Important Gamma Rays Glow in the Galaxy
Using Fermi Gamma-ray Space Telescope observations of the diffuse gamma-ray emission that fills our Galaxy, scientists are learning crucial information about the gas, dust, and high energy charged particles that form the interstellar medium of space.
Fermi-LAT observed gamma-ray counts in the energy range from 200 MeV to 100 GeV. The signal is dominated by the diffuse Galactic emission, which is strongest in the plane of our Galaxy and toward the Galactic center but present all over the sky.
The Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope, which was built at SLAC and in which KIPAC is the lead scientific institution, has revolutionized gamma-ray astronomy and provided a new window onto the Universe in the highest energy kind of light. It has observed gamma rays from the relentless regularly beeping pulsars in our own Galaxy, from our galactic neighbors which have not been seen in gamma rays before, and the blazars arising from the enormous jets of supermassive black holes in the distant Universe. However, it is fascinating to consider that even given the over a hundred pulsars, several external galaxies, and over a thousand blazars that the Fermi-LAT has seen, only about 10% of the photons it has detected from the cosmos have come from these identified sources. Another 15% of the LAT's total observed gamma rays are from the extragalactic diffuse emission, a glow that pervades the Universe and comes from distant sources too dim to see individually. Exactly what combination of distant objects is responsible the extragalactic diffuse emission is a cosmic mystery in itself which has been the subject of many papers. Finally, almost three-fourths of the LAT's total gamma rays are from yet another fascinating source, the Galactic diffuse emission which comes from all over our Galaxy.
The Galactic diffuse gamma-ray emission results primarily from high energy charged particles flying through our Galaxy and interacting with other matter. The charged particles, known as cosmic rays, have been flung outward by the winds from pulsars, the remnants of supernova explosions, and other powerful astrophysical sources. Beyond this 'standard' Galactic diffuse emission, there may be an additional contribution from annihilating dark matter particles or the black hole in the center of our Galaxy. One of the major science goals of the Fermi-LAT collaboration is to use Fermi's measurements of the Galactic diffuse gamma-ray emission to constrain its origin and the properties of the interstellar medium that permeates our Galaxy. In addition to deepening our understanding of the interstellar medium, a full understanding of the diffuse Galactic emission is crucial for interpreting any potential dark matter annihilation signals, Galactic or extragalactic, that Fermi may hone in on.
The most comprehensive study of the Galactic diffuse emission to date, using 21 months of LAT data, has been carried out by the Fermi-LAT team, and led by former KIPAC postdoc and current affiliate Gudlaugar Johannesson, now at the University of Iceland, KIPAC scientist Troy Porter, and Andrew Strong of the Max Planck Institute in Garching, Germany. The team has used the LAT observations of the Galactic diffuse emission to determine such quantities as the energy spectrum and composition by particle type of cosmic rays, the density and gas and dust properties of the interstellar medium, and the propagation of cosmic rays through it. An important tool in the analysis is the GALPROP computer code which simulates the interactions of cosmic rays and the interstellar medium given the known physics, and which was developed by Strong and KIPAC scientist Igor Moskalenko.
The LAT team's new analysis leads to several conclusions. One is that they favor a larger than previously expected density of cosmic rays in the outer regions of the Galaxy and out of the Galactic plane. Another is that estimates of the densities of gas in the Galaxy obtained traditionally from the direct observation of emission from neutral atomic hydrogen and carbon monoxide need to be augmented with considerations based on the absorption of interstellar dust particles in order to match the observed diffuse gamma-ray data. The team also shows that Galactic cosmic rays radiate a larger fraction of their energy through interactions with the ambient infrared and optical light than previously assumed. Another example of the conclusions obtained is that the composition of molecular clouds of gas in local regions of the Galaxy is different on average than in the Galaxy as a whole, having a lower level of emission from carbon monoxide relative to atomic hydrogen. The diffuse Galactic gamma-ray emission that Fermi has zeroed in on is a crucial input for studying these and other properties of the interstellar medium of our Galaxy, whose secrets are well guarded.
This work is based in part on a paper accepted to The Astrophysical Journal and available from astro-ph at arXiv:1202.4039.
Tidbit author: Jack Singal