Modified and pure Modified and pure dynamo field in dynamo field in nearby galaxiesnearby galaxies

Modified and pure Modified and pure dynamo field in dynamo field in nearby galaxiesnearby galaxies

Krzysztof Chyży (Jagiellonian University, Kraków)

OutlineOutline

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) and the 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

In collaboration with R. Beck, R. Buta, M. Ehle

Outer magnetic arm

Chyży, Ehle & Beck 2007, A&A, 474, 415

VLA+EFF4.86 GHz (6.3cm)

NGC 4254NGC 4254

Virgo cluster spiral, weakly disturbed

NGC 4254NGC 4254

PI 4.86 GHz

The ram-pressure of the cluster gas - compression?(anisotropic random field?)

Vir A1.2 Mpc

HI (WSRT) - VIRGOHI 21 (Minchin et al. 2007)

The tidal interaction - stretching/shearing (anisotropic random field)?

A range of HI blobs. The largest one: M=108

HI data cube (xyv)

Ram-pressure or tidal interaction?Ram-pressure or tidal interaction?

Vir A

Magnetic maps (B corrected for projection and Faraday rotation)Magnetic maps (B corrected for projection and Faraday rotation)

Chyży 2008, A&A in press, astro-ph 0712.4175

Projectioni=42o

P.A.=68o

TPPIpolar. angle

Farad. rot. (from 3 and 6 cm data)

The mThe modelodellling of compression (ram-pressure) ing of compression (ram-pressure) and stretching (tidal effects) in the polarized ridgeand stretching (tidal effects) in the polarized ridge

B2tot = B2

reg+B2 ran = B2

coh +B2ani+B2

ran

Compression – enhances Bran too much! Stretching well corresponds => tidal interaction

stretching/shearing compression

- geometry of the ridge and acting forces- scaling of the field components following Sokoloff et al. (1998), Beck et al. (2005)

degree of polarization

intensityratio

B-components – a sensitive tool to reveal physical processes! Chyży 2008, A&A in press, astro-ph 0712.4175

Magnetic field components - SFRMagnetic field components - SFR

Why Why the the field regularity anicorrelates with SFR?field 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!

RRinged galaxy inged galaxy NGC 4736NGC 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/s

Magnetic field?UV

opt

Waller et al. 2001

NGC 4736NGC 4736 radio contours at 8.5 GHz (VLA+EFF)infrared 24 m – colors (SPITZER)

8”x8” (200pc x 200pc)

NGC 4736NGC 4736

Chyży and Buta 2008, ApJLetters, in press, astro-ph 0803.0555

Polarized intensity 8.5 GHz + B (of PP)Hα image

rms=6μJy

Magnetic field vectors do ignore the ringed morphology altogether!

large p=35o

UndistortedUndistorted dynamo? dynamo? - Breg in the galaxy’s plane, free of projection effects- contours of PI- Hα image in colors

NGC 4736 Sombrero Btot

30 G 6G Breg 13 G 3G

Sombrero (Sa)Krause et al. 2006

The dynamo process must be very efficient!

Magnetic field Magnetic field strengthstrength

Btot Bran Breg

NGC 4736 – B stronger than in typical spirals (Beck et al. 1996)

r =440 pc, v=200 km/s – rotation curve of CO and HI (Wong & Blitz 2000)

T= 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?

Magnetic pitch angle in NGC 4736Magnetic pitch angle in NGC 4736

According to Elstner (2005)

SummarySummary

NGC 4254• Its magnetic fields are disturbed and match density waves • The total and random fields correlate but the field regularity

anticorrelates with SFR, while the regular field very weakly depends on SFR

• The magnetic field components discriminate between the tidal and ram-pressure effects

NGC 4736• Its radio emission closely associated with the inner ring• Breg ignores the galaxy’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 the dynamo theories and simulations! (?)

RRegular fieldegular field pattern in NGC 4254 pattern in NGC 4254

• B corrected for Faraday rotation• Coherent structure – matching the density waves

Other wavebandsOther wavebands

X (XMM) – contours Hα - image

HI (VLA archive) - image 1.4 GHz - contours, + B (PI)

+ Effelsberg+ Effelsberg

AIPS + NOD + MIRIADAIPS + NOD + MIRIAD

8.5, 4.8 GHz(3, 6 cm)

Radio polarimetric observations:Radio polarimetric observations:

VLAVLA