What would happen if two galaxies collided? Is it possible? It is very common for galaxies to collide and interact with other galaxies. In fact, it is now believed that collisions and mergers between galaxies are one of the main elements that drive their evolution in time. Most galaxies probably had interactions with other galaxies since the time they formed. A galaxy is made of roughly 100 billion stars. So you would think that in a head on collision between two galaxies, there would be countless collisions between those stars, right? The fact is that in such a collision, the probability of two stars colliding is almost 0. This is because even though there are an incredibly large number of stars in the galaxies, the density of stars is not very big since the galaxies are extremely big. In other words, the sizes of the stars are very small compared to the average distance between them. This means that if galaxies were made only of stars, and that two of them would go on a head on collision, they would pass one through another without being much affected! This is however not what we observe when we look at galaxies interacting. The reason is that the space between stars in galaxies is not empty: it is full of gas and dust. This material will interact when the galaxies collide. It can interact gravitationally, the galaxies can pull on the material in the other galaxies and disrupt their morphologies. There is also friction between the gas in the colliding galaxies, causing shock waves that can trigger some star formation in the galaxies. These processes can radically affect the galaxies. For example, two spiral galaxies can merge to form an elliptical galaxy. Have a look at the series of images below that guide you through such a merger. Be careful about your interpretation of such images though! Since galaxies that collide with one another will take millions of years to merge (which is very quick on the astronomical time scale!), we cannot observe their evolution. When we catch two galaxies in the process of merging, we can only get a snapshot of one step in ther interaction. In order to produce a series of pictures showing the evolution, we have to observe many pairs of similar galaxies at different points in the history of their merging, and then play a game of putting these images in a time sequence. So in the image below, you are in fact looking at 8 different pairs of galaxies, placed in a sequence showing you the different steps in the merging process. These sequences are coherent from what we get from computer simulation (have a look at this one for example).  Credit: Andy Read, Trevor Ponman, Ewan O'Sullivan, Extragalactic Astronomy Group, University of Birmingham. The pictures I've pointed out so far represent one scenario: two similar spiral galaxies merging into one elliptical galaxies. There are other scenarios depending on the masses of the galaxies, but basically when two galaxies of similar mass collide they become a large elliptical galaxy. When a massive galaxy encounters a less massive galaxy, the effects of the merger are smaller and the massive galaxy can maintain its shape. This page was last updated on June 27, 2015
Amelie is working on ways to detect the signals of galaxies from radio maps.
Galaxies may look pretty and delicate, with their swirls of stars of many colours - but don’t be fooled. At the heart of every galaxy lies a supermassive black hole, including in our own Milky Way. Black holes in some nearby galaxies contain ten billion times the mass of our sun in a volume a few times the size of our solar system. That’s a lot of mass in a very small space - not even light travels fast enough to escape a black hole’s gravity. So how did they get that big? In the journal Science today, we tested a commonly-held view that black holes become supermassive by merging with other black holes - and found the answer is not quite that simple. Searching for gravitational wavesThe answer may lie in a related question: when two galaxies collide to form a new galaxy, what happens to their black holes? When galaxies collide, they form a new, bigger galaxy. The colliding galaxies’ black holes sink to the centre of this new galaxy and orbit each other, eventually combining to form a new, bigger black hole. Black holes, as the name suggests, are very hard to observe. But orbiting black holes are the strongest emitters in the universe of an exotic form of energy called gravitational waves. Gravitational waves are a prediction of Einstein’s General Theory of Relativity and are produced by very massive, compact objects changing speed or direction. This, in turn, causes the measured distances between objects to change. For example, a gravitational wave passing through your computer screen will cause it to first stretch in one direction, then in a perpendicular direction, over and over again. Fortunately for your laptop, but unfortunately for astronomers, gravitational waves are very weak. Gravitational waves from a pair of black holes in a nearby galaxy causes your screen size to change by one atomic nucleus over ten years. But fear not - a way to detect these waves exists by using other extreme astronomical objects: pulsars, which are leftovers of massive stellar explosions called supernovae. While they’re not quite as extreme as black holes, pulsars are massive and compact enough to crush atoms into a sea of nuclei and electrons. They compress up to twice the mass of our sun into a volume the size of a large city. So how do pulsars help? First, they rotate very quickly - some of them up to 700 times per second - and very predictably. They emit intense lighthouse-like beams of radio waves, which, when they sweep by the Earth, appear as regular “ticks” - see the video below. So here’s the punchline: gravitational waves from pairs of black holes throughout the universe will disrupt the otherwise extremely regular ticks from pulsars in a way we can measure. Our pulsar measurementsWe found that the theory that black holes grew mainly by absorbing other black holes is not consistent with our data. If the theory was right, gravitational waves would exist at a level that would cause the ticks to appear less regularly than our measurements. This means that black holes must have grown by other means, such as by consuming vast swathes of gas churned up during galaxy mergers. We used measurements of pulsar ticks from the CSIRO Parkes Radio Telescope (the Dish) collected by the Parkes Pulsar Timing Array project led by the CSIRO and Swinburne University of Technology. The measurements span over ten years, and are some of the most precise in existence. These data are being collected to eventually directly observe gravitational waves. In our work, however, we compared the data with gravitational wave predictions from various theories for how black holes grew. Our work gives us great encouragement for the prospects for using pulsars to detect gravitational waves from black holes. We are confident that gravitational waves are out there - galaxies, after all, do collide - and we have shown that we can measure pulsar ticks with sufficient accuracy to be able to detect gravitational waves in the near future. In the meantime, we can even use the absence of gravitational waves to study elusive super-massive black holes. If so, you’ll be interested in our free daily newsletter. It’s filled with the insights of academic experts, written so that everyone can understand what’s going on in the world. With the latest scientific discoveries, thoughtful analysis on political issues and research-based life tips, each email is filled with articles that will inform you and often intrigue you.
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One important way astronomers can learn in some detail about what happens when galaxies collide is
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