Milky Way Surrounded By Halo
September 24, 2012

Chandra Finds Evidence Milky Way Surrounded By Halo

Lee Rannals for — Your Universe Online

The Milky Way Galaxy is known for its spirals, as well as Earth's home in the cosmos, but now it will also be known as a galaxy that features a halo.

Astronomers using NASA's Chandra X-ray Observatory found evidence that the Milky Way Galaxy is embedded in a halo of hot gas that extends for hundreds of thousands of light years.

Scientists estimated that the mass of the halo found is comparable to the mass of all the stars in the galaxy.

NASA said if the size and mass of this gas halo is confirmed, it could explain the "missing baryon" problem for the galaxy.

Baryons are particles that make up more than 99.9% of the mass of atoms found in the universe. Measurements of extremely distant gas halos and galaxies indicate the baryonic matter present when the universe was just a few billion years old presented about a sixth of the mass and density of the existing dark matter.

About 10 billion years later, a census of the baryons found in stars and gas in our galaxy and nearby galaxies shows at least half the baryons are unaccounted for.

In the latest study, data revealed x-rays from these distant sources are absorbed by oxygen ions in the vicinity of the galaxy. The astronomers determined the temperature of the halo is between 1 million and 2.5 million kelvins, which is a few hundred times hotter than the surface of the sun.

Other studies show that the Milky Way and other galaxies are embedded in warm gas with temperatures between 100,000 and 1 million kelvins.

These studies have indicated the presence of hotter gas with a temperature greater than 1 million kelvins. The latest research provides evidence that the hot gas halo enveloping the Milky Way is more massive than the warm gas halo.

"We know the gas is around the galaxy, and we know how hot it is," Anjali Gupta, lead author of The Astrophysical Journal paper describing the research, said in a statement. "The big question is, how large is the halo, and how massive is it?"

The authors concluded that the mass of the gas is equivalent to the mass in more than 10 billion suns, and may even be as large as 60 billion suns.

"Our work shows that, for reasonable values of parameters and with reasonable assumptions, the Chandra observations imply a huge reservoir of hot gas around the Milky Way," co-author Smita Mathur of The Ohio State University, said in a statement. "It may extend for a few hundred thousand light-years around the Milky Way or it may extend farther into the surrounding local group of galaxies. Either way, its mass appears to be very large."

The predated mass depends on factors like the amount of oxygen relative to hydrogen.  The estimation represents an important step in solving the case of the missing baryons.

NASA said the team's study helps to provide the best evidence yet that the galaxy's missing baryons have been hiding in the halo of gas.