Fermi LAT studies Cosmic-Ray Interactions in Distant Galaxies

May 5, 2015

The Fermi LAT has observed, for the first time, gamma-rays produced in cosmic-ray interactions in several neighboring galaxies - and is even able to spatially resolve one of those galaxies. This has given us a unique global view of cosmic ray acceleration, that previous Milky Way studies could not provide.

Gamma-ray emission from the LMC

While the Fermi LAT has found more than a thousand individual gamma-ray sources in its first year of survey, the vast majority of the observed gamma-ray flux is diffusely distributed across the sky, produced by the interaction of cosmic rays with the interstellar medium and radiation field. Cosmic rays are particles that are accelerated to high energies by interstellar shock waves, which are generated, for example, in supernova explosions. The link between cosmic-ray acceleration and star formation and death, as well as the distribution and propagation of the cosmic rays inside galaxies, is a scientific topic of considerable interest. Cosmic-ray interactions are an important ingredient in the dynamics and evolution of galaxies: the energy density in cosmic rays is similar to the energy density in the magnetic field and in the turbulent motions of the interstellar gas. Cosmic rays are key sources of heating and ionization of the interstellar medium, and may also play a significant role in the regulation of star formation during the evolution of galaxies.

The sensitivity of the LAT allows us for the first time to study gamma rays from the interactions of cosmic rays in not only our own, but also several external galaxies. The outside viewpoint enables us to accurately determine the luminosity of these Galaxies in gamma rays, and to investigate the correlations of the gamma-ray luminosity with the mass of the interstellar gas in these galaxies, as well as their star formation rate. The galaxies observed so far are the relatively nearby Milky Way satellites, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), plus two more distant "Starburst" galaxies, which feature a very high star formation activity. For these galaxies (including the Milky Way), an analysis with significant contribution from KIPAC researcher Keith Bechtol finds a very good correlation between the gamma ray luminosity of a galaxy and its rate of star formation. In the case of the LMC, it is even possible to resolve the gamma-ray emission and study in some depth the spatial correlation of gamma-ray intensity with the distribution of gas and individual regions of high star formation activity within this galaxy. Again, a good spatial correlation of the gamma-ray emission with the star forming regions is found.

There is a second important aspect to the gamma-ray emission from cosmic-ray interaction in galaxies. While the LAT can now resolve the closest galaxies, trillions of them remain unresolved, but nevertheless contribute to the diffuse extragalactic gamma-ray background (EGB). A previous LAT study showed that after taking into account the known populations of gamma-ray emitters, more than half of the observed diffuse extragalactic emission is still unaccounted for: the determination of the luminosity of the galaxies in our neighborhood provides an important input to the modeling of the total contribution of star-forming galaxies to the EGB. If such galaxies still cannot provide the gamma-rays not yet accounted for, then less conventional gamma-ray sources like annihilating or decaying dark matter may be needed to explain the observed EGB. 

The on-going sky survey of the LAT continuously improves the LAT sensitivity, and in the near future will provide an even larger sample of galaxies to further improve the accuracy of these studies.