2007A&A...470..539B


Query : 2007A&A...470..539B

2007A&A...470..539B - Astronomy and Astrophysics, volume 470, 539-556 (2007/8-1)

Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes, and helical fields.

BECK R.

Abstract (from CDS):

The spiral galaxy NGC 6946 hosts magnetic spiral arms, highly aligned magnetic fields between the gas/optical arms. The origin of the magnetic phenomena and their relation to the interstellar gas are investigated. NGC 6946 was observed in total intensity and linear polarization in five radio bands between 3 cm and 21 cm. Maps of spectral index, Faraday rotation and depolarization were derived. At the inner edge of the inner gas spiral arm the ordered magnetic field is only mildly compressed and turns smoothly, to become aligned along the gas arm. Hence the field is not shocked and is probably connected to the warm, diffuse gas. At larger radii, two bright magnetic arms between the optical arms are visible in polarized intensity. The field in the northern magnetic arm is almost totally aligned. Faraday rotation measures (RM) in these arms are consistent with the superposition of two low azimuthal dynamo modes. Three more magnetic arms are discovered in the outer galaxy, located between HI arms. The RM structure function confirms large-scale coherent fields. The observed anti-correlation between the field's pitch angles and the RM values is a possible signature of helical fields. - Due to strong Faraday depolarization the galaxy is not transparent to polarized waves at λ18cm and λ20cm. The large-scale asymmetry in depolarization with respect to the major axis may be another indication of large-scale helical fields. Three depolarization rings of almost zero polarization seen at λ20cm are probably generated by differential Faraday rotation in HII complexes in NGC 6946 of 300-500pc size. - In the gas/optical spiral arms, the total (mostly turbulent) magnetic field is amplified to ≃20µG. Its energy density is ≃10 times larger than that of the ionized gas and is similar to that of the turbulent gas motions in the inner galaxy. The magnetic energy exceeds that of the turbulent energy in the outer galaxy. All energy densities in NGC 6946 are about one order of magnitude larger than those in the Milky Way. Density waves in the inner gaseous spiral arms mildly compress the field. Dynamo action probably generates the magnetic spiral arms. The magnetic field is dynamically important, interacts with the gas flow and possibly determines the properties of the gas spiral arms.

Abstract Copyright:

Journal keyword(s): galaxies: spiral - galaxies: magnetic fields - galaxies: ISM - galaxies: individual: NGC 6946 - continuum: galaxies - radio continuum: ISM radio

Nomenclature: Text, paragr.3.7: [B2007b] source A (Nos A-B), [B2007b] JHHMMSS.s+DDMMSS N=1.

Simbad objects: 17

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Number of rows : 17
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2023
#notes
1 M 31 G 00 42 44.330 +41 16 07.50 4.86 4.36 3.44     ~ 12107 1
2 M 33 GiG 01 33 50.8965749232 +30 39 36.630403128 6.17 6.27 5.72     ~ 5610 1
3 NGC 1097 LIN 02 46 19.059 -30 16 29.68 10.46 9.97 9.48 8.72 9.8 ~ 1278 3
4 NGC 1365 Sy1 03 33 36.458 -36 08 26.37 10.48 10.08 9.63 8.79 9.7 ~ 1687 2
5 IC 342 SBG 03 46 48.514 +68 05 45.98   10.5       ~ 1470 1
6 ICRF J052109.8+163822 Sy1 05 21 09.88596258 +16 38 22.0515459   19.37 18.84 18.480   ~ 819 1
7 NGC 2997 GiG 09 45 38.7542237472 -31 11 27.345810192   9.97 9.41 8.84 9.9 ~ 403 0
8 M 81 Sy2 09 55 33.1726556496 +69 03 55.062505368   7.89 6.94     ~ 4265 3
9 NGC 4449 EmG 12 28 11.118 +44 05 36.81 10.11 9.98 9.64 9.45   ~ 1086 1
10 M 51 GiP 13 29 52.698 +47 11 42.93   9.26 8.36 8.40   ~ 4165 4
11 3C 286 Sy1 13 31 08.2883506368 +30 30 32.960091564   17.51 17.25     ~ 4172 2
12 M 83 SBG 13 37 00.91920 -29 51 56.7400 8.85 8.11 7.52 7.21   ~ 2456 2
13 ICRF J202206.6+613658 Sy2 20 22 06.68174620 +61 36 58.8047323     19.5 17.58   ~ 312 1
14 [B2007b] source A Rad 20 34 24.2 +60 10 41           ~ 1 0
15 [B2007b] source B Rad 20 34 26.1 +60 10 32           ~ 1 0
16 NGC 6946 H2G 20 34 52.332 +60 09 13.24   10.5       ~ 2428 2
17 [B2007b] J203519.2+600208 Rad 20 35 19.2 +60 02 08           ~ 1 0

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2023.02.04-10:37:15

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