Characterizing cosmic-ray propagation in massive star-forming regions: the case of 30 Doradus and the Large Magellanic Cloud.
MURPHY E.J., PORTER T.A., MOSKALENKO I.V., HELOU G. and STRONG A.W.
Abstract (from CDS):
Using infrared, radio, and γ-ray data, we investigate the propagation characteristics of cosmic-ray (CR) electrons and nuclei in the 30 Doradus (30 Dor) star-forming region in the Large Magellanic Cloud (LMC) using a phenomenological model based on the radio-far-infrared correlation within galaxies. Employing a correlation analysis, we derive an average propagation length of ∼100-140 pc for ∼3 GeV CR electrons resident in 30 Dor from consideration of the radio and infrared data. Assuming that the observed γ-ray emission toward 30 Dor is associated with the star-forming region, and applying the same methodology to the infrared and γ-ray data, we estimate a ∼20 GeV propagation length of 200-320 pc for the CR nuclei. This is approximately twice as large as for ∼3 GeV CR electrons, corresponding to a spatial diffusion coefficient that is ∼4 times higher, scaling as (R/GV)δ with δ ~ 0.7-0.8 depending on the smearing kernel used in the correlation analysis. This value is in agreement with the results found by extending the correlation analysis to include ∼70 GeV CR nuclei traced by the 3-10 GeV γ-ray data (δ ~ 0.66±0.23). Using the mean age of the stellar populations in 30 Dor and the results from our correlation analysis, we estimate a diffusion coefficient DR~ (0.9-1.0)x1027(R/GV)0.7 cm2/s. We compare the values of the CR electron propagation length and surface brightness for 30 Dor and the LMC as a whole with those of entire disk galaxies. We find that the trend of decreasing average CR propagation distance with increasing disk-averaged star formation activity holds for the LMC, and extends down to single star-forming regions, at least for the case of 30 Dor.
cosmic rays - galaxies: individual: LMC - gamma rays: galaxies - H II regions - infrared: galaxies - radio continuum: galaxies - stars: formation