There are two enormous outflows of high-energy gas above and below the disk of the Milky Way. They come from a powerful flare by the galaxy’s supermassive black hole and extend for about 25,000 light-years. They are known as the Fermi Bubbles after the telescope that first observed them.
The Bubbles were discovered a decade ago, and since then scientists have been trying to understand their properties and origin. Not an easy task. The gas in question is rarefied, making direct detection difficult. But there are ways around this conundrum. One is with the all-sky Wisconsin H-Alpha Mapper, which has garnered astronomers the first optical measurements of the Fermi Bubbles' properties.
The optical measurements focused on the subtle emission of light from hydrogen and nitrogen in the Bubbles and were conducted in the same region as observations by the Hubble Telescope of absorption signatures from the gas in ultraviolet.
"We combined those two measurements of emission and absorption to estimate the density, pressure and temperature of the ionized gas, and that lets us better understand where this gas is coming from," lead author Dhanesh Krishnarao, astronomy graduate student at UW-Madison, said in a statement.
The latest hypothesis on the formation of the Fermi Bubbles suggests a dramatic flare from the center of the Milky Way about 3.5 million years ago. Gas was thrown out at millions of kilometers per hour and the light was so intense that it lit up the gas clouds trailing behind another galaxy.
Upcoming measurements will help confirm how these events might have contributed to the formation of the Fermi Bubbles. The findings were presented by the author at the 236th meeting of the American Astronomical Society, held virtually in response to the Covid-19 pandemic.