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Magnetic fields and spiral arms in the galaxy M51 | | A. Fletcher
; R. Beck
; A. Shukurov
; E. M. Berkhuijsen
; C. Horellou
; | | Date: |
28 Jan 2010 | | Abstract: | (Abridged) We use new multi-wavelength radio observations, made with the VLA
and Effelsberg telescopes, to study the magnetic field of the nearby galaxy
M51. We compare the spatial distribution of the radio emission with
observations of the neutral gas, derive radio spectral index and Faraday
depolarization maps, and model the large scale variation in Faraday rotation in
order to deduce the structure of the regular magnetic field. We derive two
components for the regular magnetic field in this galaxy: the disc is dominated
by a combination of azimuthal modes, m=0+2, but in the halo only an m=1 mode is
required to fit the observations. We suggest a simple method to estimate the
turbulent scale in the magneto-ionic medium from the dependence of the standard
deviation of the observed Faraday rotation measure on resolution. We thus
obtain 50pc for the size of the turbulent eddies; to our knowledge, this is the
first determination of the turbulent scale in an external galaxy. We disuss how
the observed arm-interarm contrast in radio intensities can be reconciled with
evidence for strong gas compression in the spiral shocks. In the inner spiral
arms, the strong arm--interarm contrasts in total and polarized radio emission
are roughly consistent with expectations from shock compression of the regular
and turbulent components of the magnetic field. However, the average
arm--interam contrast, representative of the radii r>2kpc where the spiral arms
are broader, is not compatible with straightforward compression: lower
arm--interarm contrasts than expected may be due to resolution effects and
decompression of the magnetic field as it leaves the arms. | | Source: | arXiv, 1001.5230 | | Services: | Forum | Review | PDF | Favorites | |
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