Discovery of dense clouds of neutral hydrogen at the center of vast structures challenges hypothesis about age and stability of Fermi bubbles
A surprising discovery at the heart of the Milky Way is forcing scientists to rethink how massive structures called Fermi bubbles formed.
Scientists have identified dense pockets of cold gas hidden within vast, extremely hot clouds called Fermi bubbles at the center of the Milky Way. The discovery contradicts existing theories about how these bubbles formed and suggests they are much younger than previously thought.
"Fermi bubbles are huge structures of hot gas that extend above and below the disk of the Milky Way, reaching about 25,000 light-years in each direction from the center of the galaxy – a total height of 50,000," says Rongmon Bordoloi, co-author of the study.
"Fermi bubbles are a relatively new discovery – they were first detected in gamma-ray telescopes in 2010 – there are different theories as to how this happened, but we know it was a very violent and sudden event, like a volcanic eruption but on a massive scale."
To study the bubbles in more detail, Bordoloi and his team used the Green Bank Telescope in the US. This radio telescope allowed them to collect high-resolution data on the composition of the gas inside the bubbles and the speed of its movement. Their observations were twice as sensitive as previous surveys, allowing them to detect much smaller details inside the giant structures.
A cold surprise in a warm environment
Most of the gas inside the Fermi Bubbles is at a temperature of about a million degrees Kelvin. But the research team also found something surprising: dense clouds of neutral hydrogen gas, each with a mass of a few thousand solar masses, like dots inside the bubbles 12,000 light-years above the center of the Milky Way.
"These clouds of neutral hydrogen are cold, relative to the rest of the Fermi Bubble," says Andrew Fox, a co-author of the paper. "About 10,000 degrees Kelvin, which is at least a hundred times colder than their surroundings. Finding these clouds inside the Fermi Bubble is like finding ice cubes in a volcano."
Their existence is surprising because the hot environment (more than a million degrees Kelvin) and rapid flow of the outflow from the core should have quickly destroyed any colder gas.
"Computer models of interactions between cold gas and hot flows in extreme environments like the Fermi bubbles show that cold clouds should be destroyed rapidly, typically within a few million years, a time frame that is consistent with independent estimates of the age of the Fermi bubbles," says Bordoloi. "The presence of the clouds would be completely impossible if the age of the Fermi bubbles were ten million years or more."
"This discovery is even more remarkable because of its synergy with UV observations from the Hubble telescope," says Bordoloi. "The clouds are along a line of sight previously observed by Hubble, which revealed highly ionized multiphase gas, in the temperature range of one million to 100,000 Kelvin—which is what we would expect to see if cold gas were evaporating."
The team was also able to calculate the speed of movement of the gases, which also verified the age.
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