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How astronomers found evidence for ancient galactic cannibalism

The Large Magellanic Cloud has been keeping a secret from us.

Large Magellanic Cloud
Zdeněk Bardon/ESO

The Large Magellanic Cloud — a small galaxy the orbits the Milky Way at a distance of about 160,000 light years — seems to have swallowed up an even smaller galaxy sometime in the cosmic past.

In a new study published Monday in Nature Astronomy, a team of astronomers studied the chemical composition of red giant stars from 11 tightly-packed groupings of stars called globular clusters in the Large Magellanic Cloud (LMC) to see whether any of the clusters seemed different from its neighbors.

The result is evidence of an ancient collision between galaxies that left one globular cluster, NGC 2005, stranded in the galaxy that “ate” its original home.

What’s New? — University of Bologna astronomer Alessio Mucciarelli and his colleagues analyzed high-resolution images of the globular clusters for their chemical spectra; different chemical elements reflect and absorb light in different wavelengths, so looking at the full spectrum of light from a distant object can reveal which elements it’s made of, and in what proportions.

Stars are mostly hydrogen, but they also contain smaller amounts of other elements, which would have been mixed into the gas cloud from which the star originally formed. The ratios of those less-common elements can tell astronomers something about the environment where the star formed.

A star cluster like NGC 2005, whose chemical fingerprint looks notably different from other globular clusters in its current galaxy, may be a relic deposited from elsewhere.

“The chemical composition of a star is like its DNA — there is no way to change it,” Mucciarelli tells Inverse. “Therefore, the chemistry of a star tells us about the genealogy of this star — the gas from which it formed. Our discovery demonstrates that NGC 2005 was born outside the LMC.”

A Hubble image of NGC 2005.


Based on computer simulations, the LMC could easily produce star clusters with chemical fingerprints similar to every cluster in the study — except for NGC 2005. When authors worked backward to see what kind of galactic environment could produce this cluster of stars, the only models that worked were small galaxies that didn’t produce many stars, especially very massive ones. In other words, NGC 2005 must have formed in a different galaxy, not the LMC.

The best explanation for NGC 2005’s presence in the midst of a galaxy it doesn’t really belong in is that sometime in the cosmic past, the LMC collided with another, smaller galaxy.

The gravitational forces of that galactic merger were enough to rip the smaller galaxy apart, but because globular clusters are so small and densely packed, NGC 2005 managed to stick together — only to find itself stranded as a bit of galactic debris in the LMC. Today, the star cluster is a relic of an intergalactic cataclysm. Or, as Mucciarelli and his colleagues put it, it’s “the only surviving witness of this ancient merger event.”

NGC 2005, which is about 750 light years from the center of the LMC, is much lower in metals than its neighboring clusters. In particular, NGC 2005 seems to contain a much lower ratio of iron to hydrogen and a much lower ratio of zinc to iron than the other globular clusters in the LMC.

Heavier elements like copper, iron, and zinc are most often formed in supernovae or even larger stellar explosions called hypernovae.

The amount and distribution of material in a newly-formed galaxy determine how often, for example, massive stars form and then collapse in supernova explosions. And that, in turn, determines how much of an element like zinc the galaxy will produce. Zinc mainly forms when massive stars (more than 30 times our Sun’s mass) collapse in hypernovae, so a star cluster without much zinc probably formed in a galaxy that wasn’t home to very many massive stars.

Based on simulations, the galaxy that spawned NGC 2005 must have been small and relatively light on matter, about 1 percent as large as the Large Magellanic Cloud. It would have been comparable in size and star formation rate to the handful of dwarf galaxies that still orbit the Large Magellanic Cloud today.

“The most probable solution is that this external galaxy merged with the LMC,” Mucciarelli says. “It is hard to find other realistic possible solutions.”

The Milky Way and its satellites, many of which formed through galactic mergers.

Wikimedia Commons

Here’s the Background — Cosmologists today are reasonably sure they understand how large galaxies form: smaller galaxies collide and merge to former larger ones, and that process repeats until several dwarf galaxies eventually add up to something like the Milky Way or Andromeda (which will experience their own merger in about 4 to 10 billion years). Mergers like the one that stranded NGC 2005 in the LMC are how galaxies grow. But NGC 2005 is the first physical evidence of the process astronomers have found so far.

Today, the LMC is one of several satellite galaxies that orbits the Milky Way. It’s about 1 percent as large as our galaxy, but still large enough to have satellites of its own, including the Small Magellanic Cloud and between 4 and 6 ultra-faint dwarf galaxies, “the lowest-luminosity, oldest, most dark-matter-dominated galaxies known so far,” Mucciarelli and his colleagues wrote. We have no way of knowing, so far, how many other galaxies the Large Magellanic Cloud has swallowed up in its past.

Astronomers have no way to tell how long ago the merger between the Large Magellanic Cloud and NGC 2005’s original home happened. The globular cluster is between 12 and 13 billion years old, Mucciarelli says, which is about the same age as the other globular clusters in the Large Magellanic Cloud. Orbital data, however, suggests that the merger was a long time ago, even on a galactic scale.

NGC 2005’s radial velocity — the speed at which it seems to move relative to observers on Earth — is about the same as the other globular clusters orbiting the center of the Large Magellanic Cloud. Any difference in their orbits “has been washed out after many orbits within the gravitational potential of the Large Magellanic Cloud,” write Mucciarelli and his colleagues.

Why It Matters — So far, NGC 2005 is the only evidence of a galactic merger astronomers have found using the chemical fingerprints of stars.

“This result opens a new window in the study of the stellar populations of the Milky Way satellites, providing us a powerful tool to reconstruct the assembly history of the small galaxies of the Local Group,” Mucciarelli says.

The results help confirm what cosmologists have long presumed about smaller galaxies merging to form larger ones, and it shows that the model applies even to the galaxies in our Milky Way’s own orbit.

What’s Next — Mucciarelli and his colleagues hope to find more remnants of devoured galaxies in the Large Magellanic Cloud and other nearby galaxies. A new spectrograph called MOONS (the Multi-Object Optical and Near-infrared Spectrograph) at the European Southern Observatory’s Very Large Telescope in Peru’s Atacama Desert will give astronomers “the ideal instrument to study the chemical composition of a huge amount of stars in the Magellanic Clouds,” he says.

And a new generation of 30-meter telescopes, including ESO’s Extremely Large Telescope, which is now under construction in Chile, will come with higher-resolution spectrographs that Mucciarelli hopes to use to study the spectra of stars even further away than the Large Magellanic Cloud. That data may eventually help astronomers reconstruct a more detailed history of the Local Group, the 3.2 million-light year-wide neighborhood of galaxies in which the Milky Way resides.

Abstract — According to the standard cosmological scenario, the large galaxies that we observe today have reached their current mass via mergers with smaller galaxy satellites1. This hierarchical process is expected to take place on smaller scales for the satellites themselves, which should build up from the accretion of smaller building blocks2. The best chance we have to test this prediction is by looking at the most massive satellite of the Milky Way: the Large Magellanic Cloud (LMC). Smaller galaxies have been revealed to orbit around the LMC3,4, but so far the only evidence for mutual interactions is related to the orbital interplay with the nearby Small Magellanic Cloud, which is the most massive LMC satellite. In this work, we report the likely discovery of a past merger event that the LMC experienced with a galaxy with a low star formation efficiency and likely a stellar mass similar to those of dwarf spheroidal galaxies. This former LMC satellite has now completely dissolved, depositing the old globular cluster NGC 2005 as part of its debris. This globular cluster, the only surviving witness of this ancient merger event, is recognizable through its peculiar chemical composition. This discovery is observational evidence that the process of hierarchical assembly has worked also in shaping our closest satellites.
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