Our Galaxy may have been swallowing "pills" - clouds of gas with a magnetic wrapper - to keep making stars for the past eight billion years.

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That’s the conclusion of CSIRO astronomer Dr Alex Hill, lead author of a study of the Smith Cloud, a large gas cloud falling into our Galaxy from intergalactic space.

“Clouds like this may provide the fuel for our Galaxy to make stars,” Dr Hill said.

“But they must be held together by something, or they’d disintegrate when they hit the warm outer part of the Galaxy — the halo. They wouldn’t reach the Galaxy’s disk, where the star-making is going on.”

Dr Hill’s team has found that the Smith Cloud has a magnetic field. It’s 50 000 times weaker than the Earth’s, “but it’s probably still strong enough to keep the cloud together,” Dr Hill said.

The finding is published today in The Astrophysical Journal.

“This is one of the few such clouds large enough for us to be able measure its magnetic field,” CSIRO’s Dr Naomi McClure-Griffiths, a member of the research team, said.

“It seems the cloud is protected by a magnetic bubble, the same way the Earth’s magnetic field protects it from the solar wind.”

Named after its discoverer, Gail Bieger (née Smith), the Smith Cloud is at least two million times the mass of our Sun. If it were visible to the naked eye, it would look 20 times wider than the full Moon.

The Smith Cloud is one of thousands of “high velocity clouds” of hydrogen gas flying around the outskirts of our Galaxy.  

Astronomers think their origins are mixed, some stemming from burst “bubbles” in the gas of our Galaxy, some being primordial gas, and some associated with small galaxies our Galaxy’s gravity is shredding from a distance. The Smith Cloud is probably either semi-primordial gas condensing from the halo of our Galaxy or gas stripped from another galaxy.

Travelling at 130 kilometres a second, the Smith Cloud is only 8000 light-years from our Galaxy’s disk and will plunge into it in less than 30 million years.

The researchers for this project were from CSIRO,  US National Radio Astronomy Observatory, and the University of Wisconsin. They observed with NRAO’s Robert C. Byrd Green Bank Telescope and the Wisconsin H-Alpha Mapper telescope.

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  • The Smith Cloud, imaged with the Robert C. Byrd Green Bank Telescope.

    The Smith Cloud, imaged with the Robert C. Byrd Green Bank Telescope.  ©NRAO/AUI/NSF, Bill Saxton

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Additional information


Hill AS, Mao SA, Benjamin RA, Lockman FJ, McClure-Griffiths NM. 2013. Magnetized Gas in the Smith High Velocity Cloud. The Astrophysical Journal, Volume 777, Issue 1, p. 55. doi:10.1088/0004-637X/777/1/55. Also online at http://arxiv.org/abs/1309.2553v2 .

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