There Was Once Another Galaxy Orbiting Ours - So Where Did It Go?

The ALMA antennae, with part of the Milky Way visible to the left, and the two Magellanic Clouds in the top-center and center-right of the shot. ESO/C. Malin/Wikimedia Commons; CC BY 4.0

Robin Andrews 18 Sep 2018, 19:45

The Magellanic Clouds have been with the Milky Way since time immemorial. These two irregular dwarf galaxies, the Large and Small varieties, may give the appearance that they’ve never not orbited their far more colossal companion at a distance of hundreds of thousands of light-years away, but a new study reminds us that this wasn’t always the case.

This is intuitively obvious: It’s known, and increasingly so, that the space around the Milky Way has involved the manipulation and destruction of other galaxies too. Sometimes, destruction doesn’t involve the disappearance of galaxies, but a merger – violent, chaotic, but a merger nonetheless.

Writing in the Monthly Notices of the Royal Astronomical Society, a pair of researchers from the International Centre for Radio Astronomy Research (ICRAR) took a closer look at the Larger Magellanic Cloud (LMC). What’s the deal with the size difference, they wondered, and could it be explained by something other than random star distributions between the two Clouds?

The team recall several enigmatic features of the LMC that have bemused astronomers for decades.

Why do most stars rotate clockwise around the center of the galaxy, but a handful rotate anti-clockwise? At the same time, why were stars in the LMC either very old or very young?

Normally, when such an anomaly is spotted in astronomical circles, a galactic merger is expected. This wouldn’t be akin to a gigantic car crash, mind you: the distances between stars are so great that such an impact event is nigh-on impossible.

Instead, gravitational perturbations would lead to some stars being ejected and others pushed nearer to their new stelliferous neighbors, a bit like adding two very spread out piles of salt together. There’s no reason that couldn’t have happened with the LMC, but what did it collide into, exactly?

Initially, the idea was that the LMC poached some stars from its smaller partner. In order to test this hypothesis, cutting-edge computer simulations were undertaken. Various galactic near-misses and collisions were replicated in an attempt to get the arrangement we see in the night sky today.

It appears that the most plausible scenario is that, between 3 and 5 billion years ago, another star-riddled dwarf galaxy collided with the LHC. This would neatly explain the presence of the so-called age-gap, with one galaxy contributing one generation of stars to a much older or younger from the other.

Stars from other galaxies could also rotate differently, particularly post-collision. The LMC also has a strangely thick disc, which hints at the addition of another galaxy’s worth of gas and dust.

If these simulations are correct, that means, as the team put it, the “Magellanic Clouds were originally a triplet system containing this companion galaxy.” Future research will clarify how plausible this scenario may be.

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