In a distant corner of the Universe, some 45 million lightyears away, billions of stars are dancing a celestial waltz.<\/p>
They are spinning, gyrating and pirouetting around and, if you squint, their collective flailing looks like the antennae of an insect.<\/p>
It is from this striking appearance that this ancient group of stars gets its name: the Antennae Galaxies.<\/p>
The plural is a clue to their true origin. This is not a single galaxy, but two in the process of merging.<\/p> Galaxy collisions or mergers like these are the lifeblood of the Universe, building the enormous structures that dominate the cosmos to this day.<\/p> More than that, galaxy collisions and mergers could have helped make the Universe a more life-friendly place.<\/p> It isn\u2019t too much of stretch to say that we wouldn\u2019t be here without them.<\/p> And yet, for something so crucial, astronomers have only recently started to pull back the curtain to reveal their secrets.<\/p> Many mysteries remain. This is the story of what we know, as well as what we don\u2019t.<\/p> “If we look at the history of all galaxies, each galaxy has had a few major mergers and some minor ones,” says Christopher Conselice, professor of extragalactic astronomy at the University of Manchester.<\/p> A major merger occurs when the two entwining objects are of roughly equal size. If a large galaxy gobbles up a tiny one, then it\u2019s classed as a minor merger.<\/p> What causes galaxy mergers? \u201cThe simple answer is gravity,\u201d says Conselice.<\/p> Nothing in the Universe is static and galaxies are moving through space.<\/p> If two galaxies undertake a close encounter, then their mutual gravitational attraction can draw them closer still.<\/p> As the gap narrows, that attraction rises yet further and eventually they are set on a collision course.<\/p> It would be wrong to think of galaxy mergers as the cosmic equivalent of a car crash.<\/p> “The stars are non-collisional,” says Conselice. The gap between the stars<\/a> inside a galaxy is typically hundreds of billions to tens of trillions of kilometres.<\/p> The chances of two stars colliding in a typical galaxy merger are less likely than you tossing a coin 27 times and them all coming down heads.<\/p> However, stars can still get shredded by the extreme tidal forces inflicted during galaxy mergers.<\/p> Debris is ripped out of the galaxy and strewn into long strands of stars like those seen in the Antennae Galaxies.<\/p> Eventually the dance abates and the remaining stars settle down into a single galaxy, often bent into a strange shape.<\/p> There are various options for what can emerge from a merger.<\/p> Two galaxies with once-distinctive spiral arms can become an almost featureless elliptical galaxy \u2013 a rugby-ball-shaped blob of stars.<\/p> “You can also get a ring galaxy,” explains Conselice. This is where a merger hollows out the inner part of a galaxy, leaving only the outer stars in a ring around the centre. <\/p> Then there\u2019s the Whirlpool Galaxy<\/a>, which is currently interacting with smaller companion galaxy NGC 5195.<\/p> Whatever unfolds, it\u2019s not a quick process. “Mergers take a billion years to finish,” says Conselice.<\/p> Galaxy mergers leave their mark in other ways too.<\/p> “The gas does become affected,” says Conselice. “It interacts with other gas.”<\/p> That leads to epic bursts of star formation as high temperatures and pressures spark new stars into life.<\/p> The star-formation rate for galaxies in the early Universe was about 50%, but this has dropped to about 30% in the modern Universe.<\/p> Without star formation \u2013 and without the galaxy mergers that helped trigger it on a massive scale \u2013 we probably wouldn\u2019t exist.<\/p> “You wouldn\u2019t have as much metallicity in the Universe,” explains Conselice.<\/p> To an astronomer, a metal is any chemical element that isn\u2019t hydrogen or helium.<\/p> Massive stars churn those two elements into heavier ones, like the nitrogen that makes up most of the air you\u2019re breathing, or the oxygen in that air that keeps you alive, or the very carbon atoms you are made from in the first place.<\/p> “Galaxy mergers connect to planets and life,” Conselice says.<\/p> Galaxy mergers may look spectacular, but there is a lot going on behind the scenes that we cannot see. <\/p> For one thing, stars, gas and dust make up just a small part of a galaxy. Most of the mass of a galaxy is in the form of dark matter<\/a>.<\/p> In fact, when groups of galaxies, called galaxy clusters<\/a>, collide, ordinary matter is often separated from dark matter, providing a strong clue to the latter\u2019s existence.<\/p> Astronomers can see dark matter\u2019s influence through an effect known as gravitational lensing<\/a>, whereby the merged object bends distant starlight around it.<\/p> Dark matter plays a big role in our current best model of the Universe\u2019s size, structure and origin.<\/p> “It\u2019s called Lambda CDM,” says Conselice. Here the CDM stands for Cold Dark Matter (more on the \u2018Lambda\u2019 later).<\/p> “Mergers are a prediction of Lambda CDM,” he says. So finding them feeds back into and reinforces our most fundamental picture of the Universe.<\/p> If we didn\u2019t see galaxy mergers, something would be amiss with the model and astronomers would have to return to the drawing board.<\/p> The other largely invisible thing hiding inside galaxies is the supermassive black holes (SMBHs) at their hearts.<\/p> In the Milky Way, for example, an SMBH known as Sagittarius A* tips the scales at four million times the mass of the Sun.<\/p> When two galaxies merge, so do their black holes.<\/p> And there are still some things that we don\u2019t fully understand about how this happens, such as the Final Parsec Problem<\/a>.<\/p> “The mass of the SMBH links to the mass of the galaxy,” explains Conselice.<\/p> “There\u2019s a tight correlation between the way the black hole forms and how the galaxy forms.”<\/p> In this way, SMBHs are the conductors of the cosmic orchestra. Yet there is still a step missing that astronomers don\u2019t quite understand.<\/p> We\u2019ve seen lots of small black holes and many enormous ones, but very few in between.<\/p> Such intermediate-mass black holes would be the building blocks of SMBHs, so perhaps they merge quickly and don\u2019t hang around for long.<\/p> Black hole marriages could also open the door to exploring galaxy mergers that are far too far away, and so too faint, to see with ordinary telescopes.<\/p> When two black holes collide, they produce gravitational waves<\/a>, ripples in the very fabric of the Universe itself.<\/p> They travel at the same speed as light and eventually wash up on Earth\u2019s celestial shoreline.<\/p> Gravitational wave astronomy is still in its infancy, as the first detection only occurred in 2015.<\/p> So far our devices, such the LIGO and VIRGO facilities, are sensitive to small black hole mergers \u2013 those where the colliding black holes have masses tens of times the mass of the Sun.<\/p> To be able to detect gravitational waves from SMBH mergers, we are going to need a bigger experiment. Much<\/em> bigger.<\/p> Thankfully, there is one already in the pipeline. The Laser Interferometer Space Antenna (LISA), due for launch in 2037, will consist of three spacecraft flying in triangular formation.<\/p> The gap between each spacecraft will be 2.5 million kilometres.<\/p> You may think that eventually all galaxies will merge as gravity inexorably draws them together.<\/p> However, there is another factor at play, and one whose potency will only grow stronger with time:Galaxy collisions and the evolution of the Universe<\/strong><\/h2>
![\"Galaxy](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2021\/01\/Colliding-Galaxy-01-bcaa8bc.jpg\")
What causes galaxy collisions<\/strong><\/h2>
What happens when galaxies collide<\/strong><\/h2>
![\"Taffy](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/05\/04.noirlab2308a-f40bdef.jpg\")
![\"A](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/07\/heic0206b-5e347af.jpg\")
![\"Messier](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/03\/m51-and-companion_0-73e2787.jpg\")
Galaxy collisions and star formation<\/strong><\/h2>
![\"Hubble](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/03\/potw1520a-HEADER-bc96025.jpg\")
Dark matter<\/strong><\/h2>
![\"This](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/09\/Re-Ionization-Galaxy_Evolution-tracking_shot-H264-720p.01327_print.jpg\"){kind=tracking}
Supermassive black holes and the Final Parsec Problem<\/strong><\/h2>
![\"Artist's](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/11\/supermassive-black-holes-merging-1024x683.jpg?fit=800%2C534\")
Gravitational waves<\/strong><\/h2>
![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2022\/03\/black-holes-colliding-gravitational-waves-7f7c970.jpg\")
Dark energy<\/strong><\/h2>
![\"Illustration](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2022\/03\/universe-expansion-3c06d8c.jpg\")