This week NASA’s Fermi space telescope made a discovery that is perplexing scientists around the world. Fermi is a space telescope which detects gamma ray radiation - the most energetic form of electromagnetic radiation. In fact it is billions of times more energetic than the type of light visible to our eyes.
This means that Fermi sees the immense energy of the most exotic and energetic phenomenon in our Universe: super massive black holes, pulsars and streams of hot gas travelling at close to the speed of light. This week Fermi and the astronomers at the Harvard -Smithsonian Center for Astrophysics discovered an astounding structure right in our own Galactic back yard. They discovered huge gamma ray emitting “bubbles” which can best be shown in this image:
The purple bubbles show this incredible and unexpected structure in our Galaxy. Here we are looking at our Milky Way Galaxy edge-on with the “bubbles” emanating from the center. The structures extend 25,000 light years (see box below) to the North and South of the center of our Galaxy. That is quite a significant structure not to have been aware of, but many astronomical phenomenon do not show themselves unless we use the right wavelength of light to detect them – in this case: gamma rays.
In total the two bubbles span 50,000 light years or half the diameter of our Galaxy. They have well defined edges and are expanding at a rate of about 2.2 million miles per hour. As mentioned already, gamma rays are emitted from the most energetic things in the universe and here we are looking at structures that hold the energy of about 100,000 supernovae. Earlier surveys aimed at detecting X-ray emission gave a hint to some sort of structure which astrophysicists assumed may also emit gamma rays. They did not however expect anything like the scale of these huge bubbles.
So what in the world (or more aptly, Galaxy) could possibly have produced so much energy?
So far astronomers are considering two possible explanations.
One explanation suggests that at some time there was a “burst” of star formation occurring near the center of the Galaxy which may have produced massive short-lived stars which in turn produced energetic winds (like a much stronger version of our Sun’s “solar wind”) capable of blasting high energy particles out into space and forming these gamma ray bubbles.
Doug Finkbeiner, an astronomer with the Harvard Smithsonian Center for Astrophysics and part of the team that made the discovery, suggests another option may be more plausible. This one is even wilder – and involves an outburst from the supermassive black hole that lives at the center of our own Milky Way. Most if not all, galaxies are thought to harbor a black hole at their center and many of these black holes are associated with high energy “jets” which eject material out of the black hole. This illustration (left) shows the concept of a black hole where there is a spinning disk of material being drawn into the black hole (the accretion disk) and at the same time powerful jets shooting high energy particles out of the back hole in opposite directions.
There is a catch – our Galaxy’s black hole is not see to possess these high energy jets. But at 400 million times the mass of the Sun, our own “local” black hole has probably been very active in its past.
In fact, these bubbles may be the first real “evidence” of an outburst at some time in the past where the black hole was accreting material at such a rate that it was spewing high energy particles back out in the form of jets which formed the structures we are now seeing – these so-called ”Fermi bubbles”. Estimates suggest it would only take a period of 10,000 to 100,000 years to produce enough energy to create these structures. (Our Galaxy is about 13.2 billion years old!)
At any rate, it is exciting to have Fermi reveal to us such enormous structures which may have been part of our Galaxy for millions of years.
Sometimes, all you need is a fresh set of (gamma ray sensing) eyes.
NASA’s Fermi is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States. Read more about Fermi at: http://fermi.gsfc.nasa.gov/
The research discussed has been accepted for publication in The Astrophysical Journal