Magnetic force fields rescue a large gas cloud that is on a collision course with the Milky Way

Previous studies have indicated that the Smith Cloud has already survived a passage through the disc of our galaxy at least once, and that it is about 8,000 light-years away from the galaxy and is beginning to return to it.

This could have been the end of the Smith Cloud, a massive stream of hydrogen gas on a collision course with the Milky Way galaxy. Astronomers using the Very Large Array (VLA) and the Green Bank Radio Telescope (GBT) have discovered a magnetic field deep within the Milky Way, which may protect the cloud during the meteoric fall of gas onto the galaxy's disk.

This discovery may explain how high velocity clouds (HVCs) usually remain intact during their merger into the disks of galaxies, where the gas serves as fresh fuel for a new generation of stars.

Currently, the Smith Cloud is making its way towards the Milky Way at a speed of over 240 kilometers per second and is expected to hit it within 30 million years. When this happens, astronomers estimate that the collision will ignite a great wave of star formation, but first the gas cloud will have to penetrate the halo, or atmosphere of the galaxy - a cloud of hot ionized gas that surrounds the Milky Way.

"The million-degree atmosphere of our galaxy should destroy the hydrogen clouds before they reach the disk, where most stars are formed," says Alex Hill, an astronomer at the Center for Scientific and Industrial Research in Australia and the lead researcher in an article published in the Astrophysical Journal. New observations reveal that one of these clouds is in the process of tearing apart, but the magnetic field protects it and may allow it to survive the transition."

Many such clouds hover around the Milky Way but their orbits are not aligned with the galaxy. This leads astronomers to speculate that the HVCs are the remnants of the building blocks left over from the creation of our galaxy or that they are remnants of galactic encounters billions of years ago.

Despite their size, the concentration of gas inside them is quite sparse, and computer simulations predict that their density is not sufficient to allow them to penetrate through the halo of the Milky Way. "For a long time we tried to understand how these clouds reach the disk of the galaxy" says Hill. "There is real reason to believe that they will burn up in the halo like meteors burn up in the Earth's atmosphere.

Although it is the best example of a magnetic field inside a gas cloud, the origin of the Smith cloud is still a mystery. "The field we discovered is too large to have survived in its current state when the cloud formed," says Hill. "The field was probably amplified due to the movement of the cloud through the halo."

Previous studies have indicated that the Smith Cloud has already survived a passage through the disc of our galaxy at least once, and that it is about 8,000 light-years away from the galaxy and is beginning to return to it. "The Smith Cloud is unique among these fast-moving clouds because it has already merged with the Milky Way," says Felix Lockman, of the National Radio Observatory in Green Bank, West Virginia. "Its comet-like appearance suggests that it has already felt the influence of the Milky Way."

Since the cloud contains no stars at all, the only way to observe it is with highly sensitive radio telescopes, such as the GBT, which can detect faint neutral hydrogen emission. If it were visible to the naked eye, the Smith cloud would cover much of the sky in the Orion group area.

When the Smith Cloud eventually merges with the Milky Way, it may produce a bright ring of stars similar to that of the nearby Gould Belt. "Our galaxy is a particularly dynamic environment," concludes Hill. "And the way it interacts with the environment determines whether stars like our Sun will continue to form."

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