The halo that surrounds our Milky Way galaxy is hotter and made up of different proportions of elements than scientists previously thought, new research shows.
The findings are the first to show that multiple gases exist in the Milky Way's halo — or in any galaxy's halo — and could shed new light on our home in the universe.
The researchers outlined their findings in a paper published in December in The Astrophysical Journal.
"When I saw the data, I just started jumping up and down, because I knew we had found something that no one had seen before," said Smita Mathur, senior author of the study and an astronomy professor at The Ohio State University. "It was one of those breakthrough 'This is why we do science' moments."
Astronomers and astrophysicists have for years studied the vast halo that surrounds the Milky Way. The halo looks almost like a hazy fog, and is made up of dust, gas and dark matter. Scientists have long known that the halo gas was hot. They didn't know it could be so hot, though, until Mathur and one of her graduate students, Sanskriti Das, began analyzing data from an X-ray observatory telescope run by the European Space Agency. That telescope, called XMM-Newton, collects data in X-rays that would have otherwise been blocked by Earth's atmosphere.
The makeup of a galaxy's halo is important: The halo connects a galaxy to the wider intergalactic universe. Scientists believe it plays a crucial role in the way a galaxy grows and changes over time.
Until Mathur and Das began analyzing the data, scientists thought the hot gas in the Milky Way's halo likely had just one temperature. But the researchers saw X-rays indicating at least three phases of gases, with the hottest having temperatures about 10 times hotter than scientists previously had thought.
"We thought that gas temperatures in galactic haloes ranged from around 10,000 to 1 million degrees, but it turns out that some of the gas in the Milky Way's halo can hit a scorching 10 million degrees," said Das, who is lead author of the study.
They saw oxygen ions — an element they expected, and one that was known to exist in the halo. But they also saw nitrogen, and neon, in unexpectedly large amounts. These elemental abundance ratios, Mathur said, tell us how the Milky Way's halo is enriched by dying stars, which fling matter into space as they die.
Previous X-ray observations of the Milky Way's halo had lasted only a couple of days. For this study, the data Mathur and Das analyzed was collected over about three weeks. The researchers said that made it possible for them to see, in the data, elements and temperatures they would normally not have seen.
The researchers have more work to do: Future analysis could try to answer the question of why the gas in the Milky Way's halo got so hot, and attempt to identify sources for the elements they found in the halo.
Mathur said their finding could also help answer the "missing matter" question, a mystery that has flummoxed astronomers and astrophysicists for years. Those scientists know roughly how much matter should be in the Milky Way galaxy, but calculations show that much of it is unaccounted for.
Mathur said it is possible that the hot areas of the Milky Way's halo hold some of that matter. It is a question she'd like to study further.
"Because we've overlooked this extremely hot gas in the Milky Way's halo in the past — because we didn't know it was there — it's possible that some of that missing matter might be in that hot gas," she said.
