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Starts With A Bang

Our Local Group Is Being Eaten, And We Just Found The Galactic Leftovers

The Splinter galaxy, NGC 5907, is shown with a stream of a recently-devoured dwarf galaxy. We have just uncovered evidence that a satellite of Andromeda, M32, suffered a similar (but incomplete) devouring at the larger galaxy’s hands just 2 billion years ago. (R. JAY GABANY / WIKIMEDIA COMMONS)

It wasn’t just the Milky Way and Andromeda two billion years ago.

Here in the Local Group, our two largest galaxies dominate: Andromeda and the Milky Way.

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. Triangulum is the 3rd largest. The Large Magellanic Cloud is the fourth biggest galaxy in the local group, with a slew of others, including M32, between 0.1% and 0.6% the Milky Way’s mass. (ANDREW Z. COLVIN)

The next largest, Triangulum, is much smaller: only 5% the Milky Way’s mass.

The Triangulum galaxy might not be as massive or impressive as ourselves or Andromeda, but it’s the farthest object from Earth visible with the naked eye, and the third largest galaxy in our local group. At least, it is now. (ROBERT GENDLER, SUBARU TELESCOPE (NAOJ))

About 60 dwarf galaxies also remain: the cosmic survivors.

Even around our own galaxy, we can see the evidence of a recently-devoured dwarf galaxy: the Sagittarius dwarf. Tidal streams, halo stars, and a burst of new star formation all accompany this part of our cosmic history. (DAVID R. LAW, UCLA)

But the history of galactic cannibalism, where larger galaxies devour smaller ones, endures.

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 Milky Way’s larger satellites — the Magellanic Clouds — are interacting, forming stars, and on track to be devoured.

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, 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))

But one of Andromeda’s satellites is even more interesting.

The two largest satellites of Andromeda, M32 and M110, are among the top 10 largest galaxies in the local group themselves. (KANÁL UŽIVATELE NASTEBNI)

M32 is the smallest galaxy in the Messier catalog: just 6,500 light years across, with ~3 billion solar masses of material.

An ultra-dense, compact satellite of Andromeda, Messier 32, at its core, has a density of stars that’s 100,000,000 as great as in our own, local neighborhood around the Sun. (NASA / WIKISKY)

Its dense core houses a multi-million solar mass black hole, extremely unusual for a small galaxy.

In the Andromeda galaxy, the oldest individual stars are found in the galaxy’s halo. However, there is a stream and a population of stars in the halo that can be traced back to M32p, using computer modeling.(NASA, ESA AND T.M. BROWN (STSCI))

It suggests that M32 was once much larger, and has been partially cannibalized.

Small galaxies are gravitationally stretched and torn apart, but most of that material will eventually fall back onto the larger one. If comparable galaxies produce a merger, a small and large one is more like an acquisition. (KATHERINE JOHNSTON)

Scientists Richard D’Souza and Eric Bell pieced together:

  • a large halo of stars,
  • a stellar stream,
  • and M32, replete with young stars,

to reveal a progenitor galaxy: M32p.

We can successfully reconstruct a progenitor galaxy that gave rise to the combination of a satellite of Andromeda, halo stars, stellar streams, and a population of stars ~2 billion years old in both M32 and Andromeda, but today, all we have left to look at are the survivors. (AMANDA SMITH, INSTITUTE OF ASTRONOMY, UNIVERSITY OF CAMBRIDGE)

It was devoured by Andromeda 2 billion years ago; only the core survives.

Stars of all ages, types, and orbital configurations, including very tight binary stars, have been discovered via Hubble’s observations of Andromeda, the largest galaxy in the local group. (FULL HUBBLE FIELD: NASA/ESA/J. DALCANTON, ET AL. & R. GENDLER; WIDE OPTICAL FIELD: ROBERT GENDLER)

This explains the large population of ~2 billion year old stars in both galaxies.

A series of stills showing the Milky Way-Andromeda merger, and how the sky will appear different from Earth as it happens. This merger will occur roughly 4 billion years in the future, with a huge burst of star formation leading to a red-and-dead, gas-free elliptical galaxy: Milkdromeda. A single, large elliptical is the eventual fate of the entire local group. (NASA; Z. LEVAY AND R. VAN DER MAREL, STSCI; T. HALLAS; AND A. MELLINGER)

This reconstruction portends the ultimate, post-merger fate of the local group.

Mostly Mute Monday tells the astronomical story of an object, phenomenon, or scientific find about our Universe in images, visuals, and no more than 200 words. Talk less; smile more.

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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