Small Magellanic Cloud revealed as 2 objects

The Small Magellanic Cloud – and its larger neighbor the Large Magellanic Cloud – are so bright that they’re easily visible to the eye in Southern Hemisphere skies. Scientists have long spoken of this pair as satellite galaxies to our Milky Way. We know they orbit each other and also orbit our galaxy’s core, over a great sweep of time lasting some 1.5 billion years. But a new study published on arXiv in December, 2023 (and not yet peer-reviewed) suggests the Small Magellanic Cloud isn’t a single dwarf galaxy, after all. It says the Small Cloud might appear to be two distinct star-forming regions, lying along a single line of sight, separated from each other by about 16,000 light-years.

That’s not a large distance considering that the Small Magellanic Cloud is some 200,000 light-years away.

The findings have been accepted into – but not yet published by – The Astrophysical Journal.

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So, what is the Small Magellanic Cloud?

Claire E. Murray at the Space Telescope Science Institute in Baltimore, Maryland, led the new study. She and her colleagues tracked the motions of young, massive stars within the Small Magellanic Cloud using a galaxy survey from the Australian Square Kilometer Array Pathfinder.

Then they used two other surveys – from Gaia and APOGEE – to trace the stars back to the interstellar medium (the material between the stars) in which they formed.

What they found was that the stars paired off into two distinct groups.

The team created a simple model to show that the Small Magellanic Cloud is really:

… two, superimposed, star-forming systems with similar gas mass separated by about 16,000 light-years along the line of sight.

The paper also suggests that the stars came from structures that have two distinct chemical compositions, both in terms of their stars and in terms of their gas.

What we knew before

Because the Small Magellanic Cloud is relatively nearby, it’s been the target of many studies over the years. It’s close enough that we can resolve individual stars and structures in its interstellar medium. Scientists explained away previous observational inconsistencies …

by the fact that the Small Magellanic Cloud has been severely disrupted by its recent interactions with the nearby Large Magellanic Cloud.

In fact, back in 1986, some scientists published a paper showing two different populations of stars in the same line of sight in the Small Magellanic Cloud.

Their conclusion at the time was that past interactions with the Large Magellanic Cloud split the smaller cloud into two. And they said the two sections of the Small Magellanic Cloud are still moving apart.

Small Magellanic Cloud as 2

But the new research suggests the Small Magellanic Cloud is two distinct clusters of stars with a clear separation between them.

The front stellar system of the Small Magellanic Cloud has different metallicities – that is, greater abundances of elements heavier than hydrogen and helium – from the one behind. The scientists also found that the front system is where most of the molecular gas appears to be.

The study suggests that the two structures could be remnants of two distinct galaxies. Another possibility is that the front portion is the main galaxy. And the portion behind could be a stellar bridge. But the scientists explained that they would need more distance constraints before they could completely solve the riddle of the Magellanic Clouds.

In the meantime, the Large and Small Magellanic Clouds will continue to orbit each other and the Milky Way galaxy.

The famous Large and Small Magellanic Clouds

The Small Magellanic Cloud has long been considered a dwarf, irregular galaxy. It holds stars, gas and dust with a total mass some 7 billion times that of our sun. Those numbers are in contrast to the 100,000-light-year width of our Milky Way, and its total of an estimated 100 to 400 billion stars.

The Small Magellanic Cloud lies within the borders of Tucana the Toucan. The Large Magellanic Cloud lies across two constellations, Mensa and Dorado. You can see both Magellanic Clouds together in the image below.

Scientists believe that the two Magellanic Clouds had a direct collision some 150 million years ago. They can see the collision from the debris it left in its wake.

Bottom line: A new study, not yet peer-reviewed, suggests the Small Magellanic Cloud may not be one galaxy but two star-forming regions superimposed along our line of sight.

Source: A Galactic Eclipse: The Small Magellanic Cloud is Forming Stars in Two, Superimposed Systems