The Milky Way Steals From Its Neighbors

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The Milky Way Steals From Its Neighbors

 

Galaxies seem eternal, yet they actually change over time. (Photo by: Photo12/Universal Images Group via

Getty Images)UNIVERSAL IMAGES GROUP VIA GETTY IMAGES

 

It’s common to think of the universe as static and unchanging, with vast spiral galaxies rotating slowly, but otherwise existing as a stately and eternal work of cosmic art. 

 

However, we know this to be false and it only seems to be true because of the vastly different timescales that govern human lives and the motion of galaxies. Galaxies can tear themselves apart in violent collisions that spread stars over millions of light years.  And it’s not just happening “out there.” For instance, our own Milky Way galaxy might have a somewhat checkered past. Recent calculations and measurements have revealed that our Milky Way has stolen dwarf galaxies from its largest galactic satellite, the Large Magellanic Cloud (LMC). 
 

The Large Magellanic cloud (top right) and Small Magellanic cloud (bottom right), set against the center of the

Milky Way galaxy (Photo by: VW Pics/Universal Images Group via Getty Images)UNIVERSAL IMAGES GROUP

VIA GETTY IMAGES

 

Stars have planets and planets can have moons. What is less well known is that galaxies can be surrounded by other and smaller dwarf galaxies. The most familiar dwarf galaxies that surround our Milky Way, are the Large and Small Magellanic Clouds (SMC), which can be seen in the Southern hemisphere as faint smudges. These two dwarf galaxies contain many stars within them (SMC, several hundred million, and LMC, about thirty billion) and are small in comparison to the Milky Way, with its 200 to 400 billion stars.

 

The Milky Way is known to have over fifty dwarf galaxies, most of them smaller and fainter than the Magellanic clouds. And dwarf galaxies are quite interesting for a surprising reason – they are inextricably tied up in our understanding of a mysterious substance in the universe called dark matter.

 

This artist’s impression depicts our home Galaxy, the Milky Way, embedded in a spherical halo of dark matter

(shown in blue). (Image credit: ESO/L Calçada.)EUROPEAN SOUTHERN OBSERVATORY/L CALÇADA.

 

Dark matter is thought to be a type of matter that does not absorb or emit light, making it completely invisible (hence the name). Dark matter has never been directly observed and its existence has been inferred only indirectly by its effect on the rotation speed of galaxies or by the motion of clusters of galaxies that are bound together by gravity. Averaged over the entire universe, astronomers believe that there is five times more dark matter than the hydrogen and helium that makes up the stars and galaxies of ordinary matter.

 

Dark matter is thought to exist in large clouds that surround the familiar galaxies seen in the dazzling pictures taken by the Hubble Telescope, and there is considerable circumstantial evidence that this is true. However, an unappreciated consequence of having the visible matter of galaxies embedded in a much larger cloud of dark matter is that a galaxy like the Milky Way should be surrounded by perhaps several hundred dwarf galaxies.

 

And for a long time, this didn’t seem to be true. Astronomers knew of the Magellanic clouds and a few others, but the numbers didn’t seem to add up. Accordingly, searches for additional dwarf galaxies surrounding the Milky Way have become common, with some success and a growing population. For instance, the Sagittarius Dwarf galaxy, which is the closest known companion to the Milky Way, was discovered as recently as 1994. The reason it took so long to find is because it is located behind the center of the Milky Way and it is therefore very difficult to image.

 

Stellar streams around the Milky Way Galaxy. Image credit: NASA / JPL-Caltech / R.

Hurt, SSC & Caltech.NASA / JPL-CALTECH / R. HURT, SSC & CALTECH.

 

In a recent study of the motion of faint galactic neighbors, researchers made a surprising discovery. Of the known dwarf galaxies orbiting the Milky Way, several of them, including the Small Magellanic Cloud, two smaller classical dwarfs (called Carina and Fornax, with a few hundred thousand stars), and at least four ultrafaint dwarfs (containing as few as five to ten thousand stars) were once gravitationally connected to the Large Magellanic Cloud. However, in the vicinity of the Milky Way, the much larger galaxy’s strong gravitational field captured them and they now orbit the Milky Way, rather than the LMC. The recent research hints that perhaps there are other ultrafaint dwarfs who have experienced a similar fate, but it will require additional data to make any conclusive assertions.

 

Further study is required, but the observation that small galaxies currently trapped in the Milky Way’s gravitational field originally orbited a different small galaxy that is now orbiting the Milky Way, has validated the techniques used in this study and may lead to a more nuanced appreciation of the interplay between dark matter and ordinary matter as the Milky Way grew to its present size. Until dark matter is directly observed, it will be these ever more sophisticated studies that will help astronomers refine our appreciation of this elusive substance.

 

An increasing understanding of the nature of dark matter in cosmological contexts is perhaps the most scientifically-substantive aspect of this study, but there is one that is a little more fun. The fact that the Milky Way has stolen tiny galaxies from its small neighbors leads us to an inescapable conclusion.

 

Our galaxy is a bully.

 

 

Source: The Milky Way Steals From Its Neighbors