Characterizing cosmic ray propagation in massive star forming regions: the case of 30 Dor and LMC E. J. Murphy et al. Arxiv:

background CRs interactions with the interstellar gas, radiation and, magnetic fields, can produce diffuse emissions from the radio to high-energy. E.g.,the synchrotron radio emission arising from the injection of CR electrons from supernova remnants. High-energy rays gives access to the dominant hadronic component in CRs via the observation of gamma rays from the decay of neutral pions produced by inelastic collisions between CR nuclei and the interstellar gas.

background Tight empirical correlation between the far- infrared and non-thermal radio continuum emission from galaxies has been proved. The underlying physics relating the process of : young massive stars dust heating inject CR electrons FIR synchrotron emission

background Because the mean free path of dust-heating photons ( ∼ 100 pc) is significantly shorter than the typical diffusion length of CR electrons ( ∼ 1 − 2 kpc). There is hypothesis that the radio image of a galaxy should resemble a smoothed version of its infrared image. This is supported by studies within nearby galaxies at kpc or few 100 pc scales, as well as in the Large Magellanic Cloud (50 kpc). Investigations within a sample of nearby spirals have even shown a dependence of the typical propagation length of CR electrons on star formation activity arising from the predominant youth of CR electron populations in galaxies with enhanced disk-averaged star formation rates.

data γ-ray : 32 months 1-10 GeV data from Fermi- LAT Infrared: Spitzer IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS bandpasses (24, 70, and 160 μm) Radio: 1.4 GHz data of Parkes 64m and interferometric data of ATCA

Image registration and smearing analysis All images cropped to the same field-of-view, regrided to common pixel scale, adjusted to the same resolution by convolving appropriate PSF. = ∫I j (l)K(r)dr Gaussian kernel Exponential kernel is the e-folding scale length.

Smearing analysis Normalized-squared residuals: here The minimum φ defines the best-fit scale length, which is taken to be the typical distance traveled by the CR electrons.

Radio imageInfrared image

Infrared-radio residualInfrared-γ-ray residual

results The corresponding best-fit exponential and Gaussian kernel scale lengths between the free- free corrected 1.4 GHz and smoothed 24 μm maps are and pc. ∼ 3 GeV CR electrons Comparison between the 1 − 3 GeV γ-ray maps and 24 μm maps, the corresponding best-fit exponential and Gaussian kernel scale lengths are and pc. ∼ 20 GeV protons

The 20 GeV CR protons are found to travel ∼ 2 times further(assuming a proton energy index of 2.1). This may because of different energy CR particle populations, arising from different diffusion speeds.

Summary Using a phenomenological image smearing model, estimated the typical propagation length of ∼ 3 GeV CR electrons to be pc and ∼ 20 GeV CR nuclei to be pc. To 1.4 GHz and 24 μm maps, exponential kernels work slightly better than Gaussian kernels. In contrast, exponential and Gaussian kernels work equally well to the 1 − 3 GeV -ray and 24 μm maps. This difference suggests that, unlike the CR nuclei, CR leptons suffer additional energy losses as they propagate through the ISM near 30 Dor on timescales less than, or comparable to the diffusion timescale.