Interplay of CR-driven galactic wind, magnetic field, and galactic dynamo in spiral galaxies
aa r X i v : . [ a s t r o - ph . GA ] J a n Cosmic Magnetic Fields: From Planets, to Stars and GalaxiesProceedings IAU Symposium No. 259, 2009K.G. Strassmeier, A.G. Kosovichev & J.E. Beckman, eds. c (cid:13) Interplay of CR-driven galactic wind,magnetic field, and galactic dynamo in spiralgalaxies
Marita Krause Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germanyemail: [email protected]
Abstract.
From our radio observations of the magnetic field strength and large-scale patternof spiral galaxies of different Hubble types and star formation rates (SFR) we conclude that –though a high SFR in the disk increases the total magnetic field strength in the disk and thehalo – the SFR does not change the global field configuration nor influence the global scaleheights of the radio emission. The similar scale heights indicate that the total magnetic fieldregulates the galactic wind velocities. The galactic wind itself may be essential for an effectivedynamo action.
Keywords. galaxies: spiral – magnetic fields – halos, radio continuum: galaxies
1. Magnetic field strength and star formation
Observations of a sample of three late-type galaxies with low surface-brightness andthe radio-weak edge-on galaxy NGC 5907 (all with a low SFR) revealed that they all havean unusually high thermal fraction and weak total and regular magnetic fields (Chy˙zyet al. 2007, Dumke et al. 2000). However, these objects still follow the total radio-FIRcorrelation, extending it to the lowest values measured so far. Hence, these galaxies havea lower fraction of synchrotron emission than galaxies with higher SFR. It is knownthat the thermal intensity is proportional to the SFR. Our findings fit to the equiparti-tion model for the radio-FIR correlation (Niklas & Beck 1997), according to which thenonthermal emission increases ∝ SF R . ± . and the total magnetic field strength B t increases ∝ SF R . ± . .No similar simple relation exists for the regular magnetic field strength. We integratedthe polarization properties in 41 nearby spiral galaxies and found that (independently ofinclination effects) the degree of polarization is lower ( < L . > × WHz − (Stil et al. 2008). The radio-brightestgalaxies are those with the highest SFR. Though a dynamo action needs star formationand supernova remnants as the driving force for velocities in vertical direction, we con-clude from our observations that stronger star formation seems to reduce the magneticfield regularity. On kpc-scales, Chy˙zy (2008) analyzed the correlation between magneticfield regularity and SFR locally within one galaxy, NGC 4254. While he found that thetotal and random field strength increase locally with SFR, the regular field strength islocally uncorrelated with SFR.
2. Vertical scale heights and CR-driven galactic wind
We determined the exponential scale heights of the total power emission at λ scale heights of the thin disk and the thick disk/halo are similar in this sample (300 pc and 1.8 kpc)(Dumke & Krause 1998, Heesen et al. 2009). We stress that our sample includes thebrightest halo observed so far, NGC 253, with a very high SFR, as well as one of theweakest halos, NGC 4565, with a small SFR.For NGC 253 Heesen et al. (this volume) argued that the synchrotron lifetime (whichis ∝ B − ) mainly determines the vertical scale height of the synchrotron emission andestimated the cosmic ray bulk velocity to 300 ±
30 km/s. As this is similar to the escapevelocity, it shows the presence of a galactic wind in this galaxy. The fact that we observesimilar averaged scaleheights at λ , and hence proportional to SFR . ± . .
3. Magnetic field structure, dynamo action, and galactic wind
In a larger sample of 11 edge-on galaxies we found in all of them (except the innerpart of NGC 4631, see Krause 2009) mainly a disk-parallel magnetic field along thegalactic midplane together with an X-shaped poloidal field in the halo. Our sampleincludes spiral galaxies of different Hubble types and SFR, ranging from 0 . ⊙ yr − SFR
27 M ⊙ yr − . The disk-parallel magnetic field is the expected edge-on projectionof the spiral magnetic field within the disk as observed in face-on galaxies. It is generallythought to be generated by a mean-field α Ω-dynamo for which the most easily excitedfield pattern is the axismmetric spiral (ASS) field (e.g. Beck et al. 1996). The poloidalpart of the ASS dynamo field alone, however, cannot explain the observed X-shapedstructures in edge-on galaxies as the field strength there seems to be comparable tothat of the large-scale disk field. Model calculations of the mean-field α Ω-dynamo for adisk surrounded by a spherical halo including a galactic wind (Brandenburg et al. 1993)simulated similar field configurations as the observed ones. New MHD simulations are inprogress (see e.g. Gressel et al. this volume, Hanasz et al. this volume) which include agalactic wind implicitely. A galactic wind can also solve the helicity problem of dynamoaction (e.g. Sur et al. 2007). Hence, a galactic wind may be essential for an effectivedynamo action, and to explain the observed similar vertical scale heights and X-shapedmagnetic field structure in edge-on galaxies.
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