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Starts With A Bang

This Is How The Milky Way Is Eating Our Galactic Neighbors

Two major galaxies in the local group are already in the process of being devoured by us… and each other.

The Milky Way is the second-largest galaxy in our local group, which contains some 60 galaxies of various sizes.

Our Local Group of galaxies is dominated by Andromeda and the Milky Way, but our cosmic neck-of-the-woods contains many dwarf galaxies clustered around each of the large members. The Large Magellanic Cloud is the fourth biggest galaxy in the local group, and the Small Magellanic Cloud is either 5th, 6th, or 7th, as the uncertainties between it, NGC 6822 and NGC 3190 make it impossible to tell who’s larger. (Andrew Z. Colvin)

Small galaxies are found all across our neighborhood, with many clustered around the largest members: Andromeda, ourselves, and Triangulum.

The Large (top right) and Small (lower left) Magellanic Clouds are visible in the southern skies, and helped guide Magellan on his famous voyage some 500 years ago. In reality, the LMC is located some 165,000 light years away, with the SMC slightly farther at 198,000 light years. (ESO/S. Brunier)

Two of the largest dwarfs, the Large and Small Magellanic Clouds, are located less than 200,000 light years from Earth.

The Atacama Large Millimetre/submillimetre Array, as photographed with the Magellanic clouds overhead. (ESO/C. Malin)

Although they’re only between 0.1%-1% the Milky Way’s mass, these irregular, dwarf neighbors are full of interesting, new stars.

The Large Magellanic Cloud, the fourth largest galaxy in our local group, with the giant star-forming region of the Tarantula Nebula just to the right and below the main galaxy. (NASA, from Wikimedia Commons user Alfa pyxisdis)

New star formation is triggered by mutual gravitational interactions combined with the Milky Way’s tug.

The Large Magellanic Cloud is home to the closest supernova of the last century. The pink regions here are not artificial, but are signals of ionized hydrogen and active star formation, likely triggered by gravitational interactions and tidal forces. (Jesús Peláez Aguado)

The gas within these galaxies gets shunted into new clusters, including the local group’s largest star-forming region: 30 Doradus.

The cluster RMC 136 (R136) in the Tarantula Nebula in the Large Magellanic Cloud, is home to the most massive stars known. R136a1, the greatest of them all, is over 250 times the mass of the Sun.(European Southern Observatory/P. Crowther/C.J. Evans)

But these gravitational interactions also strip the gas away from these dwarfs, where the Milky Way will devour it.

In an interaction between two galaxies, small galaxies are gravitationally stretched and torn apart. Most of that material will eventually fall back onto the larger one. If a larger, third galaxy is present, it can help siphon off and steal the gas from the smaller, closely bound dwarfs. (Katharine Johnston)

The largest gas stream seems to connect both galaxies, but which cloud it originated from was a mystery.

In a cosmic tug-of-war between two dwarf galaxies orbiting the Milky Way, only NASA’s Hubble Space Telescope can see who’s winning. The players are the Large and Small Magellanic Clouds, and as they gravitationally tug at each other, one of them has pulled out a huge amount of gas from its companion. This shredded and fragmented gas, called the Leading Arm (in pink, as imaged in the radio part of the spectrum), is being devoured by the Milky Way and feeding new star birth in our galaxy. (Illustration: D. Nidever et al., NRAO/AUI/NSF and A. Mellinger, Leiden-Argentine-Bonn (LAB) Survey, Parkes Observatory, Westerbork Observatory, Arecibo Observatory, and A. Feild (STScI); Science: NASA, ESA, and A. Fox (STScI))

Until, that is, scientists led by Andrew Fox looked at the absorption effects of this gas from background quasar light.

An ultra-distant quasar will encounter gas clouds on the light’s journey to Earth, allowing us to measure all sorts of parameters, including absorption abundances. The ‘leading arm’ arising from the interaction of the LMC and SMC shows the cosmic fingerprint of the gas, and it matches the small cloud, not the large one. (Ed Janssen, ESO)

The cosmic fingerprints matched the Small, not Large, Magellanic Cloud.

The Small Magellanic Cloud, viewed in infrared light through the ESO’s VISTA instrument, consists of a variety of stars, including new stars, but very little gas. What little gas remains inside matches the cosmic fingerprint of the ‘leading arm’ stream found around the Milky Way. (ESO/VISTA VMC)

While the Milky Way will eventually devour both, large dwarfs strip the gas away from smaller ones, hastening their demise.

Mostly Mute Monday tells the astronomical story of an object, image, or phenomenon in the Universe in pictures, visuals, and no more than 200 words.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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