Astronomers have used the Hubble Space Telescope to study the Fermi Bubbles at the center of the Milky Way in order to learn more about their velocity and composition. This data will hopefully help answer some questions about the origin of these mysterious entities. The study was led by Andrew Fox of the Space Telescope Science Institute (STScI) in Baltimore. The paper will be published in The Astrophysical Journal Letters, and was presented on Monday at the 225th meeting of the American Astronomical Society in Seattle.
The Fermi Bubbles are two giant gamma-ray outbursts protruding 25,000 light-years above and below the core of the Milky Way. They were only discovered in 2010, so there’s still a lot of mystery about them. They are believed to be remnants from a massive explosion that occurred at the galaxy’s center about 2 million years ago, but it isn’t clear if it was caused by the formation of the supermassive black hole or a copious amount of star formation.
Previous research has observed the Fermi Bubbles in radio and x-ray wavelengths, but this new study used data from Hubble’s Cosmic Origins Spectrograph instrument in order to view them in the ultraviolet light. This light used to make the observations actually originated from a far-off quasar. Quasars are well-studied outbursts of light from the centers of other galaxies, making it serendipitous that it can be used to help answer questions about the core of our own galaxy.
Image credit: NASA, ESA, and A. Feild (STScI)
"When you look at the centers of other galaxies, the outflows appear much smaller because the galaxies are farther away," Fox said in a press release. "But the outflowing clouds we're seeing are only 25,000 light-years away in our galaxy. We have a front-row seat. We can study the details of these structures. We can look at how big the bubbles are and can measure how much of the sky they are covering.”
Fox’s team was able to find that the gas of the Fermi Bubbles is moving from the center of the galaxy at a rate of about 3 million km/h (2 million mph), which is about like flying from New York to L.A. in five seconds. They also confirmed that some of the gas is moving in our direction, while other gas is moving in the opposite direction.
"This is exactly the signature we knew we would get if this was a bipolar outflow," added co-author Rongmon Bordoloi, also from STScI. "This is the closest sightline we have to the galaxy's center where we can see the bubble being blown outward and energized.”
The composition of the gas included carbon, silicon, and aluminum. These elements are too heavy to have been created anywhere else except in the cores of stars. The temperature of the gas was found to be roughly 9,700 degrees Celsius, which is about a tenth of what is in the outflow.
"We are seeing cooler gas, perhaps interstellar gas in our galaxy's disk, being swept up into that hot outflow," Fox explained.
The current paper is based off of the results from the light of one quasar, though there will ultimately be 20 quasars utilized. These different sources will allow for a more thorough investigation. As the quasars’ light will come from different angles and sample different parts of the Fermi Bubbles, a more complete picture can be made. Ultimately, the data will be combined in order to determine the mass, which will play a big role in understanding their origin.