J/A+A/592/A123ivo://CDS.VizieR/J/A+A/592/A123doi:10.26093/cds/vizier.35920123CDSMulcahy D.D.Fletcher A.Beck R.Mitra D.Scaife A.M.M.2017-09-12T12:46:15Z2016-08-11T07:23:32ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frgalaxiesradio-continuum-emissionCosmic ray electrons (CREs) are a crucial part of the ISM 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 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. 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 325MHz obtained with the GMRT. 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+/-0.2Dunits is found to fit best and is similar to what is seen in the Milky Way. We estimate an escape time of 11Myr from the central galaxy to 88Myr in the extended disk. It is found that an energy dependence of the diffusion coefficient is not important for CRE energies in the range 0.01GeV-3GeV. We are able to reproduce the dependence of the observed synchrotron scale-lengths on frequency, with l{propto}{nu}^-1/4^ in the outer disk and l{propto}{nu}^-1/8^ in the inner disk.2016A&A...592A.123Mhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/592/A123CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/568/A74 : Low-frequency (115-175MHz) image of M51 (Mulcahy+, 2014)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/592/A123https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/592/A123http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/592/A123https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/592/A123https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/592/A123http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/592/A123GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/592/A123/list (Information on fits image)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/592/A123/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/592/A123/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/592/A123/list?GETtext/xml+votable180.050000true202.4685647.195130.005555555555555556Associated datahttps://cdsarc.cds.unistra.fr/assocdata/?obs_collection=J/A+A/592/A1236/11027https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/592/A123?format=asciiRadiodefaultJ/A+A/592/A123/listInformation on fits imagerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right Ascension of center (J2000)degpos.eq.ra;meta.mainDEJ2000Declination of center (J2000)degpos.eq.dec;meta.mainNxNumber of pixels along X-axismeta.numberintNyNumber of pixels along Y-axismeta.numberintObs.dateObservation datetime.epoch;obsnullableFreqObserved frequencyMHzem.freqfloatsizeSize of FITS fileKibytephys.size;meta.fitsintFileNameName of FITS filemeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitschar