Thanks to the Chandra X-ray observatory, astronomers have recently discovered that the Milky Way galaxy is enshrouded by a massive cloud of gas that's a few hundreds of thousands of light-years across. In fact, the halo of hot gas is estimated to be similar in overall mass to all of the stars in the galaxy COMBINED. If the finding is verified and the estimates are correct, it may provide astronomers with a solution to the "missing-baryon' problem observed in our galaxy.
The missing-baryon mystery has captivated astronomers that are looking to understand the evolution of both normal matter and the the elusive dark matter, which is thought to account for a total 84% of the matter in the universe today. Baryons are composite subatomic particles from the hadron family that are made up of three quarks. Together, along with protons and neutrons, they make up approximately 99% of the mass in all atoms in the universe.
As physicists make measurements of dense gas halos surrounding distant galaxies, they learned that the amount of baryonic matter that was present when the universe was still very young (a few billion years old, to be exact) represented around one-sixth of the density and mass of the unobservable matter. Today, measurements taken of stars and interstellar gases in our galaxy doesn't account for over half of the baryons that should be present in the Milky Way and other nearby galaxies. So, where is it? Well, According to the new(ish) paper -- it isn't missing at all. We just haven't been looking in the right place for it.
Using a variety of data collected on the temperatures, extent and mass of the gas halo -- from the European Space Agency's XMM-Newton space observatory, Chandra X-ray Observatory and Japan's Suzaku satellite, astronomers saw eight exceptionally bright sources of x-rays being emitted from a location a few hundreds of millions of light-years out from the outskirts of the Milky Way galaxy, which extends about 100,000 light-years across itself. It was discovered that these distant x-rays that were detected are being 'selectively absorbed' by oxygen ions in the immediate area of the galaxy. From there, they were able to deduce the temperature of the halo, which clocks in at about 1 million and 2.5 million K -- hundreds of times hotter than the sun's surface.
In other studies on the subject, it was discovered that the Milky Way and other galaxies observed were encompassed in gas with temperatures between 100,000 and 1 million K, but this new study amplifies those estimated temperatures and mass exponentially. Thus concluding that the 'missing' baryons aren't missing at all, but instead they are hiding in the halo of hot gas that's surrounding our galaxy, but escaped detection due to the low density of the said halos.
Nevertheless, astronomers had to find out the precise mass of the halo to determine how many baryons may be present, which depends on a few factors, but most importantly, the amount of oxygen relative to the amount of hydrogen in the cloud. Using data collected from Chandra, the authors of the paper measured how much absorption was taking place by the oxygen ions from the x-rays being emitted by the large halo of gas. The astronomers ultimately determined the approximate mass of the halo to be between 10 - 60 billion solar masses, which goes a long way in accounting for all of the missing baryons.