Leave It To Smith

In Milky Way by Brian Koberlein2 Comments

There’s an interstellar cloud of gas heading for the Milky Way. It’s about 10,000 light years across, has a mass of a million Suns, and impact our galaxy in about 30 million years. Until now, it’s origin has been a mystery. 

First discovered in 1963 by Gail Smith, the faint cloud of gas was thought to be a failed dwarf galaxy, or simply a large cloud from intergalactic space. We’ve known its trajectory for some time, and found that it was likely in our galaxy about 70 million years ago. For a cloud of its size to pass through our galaxy relatively intact, it would need to have a much higher mass that it seems to have, leading some to speculate that it might be a dark matter galaxy.

But recently a team of astronomers studied the composition of Smith’s cloud by observing ultraviolet light passing through the cloud, they could determine which wavelengths were most absorbed. From this they determined that it has quantities of sulfur at levels similar to that of the outer region of the Milky Way. This means it likely originated from our galaxy about 70 million years ago.

Just how the large cloud was ejected from the Milky Way is still a mystery. One possibility is that dark matter has played a role, capturing material from our galaxy as it passed through. To test that idea, however, we’ll need to study Smith’s cloud a bit more.

Paper: Andrew J. Fox, et al. On the Metallicity and Origin of the Smith High-velocity Cloud.  The Astrophysical Journal Letters, Volume 816, Number 1 (2016)


  1. Enter your comment here…please, where is our solar system and earth.

  2. Not sure how the authors conclude that the cloud is in “an advanced state of accretion”. It’s either gaining mass, losing mass, or maintaining its mass. Which of those isn’t clear to me from the text. I do agree, based on what was presented, that this is an unusual gas cloud.

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