• Authors: E.J. Murphy, T.A.Porter, I.V.Moskalenko, G.Helou, and A.W.Strong
• First Author’s Institution: Observatories of the Carnegie Institution for Science, Pasadena, CA
Cosmic rays have recently become a key ingredient in allowing us to learn about the interstellar medium. Many particles, however, can be called cosmic rays, whether they are simply protons, electrons, or atomic nuclei. When studying cosmic rays it is therefore important to characterize the type of cosmic ray as this causes their origin and propagation in the interstellar medium to differ. Recent successes have led us to a better understanding of their role in galaxy formation.
The role cosmic rays play within a galaxy is dynamically important. Not only is the interstellar medium partially composed of cosmic rays (as well as dust, and gas) but cosmic rays also heat the interstellar medium. They have the ability to penetrate deep into molecular clouds where they transfer energy to the gas via excitation or ionization of the free electrons. Essentially, their energy density is similar to that of magnetic fields, radiation fields, and even the turbulent motions of the interstellar gas. Thus, both the cosmic rays and the magnetic fields create a relativistic plasma whose interactions with the interstellar gas shape the overall chemistry of the interstellar medium.
Due to the varying types of cosmic rays, we can observe these interactions across the electromagnetic spectrum. For example, those produced from supernova remnants can be viewed in the radio, while those that scatter off the interstellar radiation field can be viewed from X-ray to Gamma-ray energies. Not only can we view the cosmic rays across the electromagnetic spectrum but there are also relationships between various parts of the spectrum.
There is a clear empirical correlation between the infrared and radio continuum emission in galaxies. The link? Cosmic rays. Young massive stars heat the dust in the interstellar medium, which produces infrared emission. These stars then end their lives as supernovae, which produce cosmic ray electrons. These electrons, in turn, interact with the interstellar medium (more precisely, its magnetic field), which then produces synchrotron emission in the radio. Hence, there is a correlation between the infrared and radio emission in galaxies.
But what exactly is this correlation? It is a spatial correlation; it has been hypothesized that in comparison to an infrared image of a galaxy, the radio image should be its smooth counterpart. This is because the mean free path of the dust-heating photons produced originally in the young massive stars is much less than the mean free path of the cosmic ray electrons. In other words, the dust-heating photons move for much shorter distances before interacting with another particle or field than the cosmic ray electrons do. So the infrared image caused by the dust-heating photons will be much clearer than the radio image caused by the cosmic ray electrons. This has been observed in multiple nearby galaxies.
The authors of this paper take a closer look at the star-forming region 30 Doradus, located in the Large Magellanic Cloud. They compiled infrared, radio, and gamma ray data from both Fermi (a space-based gamma ray telescope) and Spitzer (a space-based infrared telescope). The team analyzed images of 30 Doradus that were cropped to a common field of view and regridded to a common pixel scale for easy comparison.
More specifically the team looked at the morphologies of 30 Doradus as measured by warm dust at 24 microns, synchrotron emission at 1.4 GHz, and gamma-ray emission at 1-3 GeV. We expect a correlation between all three.
The primary results depicted in Figure 1 show that the peak of the gamma ray emission is clearly offset from the peak of the radio and infrared emission. The difference is hypothesized to be due to the different cosmic rays. The gamma ray emission is due to the cosmic ray nuclei, while the radio emission is due to the cosmic ray electrons. This suggests that the differences in the appearance between the gamma ray and radio images are due to differences in the distributions of the cosmic ray nuclei and the cosmic ray electrons. It is necessary then to look at these different cosmic rays and analyze their propagation lengths.
The Large Magellanic Cloud
Assuming that the cosmic ray electrons and nuclei are produced by the same sources, and that they have been propagating through the interstellar medium for the same amount of time, the authors found the propagation distances of cosmic ray electrons to be much shorter than those of cosmic ray nuclei. In fact cosmic ray electrons propagate for 100 – 140 pc, while cosmic ray nuclei propagate for 200 – 320 pc.
The authors compared propagation distances for the entire Large Magellanic Cloud. It has previously been noted that there is a correlation between the cosmic ray electron propagation distance and the galaxy surface brightness. The slope in Figure 2 shows that as the propagation lengths of cosmic ray electrons shorten, the star formation rate increases. The team verified this slope and placed the the Large Magellanic Cloud slightly beneath the line and 30 Doradus along the line (as can be seen in Figure 2). Think about it this way: if there is a high star formation rate within a galaxy, a cosmic ray electron will most likely interact with something earlier on.
All in All
Cosmic rays yield vast amounts of information about the interstellar medium through their various interactions. Emitted across the electromagnetic spectrum, the varying types depict varying processes within galaxies. The case of 30 Doradus in the Large Magellanic Cloud shows clear evidence for the infrared-radio correlation present in galaxies as well as the link between cosmic ray electron propagation distance and star formation rates. The varying types of cosmic rays are only beginning to prove to be unique probes in studying varying astrophysical processes throughout the Universe.
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