Lookup NU author(s): Dr Andrew Fletcher,
Dr Dipayan Mitra
This is the final published version of an article that has been published in its final definitive form by EDP Sciences, 2016.
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Context. Cosmic ray electrons (CREs) are a crucial part of the interstellar medium and are observed via synchrotron emission. While much modelling has been carried out on the CRE distribution and propagation of the Milky Way, little has been done on normal external star-forming galaxies. Recent spectral data from a new generation of radio telescopes enable us to find more robust estimations of the CRE propagation.Aims. To model the synchrotron spectral index of M51 using the diffusion energy-loss equation and to compare the model results with the observed spectral index determined from recent low-frequency observations with LOFAR.Methods. We solve the time-dependent diffusion energy-loss equation for CREs in M51. This is the first time that this model for CRE propagation has been solved for a realistic distribution of CRE sources, which we derive from the observed star formation rate, in an external galaxy. The radial variation of the synchrotron spectral index and scale-length produced by the model are compared to recent LOFAR and older VLA observational data and also to new observations of M51 at 325 MHz obtained with the GMRT.Results. We find that propagation of CREs by diffusion alone is sufficient to reproduce the observed spectral index distribution in M51. An isotropic diffusion coefficient with a value of 6 : 6 +/- 0 : 2 x 10(28) cm(2) s(-1) is found to fit best and is similar to what is seen in the Milky Way. We estimate an escape time of 11 Myr from the central galaxy to 88 Myr in the extended disk. It is found that an energy dependence of the diffusion coefficient is not important for CRE energies in the range 0 : 01 GeV-3 GeV. We are able to reproduce the dependence of the observed synchrotron scale-lengths on frequency, with l proportional to gamma(-1/4) in the outer disk and l proportional to gamma(-1/8) in the inner disk.
Author(s): Mulcahy DD, Fletcher A, Beck R, Mitra D, Scaife AMM
Publication type: Article
Publication status: Published
Journal: Astronomy & Astrophysics
Print publication date: 11/08/2016
Online publication date: 11/08/2016
Acceptance date: 30/04/2016
Date deposited: 06/07/2018
ISSN (print): 0004-6361
ISSN (electronic): 1432-0746
Publisher: EDP Sciences
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