magnetic spiral arms in galaxies

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  • Magnetic spiral arms in galaxies

    Anvar ShukurovDepartment of Mathematics, University of Newcastle, Newcastle upon Tyne NE1 7RU

    Accepted 1998 July 1. Received 1998 June 15; in original form 1998 March 30

    A B S T R A C TLarge-scale magnetic fields in spiral galaxies are strongest in spiral-shaped regions, themagnetic arms. It was recently discovered for the galaxy NGC 6946 that magnetic arms can beinterlaced with the gaseous arms, rather than coinciding with them. There are indications thatthe magnetic and gaseous arms may cross in some other galaxies. We suggest that magneticarms can be located away from regions of higher gas density (gaseous arms) becauseinterstellar turbulence is stronger in the latter. We predict magnetic arms interlaced withgaseous arms in galaxies with weak dynamos, whereas the two spiral structures should overlapin galaxies with strong dynamo action; in an intermediate case, the magnetic and gaseousspiral structures overlap in the inner galaxy and are interlaced in the outer parts (as, possibly, inM51). Another plausible mechanism to produce displaced magnetic and gaseous spiralpatterns results from a delay in the dynamo response to the enhancement of turbulence inthe gaseous arms. This should lead to the magnetic and gaseous arms crossing at the corotationradius, as possibly observed in the galaxies IC 342 and M83. We also argue that spiral armsonly weakly affect the local scaleheight of the galactic gas layer.

    Key words: magnetic fields MHD ISM: magnetic fields galaxies: ISM galaxies:magnetic fields radio continuum: galaxies.

    1 I N T RO D U C T I O N

    A traditional understanding of the interaction between interstellarmagnetic fields and galactic spiral arms considers the fields as apassive component of the interstellar medium. The magneticdiffusion time owing to the molecular diffusivity by far exceedsthe galactic lifetime. Therefore magnetic field is frozen into theinterstellar gas, and responds instantaneously and directly to anychanges of the gas density, being controlled by magnetic fluxconservation. Because of this, galactic magnetic fields wereexpected to be stronger in gaseous spiral arms where the plasmadensity is enhanced (Roberts & Yuan 1970). Interstellar turbulencedramatically changes this picture, e.g. by an enhancement of themagnetic diffusivity at large scales. With the observed scaleof interstellar turbulence l . 100 pc and its rms velocity v . 1030 km s1, the turbulent magnetic diffusivity isb . 13 lv . 10

    26 cm2 s1. The turbulent magnetic diffusion timeh2=b . 530 108 yr based on the scaleheight of the ionizedgalactic disc, h 4001000 pc, is significantly shorter than thegalactic lifetime. Hence magnetic fields can behave differently atlarge and small scales: they are frozen into the gas at scales smallerthan l (but larger than a fraction of a parsec), whereas at scales largerthan l their behaviour is more independent of the gas densityvariations.

    A striking example of the departure of large-scale galacticmagnetic fields from a tight correlation with the ambient plasma

    density is given by magnetic spiral arms located between thegaseous arms, as discovered in the spiral galaxy NGC 6946 (Beck& Hoernes 1996). A similar phenomenon was observed previouslyin the galaxy IC 342 (Krause, Hummel & Beck 1989a; Krause1993) and suspected for M81 (Krause, Beck & Hummel 1989b) andM83 (Sukumar & Allen 1989). In these galaxies, polarized syn-chrotron emission, a tracer of the large-scale (regular) magneticfield, is (or is suspected) to be strongest in spiral-shaped regions,called magnetic arms, displaced from the gaseous arms. It is stillunclear how widespread this phenomenon is, and which propertiesof the galaxies control the location of magnetic arms with respect tothe gaseous ones.

    The distinction between the scalings of the large-scale and small-scale magnetic fields with density is highlighted by the fact that thetotal equipartition magnetic fields are well correlated with the gasdensity at scales as large as about 1 kpc (Berkhuijsen 1997).Therefore the total magnetic field (regularturbulent) has mildmaxima in the gaseous arms even if the regular magnetic field isdisplaced from them.

    In this Letter we propose a simple explanation of galacticmagnetic arms based on the natural assumption that the generationof the large-scale galactic magnetic field is a threshold phenom-enon, i.e. that the field can be maintained only provided that someparameter exceeds a certain critical value. In the case of mean-fielddynamo theory, this parameter is the dynamo number.

    Our arguments use explicitly the language of mean-field dynamo

    Mon. Not. R. Astron. Soc. 299, L21L24 (1998)

    q 1998 RAS

  • theory. However, we do not rely on any detailed properties of thedynamo, but rather appeal to generic properties of self-excitationsystems, where the magnetic field arises in a simple bifurcationwhen a certain control parameter exceeds the generation threshold.It is only natural to assume that the control parameter decreaseswith v, since v determines the turbulent magnetic diffusivity. Non-linear behaviour of mean-field dynamos is a matter of debate now(see recent reviews by Childress & Gilbert 1995 and Beck et al.1996), but our conclusions are independent of the details of themean-field dynamo theory.

    2 T H E S T E A DY- S TAT E M AG N E T I C F I E L D

    The regeneration of the large-scale (regular) magnetic field in aturbulent medium is a result of competition between the conversionof turbulent kinetic energy into magnetic energy and the turbulentmagnetic diffusion that destroys the field. In the framework of themean-field dynamo theory (Moffatt 1978; Parker 1979; Krause &Radler 1980), the control parameter is the dynamo numberD a0Gh

    3b2, where a0 . l2Q=h is a measure of the deviations

    of the turbulent motions from mirror symmetry, G r dQ=dr is therotational shear, Q is the angular velocity of rotation and r is thegalactocentric distance. Using the above expressions for a0 and bwe obtain (Ruzmaikin, Shukurov & Sokoloff 1988; Beck et al.1996)

    D . 10hv

    2GQ : 1

    The dynamo can maintain the large-scale magnetic field againstturbulent magnetic diffusion provided that jDj $ Dcr with Dcr < 10.For jDj < Dcr , the averaged induction equation has only the trivialsolution B 0 for the large-scale magnetic field B. At jDj Dcr, anon-trivial non-linear solution bifurcates. For jDj slightly aboveDcr, the steady-state strength of the magnetic field is given by

    B < B0jDjDcr

    1 1=2

    for jDj $ Dcr : 2

    Here B0 is the characteristic regular field strength which is plausiblydetermined by equipartition between magnetic and turbulentkinetic energies,

    B0 . K4prv21=2 ; 3

    where r is the total gas density and K is some number of order unity.Equation (2) describes the generic behaviour of the non-linearsolution near the bifurcation. More specifically, it can be obtainedfrom a widespread parametrization of the back-reaction of theLorentz force (which is quadratic in magnetic field) on the turbu-lence in terms of the quenched dynamo number

    DB D 1 B2=B0

    2 1 4(a particular case of this parametrization is alpha-quenching Krause & Radler 1980). A steady state is reached when jDBj Dcr,which immediately yields (2). This simple description of magneticfield generation embraces the main features and, as we shall see, itprovides insight into the origin of magnetic spiral arms.

    3 T H E A R M I N T E R A R M C O N T R A S T I N T H EL A R G E - S C A L E M AG N E T I C F I E L D

    According to simulations of spiral density waves in a cloudyinterstellar medium (Levinson & Roberts 1981; Roberts & Haus-man 1984), a typical arminterarm density contrast for H I clouds is

    ra=ri . 4, where subscripts a and i henceforth refer to regionsinside the gaseous arms and their interarm regions, respectively. Inresponse to the enhanced gas density, young massive stars and H IIregions concentrate in the gaseous arms as star formation is moreactive there. Because of the enhanced supernova rate and morevigorous gravitational interactions between interstellar clouds, theturbulent velocity is larger in the gaseous arms, typically va=vi . 2(Roberts & Hausman 1984). Observational evidence for strongerturbulence in the arms (Rohlfs & Kreitschmann 1987; Garca-Burillo, Combes & Gerin 1993) is less straightforward, albeitcompatible with the above estimate of va=vi. The resulting ratiobetween the dynamo numbers is

    DaDi

    .vahiviha

    2.

    14; 5

    where we have assumed for numerical estimation that ha hi (seeSection 4). Thus the dynamo action in the spiral arms is weaker thanbetween them, provided that v=h in the arms is larger than in theinterarm regions.

    The arminterarm contrast in the large-scale magnetic fieldsfollows from equations (2) and (5) as

    BaBi

    .rari

    1=2vavi

    jDaj=Dcr 1jDij=Dcr 1

    1=26

    . 4jDaj=Dcr 14jDaj=Dcr 1

    1=2:

    Together with equation (2), this relation is applicable whenjDj $ Dcr, and it shows that the field in the arms is weaker thanbetween them, Ba=Bi < 1, if

    jDaj 1 independently of va=vi. This limitingcase may occur in some galaxies and in the central parts of manygalaxies. Although equation (2) is applicable only near the gen-eration threshold this asymptotic result may still be viable, as weonly need that B does not decrease with jDj in the steady state. Note,however, that a steady state with Ba=Bi > 1 may require a time ofseveral Gyr to be established.

    In those galaxies where the dynamo number (1) decreases withradius, magnetic arms can coincide with the gaseous ones in theinner parts of the galaxy and be interlaced in the outer galaxy.Possibly this occurs in M51, where strong la

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