? (Otmianowska et al.)

10 M 51 6cm VLA+Effelsberg Total intensity + B-vectors (Fletcher et al. 2010) Spiral fields are roughly parallel to the optical spiral arms Sterne und Weltraum

11 M 51 6cm VLA+Effelsberg Total intensity + B-vectors (Fletcher et al. 2010) Field lines parallel to the inner spiral arms: density-wave compression or shear

12 Origin of polarized emission: Regular field (dynamo) or anisotropic field (compressed isotropic field)?

13 Fletcher 2004 Regular field Anisotropic field Polarization : strong strong Faraday rotation : high low

14 M51 VLA+Effelsberg RM 3/6cm (Fletcher et al. 2010) 150 rad m -2 0 rad m -2 Complicated RM pattern: Two weak dynamo modes (m=0+2), plus strong anisotropic fields -150 rad m -2

15 M31: The dynamo is working Berkhuijsen et al Fletcher et al The spiral field of M31 is coherent and axisymmetric (small spiral pitch angle)

16 NGC cm VLA Polarized intensity + B-vectors (Chyzy & Buta 2007) Spiral fields in a ring galaxy: dynamo action

17 NGC 1097 SPITZER IRAC Barred galaxies

18 NGC cm VLA Total intensity + B-vectors (Beck et al. 2005) Strong ordered field in front of the bar: amplification by shearing gas flows The field lines map the gas flow

19 NGC 1097 Circumnuclear ring VLT (Prieto et al. 2005) How to feed an AGN?

20 NGC 1097 Circumnuclear ring 3.6cm VLA Total intensity + B (Beck et al. 2005) Strong field in the circumnuclear ring: Magnetic stress leads to inflow which can feed the active nucleus

21 Accretion by magnetic stress Basic MHD theory of rotating disks (Balbus & Hawley 1998) : Mass inflow: dm/dt = -h/ω (< B tot,r B tot,φ > + < B reg,r B reg,φ >) NGC 1097 (Beck et al. 2005) : h=100 pc, v=450 km/s, B tot,r B tot,φ 50μG, B reg,r B reg,φ 10μG: dm/dt 1 M o /yr

22 NGC 891 3cm Effelsberg Total intensity + B-vectors (Krause 2007) Bright radio halo with X-shaped field pattern: Quadrupolar dynamo field in disk and halo?

23 Possible tasks for the SPP - in cooperation with the new magnetic DFG research group FOR1254 Physics of the galactic dynamo in the ISM Origin of the "magnetic arms" Feeding active nuclei by magnetic stress Field amplification by shear flows IGM magnetization by outflows

24 Spektrum der Wissenschaft/Emde-Grafik LOw Frequency ARray MHz MHz

25 SKA Project Office + Swinburne Astr. Prod. SKA

26 ALMA Antennas: 54x 12-m plus 12x 7-m Receivers: GHz ( mm), mostly with pol Construction:

27 Submm polarization by aligned dust grains: Ordered magnetic fields in a molecular gas disk (SMA 870 μm, 0.02 pc resolution) Tang et al Hourglass field: Ambipolar diffusion?

28 E-ELT Construction: Site: Cerro Armazones Polarimetry needed but still needs efforts

29 Summary: Future observations Polarization of diffuse emission (EVLA, SKA): Detailed field structure Low-frequency emssion (LOFAR, SKA): Extension of fields in outer disks and halos IR/submm polarization by dust grains (ALMA, E-ELT): Detailed field structure of molecular clouds