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Characterizing cosmic ray propagation in massive star

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Characterizing cosmic ray
propagation in massive star forming
regions: the case of 30 Dor and LMC
E. J. Murphy et al.
Arxiv:1203.1626
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 farinfrared 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 FermiLAT
• 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.
= ∫Ij(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 image
Infrared image
Infrared-radio residual
Infrared-γ-ray residual
results
• The corresponding best-fit exponential and
Gaussian kernel scale lengths between the freefree 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 100-140 pc and ∼ 20 GeV CR nuclei to
be 200-320 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.
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