Giant Gas Cloud Magnetic Shield Milky Way
November 1, 2013

Magnetic Force Field May Protect Giant Gas Cloud During Milky Way Collision

redOrbit Staff & Wire Reports - Your Universe Online

Astronomers have discovered a magnetic field deep within the Smith Cloud’s interior that may protect the giant galactic streamer of hydrogen gas during its projected collision with the Milky Way galaxy. The discovery, which was made using the Karl G. Jansky Very Large Array (VLA) and Robert C. Byrd Green Bank Telescope (GBT), could help explain how so-called high velocity clouds (HVCs) remain mostly intact during their mergers with the disks of galaxies, where they provide fresh fuel for a new generation of stars.

The massive Smith Cloud is currently racing toward the Milky Way at more than 150 miles per second and is estimated to make impact in approximately 30 million years. When that happens, astronomers believe it will set off a spectacular burst of star formations. Before that can happen, however, it has to survive careening through the halo, or atmosphere, of hot ionized gas that surrounds the Milky Way.

“The million-degree upper atmosphere of the Galaxy ought to destroy these hydrogen clouds before they ever reach the disk, where most stars are formed,” said Alex Hill, an astronomer at Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) and lead author of a paper about the findings published in the Astrophysical Journal. “New observations reveal one of these clouds in the process of being shredded, but a protective magnetic field shields the cloud and may help it survive its plunge.”

Many hundreds of HVCs zoom around our Galaxy, but their orbits seldom correspond to the rotation of the Milky Way. This leads astronomers to believe that HVCs are the leftover building blocks of galaxy formation, or the splattered remains of a close galactic encounter billions of years ago.

Though massive, the gas that makes up HVCs is highly fragile, and computer simulations predict they lack the necessary wherewithal to survive plunging through the halo and into the disk of the Milky Way.

“We have long had trouble understanding how HVCs reach the Galactic disk,” said Hill. “There's good reason to believe that magnetic fields can prevent their ‘burning up’ in the halo like a meteorite burning up in Earth’s atmosphere.”

Despite being the best evidence yet for a magnetic field inside an HVC, the origin of the Smith Cloud’s field remains a mystery. “The field we observe now is too large to have existed in its current state when the cloud was formed,” said Hill. “The field was probably magnified by the cloud's motion through the halo.”

Previous research indicates the Smith Cloud has already endured punching through the disk of our galaxy once, and is now at about 8,000 light-years from the disk beginning its re-entry.

“The Smith Cloud is unique among high-velocity clouds because it is so clearly interacting with and merging with the Milky Way,” said Dr. Felix Lockman, an astronomer at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. “Its comet-like appearance indicates it’s already feeling the Milky Way’s influence.”

Since the Smith Cloud appears to be devoid of stars, the only way to observe it is with highly sensitive radio telescopes like the GBT, which can detect the faint emission of neutral hydrogen. If it were visible with the naked eye, the Smith Cloud would cover almost as much sky as the constellation Orion.

When the Smith Cloud ultimately merges with the Milky Way, it could produce a bright ring of stars similar to the one relatively close to our Sun known as Gould's Belt, the researchers predict.

“Our Galaxy is in an incredibly dynamic environment,” said Hill. “How it interacts with that environment determines whether stars like the Sun will continue to form.”