Whether Fermi Sees Galaxies In Gammas Or Not, We Learn A Lot

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Gamma rays are the highest energy kind of light and are therefore frequently associated with spectacular phenomena of intensely beamed energy in the Universe such as pulsars, blazars, and gamma ray bursts. The study of pulsars and blazars has been revolutionized by the Fermi Gamma-ray Space Telescope, and its major science instrument the Large Area Telescope (LAT). However, fully 75% of the total gamma rays received by the LAT are not from these beamed behemoths near and far but rather come from the diffuse gamma-ray glow of our Milky Way Galaxy. The Galactic gamma-ray glow is the result of cosmic ray charged particles from supernova explosions flying around and slamming into gas in the Galaxy, causing the production of pions which decay into gamma rays, in a process reminiscent of the decays and reactions familiar from particle physics experiments.

That our own Galaxy produces such a gamma-ray glow across our sky implies that other similar galaxies do as well. Because we are not in them and they are farther away, seeing the gamma rays from these galaxies is more difficult. Since more star formation means more supernovae which means more cosmic rays, the galaxies that are brightest in gamma rays will presumably be those forming the most stars, and also those with the most gas clouds for cosmic rays to crash into.

Following on the heels of previous confirmations of gamma rays from from our Milky Way's three closest neighbors and four farther 'starburst' galaxies, the Fermi-LAT team set out to see what other galaxies like these had to tell us in gamma rays. The analysis was lead by KIPAC graduate student Keith Bechtol and Professor Stefan Funk.

The LAT team combed the LAT gamma-ray data for 64 galaxies that have infrared and radio emission suggesting star formation activity and the presence of gas clouds. While none of the 64 had enough gamma-ray emission to be considered a verified source by the LAT's standards, the upper limits to their gamma-ray brightness provide crucial information, especially when combined with the seven galaxies - eight including the Milky Way itself - with previously confirmed gamma-ray detections.

There appears to be a simple scaling relation between gamma-ray luminosity and star-formation rate as estimated by radio or infrared luminosity. Given this relation, starburst galaxies seem to be able to convert, on average, between 30 and 50 percent of the energy of their high energy cosmic rays into gamma rays before the cosmic rays escape. The team also used the data to estimate the contribution of non-active galaxies to the diffuse extragalactic gamma-ray background radiation (EGRB) which fills the Universe. Most of the EGRB seems to come from blazars, but how much comes from regular - non-active - galaxies is a subject of much debate. The analysis used the correlations between gamma-ray emission and that at other wavelengths of the eight detected galaxies along with the upper limits for the other galaxies to conclude that between 4 and 23 percent of the EGRB comes from non-active galaxies. Another intriguing conclusion of the analysis is that if LAT data is collected for 10 years, there will be enough photons to confirm detections for around 10 additional galaxies, which will allow more precise determinations of the cosmic ray behavior of galaxies and their contributions to the EGRB.

This work is described in a paper to appear in the Astrophysical Journal and available from astro-ph at arXiv:1206.1346. The Fermi-LAT was designed and built by an international collaboration with members from France, Italy, Japan, Sweden and the United States. SLAC managed the construction and integration of the instrument and KIPAC is the lead for ongoing operation of the instrument. Research at KIPAC is supported by the Department of Energy, the Kavli Foundation, the National Aeronautics and Space Administration, the National Science Foundation and Stanford University, as well as private donors. We are grateful to each of these sponsors for their continued interest and support.

Science Contact:
Keith Bechtol
KIPAC
email: bechtol@stanford.edu

Tidbit Author: Jack Singal