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For refcode 1989A&A...211..280K:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1989A&A...211..280K A new radio continuum survey of the Magellanic Clouds at 1.4 GHz II. The radio morphology, and thermal and nonthermal emission of the LMC U. Klein, R. Wielebinski, R. F. Haynes, and D. F. Malin Radioastronomisches Institut der Universitat Bonn, Auf dem Hugel 71, D-5300 Bonn 1, Federal Republic of Germany Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-5300 Bonn 1, Federal Republic or Germany CSIRO Division of Radiophysics, P.O. Box 76, Epping, N.S.W. 2121, Australia Anglo-Australian Observatory, P.O. Box 296, Epping, N.S.W. 2121, Australia Received May 16, accepted August 30, 1988 Summary. The distribution of the radio continuum emission of the LMC is investigated. The overall distribution is characterized by an east-west asymmetry: in the west, and especially in the south-west, faint radio emission extends in the direction of the SMC and is partly accompanied by weak optical radiation visible on deep plates. In contrast, the conspicuous weak optical emission extending away from the LMC in the east has no radio counterpart at all. It is argued that the magnetic field is pushed into the LMC there by ram pressure, thus inhibiting the diffusion of relativistic electrons, while towards the SMC the field is dragged into intergalactic space, thus allowing particle diffusion in that direction. Long filaments in the LMC which have been previously identified mainly using young stars as tracers are clearly identifiable in the radio maps of the LMC with sufficient angular resolution, as well as in the IRAS 100 micron and 60 micron maps. Continuations of the two main filamentary features can be seen further out on deep optical plates. Using the radio continuum surveys of the LMC at 408, 1400 and 2300 MHz the distribution of the spectral index as well as a reliable spectrum of the radio emission of the LMC is derived for the first time. The mean spectral index between 20 MHz and 2.3 GHz is <{alpha}> = -0.56 +/- 0.05, significantly flatter than that typical of normal spirals, indicating a larger relative amount of thermal emission. The data are consistent with a nonthermal spectral index {alpha}_nth_ = -0.84 +/- 0.20 (S_{nu}_ is proportional to {nu}^{alpha}^), and a thermal fraction of f_2.3 GHz_ = 65 +/- 20% . For the first time a number for the Lyman continuum flux from the LMC can be assessed. Based on the thermal fraction derived here, this flux amounts to n_Lyc_ ~ 9 10^52^ photon s^-1^, about 1/3 the galactic value. The distribution of the spectral index across the LMC reveals a close positional agreement between flat spectrum and H{alpha} emitting regions. The distribution of the thermal radio emission closely resembles that extrapolated from H{alpha}/H{beta} photometry. The integrated thermal flux density is in good agreement with that obtained from the total radio spectrum of the LMC. Rough estimates of the supernova rate of the LMC indicate that a significant fraction (~50%) of the observed nonthermal emission can be accounted for by supernovae and their remnants (SNR), although only 8% of this emission is found in identified SNR. The total nonthermal radio luminosity derived in the present paper leads to a total equipartition magnetic field strength of ~6 microG, less than that typically found for grand-design spiral galaxies, but comparable to that of the SMC (Loiseau et al., 1987). A comparison with the IRAS 100 micron and 60 micron far infrared (FIR) maps reveals a close correlation with the radio continuum maps. In particular, the filaments and the east-west asymmetry in the brightness as established for the radio continuum is also found for the FIR emission. The ratio of integrated radio-to-FIR luminosity of the LMC is well within the range established for large samples of spiral and dwarf irregular galaxies. We have determined accurate thermal radio flux densities of the most prominent LMC H II complexes. A comparison with the FIR fluxes reveals that the ratio of radio-to-FIR luminosity of LMC H II regions is significantly higher than for galactic H II regions which agrees with the CO deficiency, low heavy-element abundance and high gas-to-dust ratio found for the LMC and dwarf irregular galaxies in general. A marginal tendency is found for the LMC H II regions whose extinction behaviour deviates from uniform interstellar extinction to exhibit lower ratios of radio-to-FIR emission. If confirmed by more accurate investigations with higher resolution this implies that there is dust intrinsic to some of the H II regions of the LMC. Key words: Large Magellanic Clouds - radio continuum - far infrared - star formation
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