30th January 2016

Giant cloud heading for Milky Way originated from within our galaxy

New observations of Smith's Cloud, which is predicted to collide and merge with our galaxy in 27 million AD, reveal that it originated from within the Milky Way. Like a boomerang effect, it now appears to be heading back to its galactic home.

Smith's Cloud – a vast cloud of hydrogen gas that is speeding towards the Milky Way – did not originate from intergalactic space, but was actually launched out of our own galaxy around 70 million years ago. That's according to a new study by astronomers at the Space Telescope Science Institute in Baltimore, Maryland, who used the Hubble Space Telescope to determine its location and trajectory with more precision.

It was previously believed that Smith's Cloud, which is 11,000 light years in length, crossed the immense void of intergalactic space on its long journey and was possibly a failed, starless dwarf galaxy. For either of these speculations to be true, it would need to contain mostly hydrogen and helium gas – not the heavier elements made by stars.

These new observations, however, looked at ultraviolet light from the bright cores of three distant galaxies, using Hubble's Cosmic Origins Spectrograph to see how this light was filtered through the cloud. The data revealed its chemical composition for the first time, confirming it to be as sulphur-rich as the Milky Way's outer disk.

"By measuring sulphur, you can learn how enriched in sulphur atoms the cloud is compared to the Sun," explained team leader Andrew Fox. Sulphur is a good gauge of how many heavier elements reside in the cloud.

This means that the Smith Cloud was enriched by material from stars, which could not happen if it were pristine hydrogen from outside the galaxy, or if it were the remnant of a failed galaxy devoid of stars. Instead, the cloud appears to have been ejected from within the Milky Way and is now returning back towards the galactic plane in a kind of boomerang effect.

"The cloud is an example of how the galaxy is changing with time," said Fox. "It's telling us that the Milky Way is a bubbling, very active place, where gas can be thrown out of one part of the disk and then return back down into another."

"Our galaxy is recycling its gas through clouds, the Smith Cloud being one example, and will form stars in different places than before. Hubble's measurements of the Smith Cloud are helping us to visualise how active the disks of galaxies are."

While this settles the mystery of the Smith Cloud's origin, it raises new questions: How did the cloud get to where it is now? What calamitous event could have catapulted it from the Milky Way, and how did it remain intact? Could it be a region of dark matter – an invisible form of matter – that passed through the disk and captured Milky Way gas? Answers to these questions may be found in future research.

Smith Cloud's is moving at nearly 200 miles per second (or about 700,000 mph) and is predicted to collide into the Perseus Arm by 27 million AD. This will trigger an intense burst of star formation where it hits, giving birth to as many as two million additional new stars. Fox and his team have published their study in the Astrophysical Journal Letters. The entry on our timeline has been updated to reflect their new findings.

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