The Milky Way, like many other galaxies, is surrounded by a halo, a spherical region containing stars, gas, and a lot of dark matter. It extends for hundreds of light-years, much further than the disk where the Solar System is located. Despite its total mass of gas and stars being similar to that of the galaxy disk, the halo is so big that everything in it is very spread out, making it very difficult to study.
One of our assumptions is the temperature of the gas in the halo. Galaxy formation models provide a range of values for how hot the gas can potentially get but new research published in The Astrophysical Journal Letters finds that some parts are at least 10 times hotter than the upper limit expected from the models.
"We thought that gas temperatures in galactic halos 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," lead author Sanskriti Das, a graduate researcher at The Ohio State University, said in a statement. "While we think that gas gets heated to around 1 million degrees as a galaxy initially forms, we're not sure how this component got so hot. It may be due to winds emanating from the disc of stars within the Milky Way."
The observations were possible thanks to the European Space Agency’s XMM-Newton X-ray observatory. Given the complexity of studying it directly, the team had to be inventive. They looked for objects known as blazars, which are supermassive black holes feeding in distant galaxies with their high-energy jets pointing right at us.
The X-ray emission from these incredible objects crosses the halo and some of the spread-out gas there can leave a signature in the light spectrum. To make sure that they were seeing everything, the team spent three weeks detecting signals that would usually be too faint to see.
"We analyzed the blazar's light and zeroed in on its individual spectral signatures,” said co-author Smita Mathur, also of The Ohio State University, and Das' advisor. "There are specific signatures that only exist at specific temperatures, so we were able to determine how hot the halo gas must have been to affect the blazar light as it did."
The team released a second paper, published in The Astrophysical Journal, that looked at the composition of the gas in the halo. This is exciting because it's not exactly what the researchers expected. We still have a lot more to learn about what the halo is truly like.