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Galaxies - Merging and Interacting

Large galaxies like the Milky Way formed out of mergers with smaller galaxies and by stealing some of their stars. Astronomers discovered that as many as 25% of galaxies are currently merging with others. Probably even more are gravitationally interacting with their neighbors, with subsequent exchanges of stars and effects on the structures of both galaxies. For that reason, researchers study merging and interacting galaxies to understand how they form and evolve.

Our Work

Center for Astrophysics | Harvard & Smithsonian scientists study galactic mergers and interactions in many different ways:

  • Hunting for signs of mergers and cannibalism in the Milky Way’s past. Astronomers have discovered traces of that history in the form of streams of stars that were pulled from other galaxies, including the Magellanic Clouds and the Sagittarius dwarf galaxy.
    Farthest Stars in Milky Way Might Be Ripped from Another Galaxy

  • Observing the effects of mergers on the stars and black holes in the interacting galaxies. Researchers use NASA’s Chandra X-ray Observatory and other telescopes to look for bursts of activity driven by galactic collisions, including increased amounts of feeding by the supermassive black holes at the centers of the galaxies.
    NASA's Chandra Finds Supermassive Black Hole Burping Nearby

  • Tracing the history of merging galaxies by studying their present structures. Using the Smithsonian’s Submillimeter Array (SMA), the Atacama Large Millimeter/submillimeter Array (ALMA), and other telescopes, astronomers observe the hidden interiors of distorted galaxies to learn what the component galaxies looked like.
    Split-Personality Elliptical Galaxy Holds a Hidden Spiral

  • Performing huge computer simulations of collisions, to learn how galaxies form and grow. This includes the Illustris Project, designed to provide a realistic understanding of the way mergers shape the large-scale structure of the universe.
    The Cosmic Evolution of Galaxies

  • Observing bursts of star formation in merging galaxies. Many galaxy mergers and interactions result in a frenzy of new stars, but not all do. Astronomers want to understand what the differences are, and how mergers might play a role in shaping the populations of stars within galaxies. Along with X-ray observations, researchers use NASA’s Spitzer Space Telescope to find those newborn stars in merging galaxies.
    Starburst Galaxies in the Early Universe

Hubble Space Telescope image of merging galaxies NGC 2623

This image from NASA's Hubble Space Telescope shows the merging galaxies NGC 2623. The individual galaxies are being pulled apart, and the process is leading to the birth of new stars, seen here in bright blue.

Credit: ESA/Hubble & NASA

What Galaxy Mergers Do

Billions of years from now, our galaxy will collide and merge with the Andromeda Galaxy, eventually forming one big galaxy. The Milky Way itself is the product of past mergers, which we see from traces of those other galaxies. And our galaxy currently is interacting gravitationally with its satellites, exchanging stars in long streams.

Astronomers see galaxy interaction and merging throughout the universe. In particular, large galaxies formed by merging with smaller galaxies over billions of years. The largest galaxies in the cosmos, the giant ellipticals, almost certainly got that way by mergers. Irregular galaxies, including our neighbors Large and Small Magellanic Clouds, probably got their distorted shapes through gravitational interactions with other galaxies. Since ellipticals make up about 15% of known galaxies and irregulars are an additional 5%, mergers and interactions play an obvious role in the types of galaxies we see.

In addition, astronomers observe galaxies that are currently merging, which is somewhere between 5% and 25% of all galaxies. In many of these cases, the galaxies undergo an intense burst of star formation, strongly implying a connection between galactic interactions and the birth of stars.


Types of Galaxy Interactions

Galaxy mergers can be spectacularly beautiful, as in the case of the Antennae Galaxies. First observed in 1785, these were originally two similarly-sized spiral galaxies that began colliding around 600 million years ago. Their original spiral shapes are mostly gone, but the merger has created huge amounts of star formation. Eventually, these galaxies will form one large galaxy.

Other galaxy mergers are more lopsided. The Milky Way is currently engaged in “galactic cannibalism”, stripping stars off our satellite galaxies. Based on studies of populations of stars, astronomers have found this isn’t the only time such cannibalism has happened.

Between larger galaxies eating smaller ones and mergers between roughly equal-sized galaxies, interactions play an obviously important part in the growth and evolution of galaxies.