For the first time, thanks to the Fermi Space Telescope, high energy gamma rays gave been detected coming from another spiral galaxy much like our own Milky Way. It is now evident that the differences in gamma-ray luminosity among galaxies show that the density of cosmic rays varies and is correlated with the formation of new stars.
The intensity of gamma-rays from outside of our Galaxy seen by the Fermi space telescope, in the direction of the Andromeda galaxy. The contour lines show the position of the Andromeda Galaxy in infrared emission.
In its two years of operation so far, the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope has seen gamma rays - very high energy photons of light - coming from hundreds of objects outside of our Milky Way galaxy. Almost all of these objects are "blazars", giant black holes which gobble up matter and eject beams of high-energy particles pointed towards us. However, blazars are found in only a small fraction of galaxies; most galaxies, like our own, don't go for that sort of drama.
Although galaxies without these giant black holes are relatively faint in gamma rays, the LAT has accumulated enough data to see some of them. There is, of course, diffuse emission produced by our Milky Way Galaxy which can be seen in great detail, as well as from the Large and Small Magellanic Clouds, two smaller satellite galaxies of ours. Beyond that, the LAT team has previously reported emission from M82 and NGC253, two so-called "starburst" galaxies which are in the process of forming a lot of stars. In spite of these achievements, it is challenging to gain a global picture of our own Galaxy precisely because of our perspective from within the disk. In order to better understand the processes that lead to gamma-ray emission in galaxies, it would be very useful to see and characterize it from an external galaxy more similar to our own.
Fermi has now done exactly that. With an analysis effort led by KIPAC graduate student Keith Bechtol along with Pierrick Martin at the Max Planck Institute in Germany and Pierre Jean and Jurgen Knodlseder at the Center for Space Radiation Studies in France, the Fermi collaboration has detected and measured the gamma-ray output of the Andromeda galaxy, the closest neighbor and 'sister' to our own.
The high energy gamma-ray luminosity of Andromeda, also known as M31, is about a third that of our Milky Way, even though Andromeda is slightly larger. This is very important information, because interesting astrophysical processes give rise to gamma-ray emission from galaxies. Diffuse gamma-ray emission is the result of cosmic rays - high energy charge particles - interacting with the atoms between stars. Gamma-ray emission is therefore a tracer of cosmic rays and the mechanisms that produce them, and a vital one, because cosmic rays themselves get deflected by the magnetic field of our Galaxy and can't usually be traced back to their origin.
When the gamma-ray luminosities of M31 and the other detected galaxies are compared, an interesting correlation can be seen. The gamma-ray production is higher in galaxies where more new stars are being created. The new stars themselves are not the primary source of the high energy cosmic rays in question, but they are linked to other phenomena that are important in creating and accelerating cosmic rays, such as supernovae. A fuller interpretation will be possible as more data is collected by Fermi. The comparison between galaxies is just beginning.
This work is described in a paper in Astronomy and Astrophysics (A&A, 2010, 523, L2).
Tidbit author: Patrick Nolan and Jack Singal