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Modified and pure dynamo field in nearby galaxies

Modified and pure dynamo field in nearby galaxies. Krzysztof Chyży (Jagiellonian University, Kraków). Outline. In collaboration with R. Beck, R. Buta, M. Ehle. A d isturbed galaxy NGC 4254 in the Virgo cluster The disturbed magnetic field and the idea of „magnetic maps”

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Modified and pure dynamo field in nearby galaxies

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  1. Modified and pure dynamo field innearbygalaxies Krzysztof Chyży (Jagiellonian University, Kraków)

  2. Outline In collaboration with R. Beck, R. Buta, M. Ehle • A disturbed galaxy NGC 4254 in the Virgo cluster • The disturbed magnetic field and the idea of „magnetic maps” • Correlations between the magnetic field components (Breg,Bran, Btot) andthe local SFR • Are the magnetic fields modified by ram-pressure or tidal forces? • An isolated, early-type galaxy of ringed morphology - NGC 4736 • The observed structure of magnetic field in the inner ring (the Lindblad resonance) • „Pure” dynamo – without support from spiral density waves

  3. NGC 4254 Virgo cluster spiral, weakly disturbed VLA+EFF4.86 GHz (6.3cm) Outer magnetic arm Chyży, Ehle & Beck 2007, A&A, 474, 415

  4. NGC 4254 PI 4.86 GHz Vir A1.2 Mpc The ram-pressure of the cluster gas - compression?(anisotropic random field?)

  5. Ram-pressure or tidal interaction? HI (WSRT) - VIRGOHI21 (Minchin et al. 2007) HIdata cube (xyv) A range of HI blobs. The largest one: M=108 Vir A The tidal interaction - stretching/shearing (anisotropic random field)?

  6. Magneticmaps (B corrected for projection and Faraday rotation) Projection i=42o P.A.=68oTP PI polar. angleFarad. rot. (from 3 and 6cm data) Chyży2008, A&A in press, astro-ph 0712.4175

  7. The modelling of compression (ram-pressure) and stretching (tidal effects) in the polarized ridge - geometry of the ridge and acting forces- scaling of the field components following Sokoloff et al. (1998),Beck et al. (2005) stretching/shearing compression degree of polarization intensityratio Compression – enhances Bran too much! Stretching well corresponds => tidal interaction B-components – a sensitive tool to reveal physical processes! Chyży2008, A&A in press, astro-ph 0712.4175 B2tot = B2reg+B2ran = B2coh +B2ani+B2ran

  8. Magnetic field components - SFR

  9. Why thefield regularity anicorrelates with SFR? • Tangling/disruption of the regular field in the regions of enhanced star formation (by turbulence) • Production of turbulent (random) field proportional to SFR • Production of regular field antiproportional to SFR • correlation length of turbulence antiproportional to SFR (Rohde 1999) • turbulent diffusivity enhanced by shearing (Moss 2001, 2007) • suppresion of the mean-field dynamo in the spiral arms (Shukurov 2005) MHD simulations needed!

  10. Waller et al. 2001 Ringed galaxy NGC 4736 • Early type Sab • The large oval causing outer and inner Lindblad resonances => mass accumulation • The outer ring – very weak • The inner ring–well-defined zone of star formation • Flocculent spiral pattern • Circular gas motions 200 km/sMagnetic field? opt UV

  11. radio contours at 8.5 GHz (VLA+EFF)infrared 24 mm – colors (SPITZER) NGC 4736 8”x8” (200pc x 200pc)

  12. Polarized intensity 8.5 GHz + B (of PP)Hαimage NGC 4736 rms=6μJy Chyży and Buta 2008, ApJLetters, in press, astro-ph 0803.0555

  13. Undistorted dynamo? - Breg in the galaxy’s plane, free of projection effects- contours of PI- Hα image in colors Magnetic field vectors do ignore the ringed morphology altogether! large p=35o

  14. BtotBranBreg Magnetic field strength NGC4736SombreroBtot 30 G6G Breg 13 G3G Sombrero (Sa)Krause et al. 2006 NGC 4736 – B stronger than in typical spirals (Beck et al. 1996) The dynamo process must be very efficient!

  15. Magnetic pitch angle in NGC 4736 According to Elstner (2005) rW =440 pc, v=200 km/s – rotation curve of CO and HI(Wong & Blitz 2000) hT= 1kpc km/s, h=500 pc – typical values for spirals (Elstner 2005, Shukurov 2005) =>pitch angle p=2o (instead of 35o) thinner disk or larger turbulent diffusion (>10 kpc km/s) Parker instability? MRI?

  16. Summary • NGC 4254 • Itsmagnetic fields are disturbed andmatchdensity waves • Thetotal and random fields correlate but thefield regularity anticorrelates with SFR, whiletheregular field very weakly depends on SFR • Themagnetic field components discriminate between thetidal and ram-pressure effects • NGC4736 • Itsradio emission closely associated with theinner ring • Breg ignores thegalaxy’s ring and shows a spiral structure • Magnetic vectors cross the ring at large pitch angle values– large turbulent diffusion? • Strong fields (efficient dynamo) in early-type galaxy • A challenge for thedynamo theories and simulations! (?)

  17. Regular fieldpatternin NGC 4254 • B corrected for Faraday rotation • Coherent structure – matching the density waves

  18. Other wavebands X (XMM)– contoursHα - image HI (VLA archive) - image1.4 GHz - contours, + B (PI)

  19. Radio polarimetricobservations: VLA + Effelsberg 8.5, 4.8 GHz(3, 6 cm) AIPS + NOD + MIRIAD

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