When is 50,000 light-years only a small distance? When three galaxies are that close to one another. At that range, they’re fiercely interacting.
In the case of the three galaxies referred to as SDSSCGB 10189, they’re 50,000 light-years apart and growing closer as they merge into a single massive galaxy.
Galaxy mergers aren’t exotic. The Hubble has caught many galaxies in the act of merging. Our own Milky Way galaxy is in on the game, as it slowly absorbs the Large and Small Magellanic Clouds. Another one of our neighbours, the Sagittarius Dwarf Spheroidal Galaxy (SDSG), is also in the process of merging and has passed through the Milky Way’s disk several times, losing mass each time.
This image based on Gaia data shows the Milky Way’s disk and the location of the Sagittarius Dwarf Galaxy and the Large and Small Magellanic Clouds. All three are slowly merging with the Milky Way. Image Credit: By ESA/Gaia/DPAC, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.php?curid=77752828
While those are all technically mergers, they’re really more like absorptions since the Milky Way is so much more massive than the Magellanic Clouds and the SDSG. Even after the Milky Way has consumed all three, our galaxy will still look pretty much the same.
Things play out differently when three massive galaxies collide, like in the Hubble image. After they merge, they’ll be one single, massive galaxy, probably an elliptical. But while they merge, they’ll wreak gravitational carnage on each other, streaming tails of gas out into space and triggering widespread star formation.
Astronomers research galaxy mergers because they play a key role in the Universe. The modern Universe contains a vast number of huge galaxies, but scientists think they only grew so large through mergers. (Although recent results from the JWST suggest that there were some massive galaxies in the Universe’s first few hundred million years.)
The Hubble has spotted lots of mergers between two massive galaxies. The image montage below is from the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC), a survey investigating star cluster formation in the extreme environments of six merging galaxies. These images show pairs of galaxies merging.
The Hubble has imaged lots of merging galaxies over the years. Galaxy mergers are spectacular events that trigger abundant star formation. Top left: NGC 3256 Credit: ESA/Hubble, NASA Top Middle: NGC 1614 Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University) Top Right: NGC 4194, also known as the Medusa merger. Credit: ESA/Hubble & NASA, A. Adamo Bottom Left: NGC 3690 consists of a pair of galaxies, dubbed IC 694 and NGC 3690, which made a close pass some 700 million years ago. Bottom Middle: NGC 6052 Image Credit: ESA/Hubble & NASA, A. Adamo et al. Bottom Right: NGC 34 Image Credit: ESA/Hubble & NASA, A. Adamo et al.
Triple galaxy mergers are rare, but Hubble has still spotted some. In February 2022, the Hubble team released an image of IC 2431, another distant trio of merging galaxies about 682 million light-years away. The center of the image is obscured by dust, but the three galaxies are still clearly visible as they interact gravitationally with each other.
IC 2431 is another triple galaxy merger almost 700 million light-years away. The tidal distortions are obvious in this image, and the merger is also triggering star formation. Image Credit: ESA/Hubble & NASA, W. Keel, Dark Energy Survey, DOE, FNAL, DECam, CTIO, NOIRLab/NSF/AURA, SDSSAcknowledgement: J. Schmidt
The leading image is from an effort to understand the most massive, brightest galaxies in the Universe. They’re called Brightest Cluster Galaxies (BCGs), and they’re the brightest galaxies in any given galaxy cluster. Some BCGs are 100 billion times more massive than our Sun, and most of them are ellipticals. BCGs are found at the kinematic and geometric center of their host cluster.
Galaxy mergers will restructure our little corner of the Universe over long periods of time. The Milky Way is in the Local Group of galaxies, along with another large spiral galaxy, Andromeda. In a few billion years, the pair will merge and form a single galaxy (Milkdromeda?) The Local Group also contains about 50 smaller galaxies and thousands of globular clusters. It’s arranged in a kind of dumbbell shape, with the Milky Way and its satellites in one lobe and Andromeda and its satellites in the other lobe.
This illustration shows the Local Group. The Milky Way is in the center, and Andromeda (M31) is the red galaxy up and to the left. Eventually, the two will merge, along with all of their satellites, into one gigantic elliptical galaxy. Image Credit: By Antonio Ciccolella – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50409931
In a few tens of billions of years, all of the objects in the Local Group will be combined into one gigantic elliptical galaxy. When Hubble spots triple-galaxy mergers, they’re just snapshots of this epic process of merging and combining matter. When will it end?
It’s tempting to think that matter will keep combining into more and more massive agglomerations. But that’s not what will happen. Gravity is ultra-powerful, and if it were left to its own devices, it might eventually gather all matter together into one super-gigantic, ultra-turbo-massive elliptical galaxy. But gravity isn’t ultra-powerful, and it’s not alone.
The Universe is expanding, driven by Dark Energy, gravity’s counterbalance. On smaller scales, gravity can work its magic on regions of the Universe that have over-densities of matter. These over-densities date back to the Big Bang. But on larger scales, these over-densities are defeated by Dark Energy. As long as Dark Energy keeps driving the expansion of the Universe, gravity can never achieve ultimate victory. It can never unify all matter.
Our Local Group is massive, but it’s still small enough to belong to an even larger structure called the Virgo Supercluster. The Universe is full of these superclusters that exist in a sort of web-like arrangement of filaments and clumps. They’re truly vast, and the Virgo Supercluster spans some 110 million light years. At those great distances, gravity is severely weakened. It has no power to reshape superclusters like Virgo.
It doesn’t end there. We can zoom out even further and see that the Virgo Supercluster is part of another structure called the Laniakea Supercluster.
The Laniakea Supercluster with our Local Group (blue) in the center. Image Credit: By Andrew Z. Colvin – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=71065242
The Universe’s continuous expansion is like a line in the sand for gravity. It simply can’t overcome it. The large-scale structure of the Universe shows why.
Image of the large-scale structure of the Universe, showing filaments and voids within the cosmic structure. The distances are so vast that gravity can’t draw all this matter together. Credit: Millennium Simulation Project
When we zoom back into the Milky Way, it seems small in comparison. Even the triple merger in the leading Hubble image seems small in comparison, and there are hundreds of billions of stars—maybe even more—involved in that merger, and who knows how many more tens or hundreds of thousands more stars are being born as a result. Don’t even try to guess the number of planets involved and if one of them might host life.
Take another look at these three merging galaxies and consider the vast scale of the Universe. Image Credit: ESA/Hubble & NASA, M. Sun
It’s highly unlikely that humanity will be around when these three galaxies finally combine into a single enormous elliptical galaxy. The same goes for the Milky Way and Andromeda merger. It’s highly unlikely that Earth will even be here since the Sun may have consumed it in its red giant phase.
But for those of us alive now and into the near future, we can gaze out into the Universe with our powerful space telescopes and watch it all unfolding.
