tag:theconversation.com,2011:/uk/topics/andromeda-3111/articlesAndromeda – The Conversation2018-08-28T20:19:51Ztag:theconversation.com,2011:article/1019772018-08-28T20:19:51Z2018-08-28T20:19:51ZCurious Kids: Where are all the other galaxies hidden?<figure><img src="https://images.theconversation.com/files/233003/original/file-20180822-149484-cs44to.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The other galaxies are there, but they are hiding a very long way away.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/galaxy-nebula-elements-this-image-furnished-530443579">www.shutterstock.com</a></span></figcaption></figure><p><em>This is an article from <a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a>, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky! You might also like the podcast <a href="http://www.abc.net.au/kidslisten/imagine-this/">Imagine This</a>, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.</em> </p>
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<p><strong>Where are all the other galaxies hidden? – Sasha, age 7, Sydney.</strong></p>
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<p>Hi Sasha, </p>
<p>Thanks for sending in your question.</p>
<p>Almost everything in our universe is high above the clouds, even higher than an aeroplane. If you keep going up, up, up… you get to outer space. There is a lot of empty space up there, as well as some fascinating things like planets, stars and beautiful clouds of gas.</p>
<p>Imagine that you could launch yourself on a very fast spaceship — travelling as fast as lightning — the first place you would reach would be the planets in our solar system: Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. At that speed, you could travel to the planets in a few minutes. Which would you visit first?</p>
<p>If you went even further, travelling for almost five years on your spaceship you would reach the nearest stars. Many of these stars live in twos or threes, and have their very own planets too. We don’t know whether there are any animals, plants, birds or fish on these planets, or perhaps creatures that we don’t have here on Earth. Maybe that’s a mission for astronaut Sasha. Can you imagine what life on other planets might be like?</p>
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Read more:
<a href="https://theconversation.com/curious-kids-will-the-universe-expand-forever-or-contract-in-a-big-crunch-96209">Curious Kids: will the universe expand forever, or contract in a big crunch?</a>
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<p>If you zoom even further on your spaceship, travelling for thousands of years, you will start to see that we live in a huge spiral galaxy containing two hundred billion stars. It’s called the Milky Way and you can see it glowing in the night sky as a band of light. Have you ever seen the Milky Way?</p>
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<a href="https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=342&fit=crop&dpr=1 600w, https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=342&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=342&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=430&fit=crop&dpr=1 754w, https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=430&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/233007/original/file-20180822-149481-1uvtm1h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=430&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">An artist’s impression of the Milky Way galaxy, which is where you find our solar system.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
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<p>Now, keep travelling out of our Milky Way, for more than two million years (you’ll be very old by then). You will finally reach the nearest spiral galaxy, called the Andromeda galaxy. It’s hard to believe, but all the other galaxies are even further away than this. Some galaxies are so far away they would take thousands of millions of years to reach.</p>
<p>So to answer your question Sasha, the other galaxies are there, but they are hiding a very long way away. That’s why we can’t easily see them when we look up at the night sky. Luckily, we don’t have to fly in a spaceship for millions of years to find other galaxies. Scientists like me, astronomers who study space, can use huge telescopes to study the light coming from galaxies and see what they are up to.</p>
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Read more:
<a href="https://theconversation.com/curious-kids-whats-going-to-happen-to-the-sun-in-the-future-will-it-explode-78029">Curious Kids: What's going to happen to the Sun in the future? Will it explode?</a>
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<p>Galaxies are fascinating. Like people, some are young and others are old. Some galaxies are made up of hot stars that have only recently been born, like baby stars. Other galaxies are colder and contain mostly older stars. Galaxies can be active too. As gravity pulls them towards one another, they can merge together and collide. This will eventually happen to the Milky Way, which will collide with the Andromeda galaxy in about four billion years’ time. When that happens, the Sun will keep shining but our night sky will look very different.</p>
<p>I hope you will keep looking up at the stars, astronaut Sasha. It’s a fascinating place out there. Let me know if you find life on those planets!</p>
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<p><em>Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to us. They can:</em></p>
<p><em>* Email your question to curiouskids@theconversation.edu.au
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<img alt="" src="https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=376&fit=crop&dpr=1 600w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=376&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=376&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=472&fit=crop&dpr=1 754w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=472&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=472&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p><em>Please tell us your name, age and which city you live in. You can send an audio recording of your question too, if you want. Send as many questions as you like! We won’t be able to answer every question but we will do our best.</em></p><img src="https://counter.theconversation.com/content/101977/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lisa Harvey-Smith has authored a book called 'When Galaxies Collide' (Melbourne University Publishing, 2018). </span></em></p>We are in the Milky Way. If you travelled on an extremely fast spaceship for more than two million years, you would reach our neighbour, the Andromeda galaxy. All other galaxies are even further away.Lisa Harvey-Smith, Astronomer, CSIROLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/527322016-01-28T10:46:32Z2016-01-28T10:46:32ZIs our Milky Way galaxy a zombie, already dead and we don’t know it?<figure><img src="https://images.theconversation.com/files/109408/original/image-20160127-26823-vlapaf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Can a galaxy (like NGC 3810 in this case) have a classical spiral structure and also be already dead?</span> <span class="attribution"><a class="source" href="https://www.spacetelescope.org/images/potw1006a/">ESA/Hubble and NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Like a zombie, the Milky Way galaxy may already be dead but it still keeps going. Our galactic neighbor Andromeda almost certainly expired a few billion years ago, but only recently started showing outward signs of its demise.</p>
<p>Galaxies seem to be able to “perish” – that is, stop turning gas into new stars – via two very different pathways, driven by very different processes. Galaxies like the Milky Way and Andromeda do so very, very slowly over billions of years. </p>
<p>How and why galaxies “quench” their star formation and change their morphology, or shape, is one of the big questions in extragalactic astrophysics. We may now be on the brink of being able to piece together how it happens. And part of the thanks goes to citizen scientists who combed through millions of galactic images to classify what’s out there.</p>
<h2>Galaxies grow by making new stars</h2>
<p>Galaxies are dynamic systems that continually accrete gas and convert some of it into stars.</p>
<p>Like people, galaxies need food. In the case of galaxies, that “food” is a supply of fresh hydrogen gas from the cosmic web, the filaments and halos of dark matter that make up the largest structures in the universe. As this gas <a href="http://www.illustris-project.org/media/">cools and falls into dark matter halos</a>, it turns into a disk that then can cool even further and eventually fragment into stars.</p>
<p>As stars age and die, they can return some of that gas back into the galaxy either via winds from stars or by <a href="https://en.wikipedia.org/wiki/Supernova">going supernova</a>. As massive stars die in such explosions, they heat the gas around them and prevent it from cooling down quite so fast. They provide what astronomers call “feedback”: star formation in galaxies is thus a self-regulated process. The heat from dying stars means cosmic gas doesn’t cool into new stars as readily, which ultimately puts a brake on how many new stars can form. </p>
<p>Most of these star-forming galaxies are disk- or spiral-shaped, like our Milky Way.</p>
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<a href="https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=304&fit=crop&dpr=1 600w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=304&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=304&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=381&fit=crop&dpr=1 754w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=381&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=381&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Left: a spiral galaxy ablaze in the blue light of young stars from ongoing star formation; right: an elliptical galaxy bathed in the red light of old stars.</span>
<span class="attribution"><a class="source" href="http://www.sdss3.org/">Sloan Digital Sky Survey</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
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<p>But there’s another kind of galaxy that has a very different shape, or morphology, in astronomer-parlance. These massive elliptical galaxies tend to look spheroidal or football-shaped. They’re not nearly so active – they’ve lost their supply of gas and therefore have ceased forming new stars. Their stars move on far more unordered orbits, giving them their bulkier, rounder shape.</p>
<p>These elliptical galaxies differ in two major ways: they no longer form stars and they have a different shape. Something pretty dramatic must have happened to them to produce such profound changes. What?</p>
<h2>Blue=young and red=old?</h2>
<p>The basic division of galaxies into star-forming spiral galaxies blazing in the blue light of massive, young and short-lived stars, on the one hand, and quiescent ellipticals bathed in the warm glow of ancient low-mass stars, on the other, goes back to early galaxy surveys of the 20th century.</p>
<p>But, once modern surveys like the Sloan Digital Sky Survey (<a href="http://www.sdss.org/">SDSS</a>) began to record hundreds of thousands of galaxies, objects started emerging that didn’t quite fit into those two broad categories. </p>
<p>A significant number of red, quiescent galaxies aren’t elliptical in shape at all, but retain roughly a disk shape. Somehow, these galaxies stopped forming stars without dramatically changing their structure.</p>
<p>At the same time, blue elliptical galaxies started to surface. Their structure is similar to that of “red and dead” ellipticals, but they shine in the bright blue light of young stars, indicating that star formation is still ongoing in them.</p>
<p>How do these two oddballs – the red spirals and the blue ellipticals – fit into our picture of galaxy evolution?</p>
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<a href="https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=423&fit=crop&dpr=1 600w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=423&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=423&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=532&fit=crop&dpr=1 754w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=532&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=532&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Galaxy Zoo allows citizen scientists to classify galaxies.</span>
<span class="attribution"><span class="source">Screenshot by Kevin Schawinski</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<h2>Send in the citizen scientists</h2>
<p>As a graduate student in Oxford, I was looking for some of these oddball galaxies. I was particularly interested in the blue ellipticals and any clues they contained about the formation of elliptical galaxies in general.</p>
<p>At one point, I spent a whole week going through almost 50,000 galaxies from SDSS by eye, as none of the available algorithms for classifying galaxy shape was as good as I needed it to be. I found quite a few blue ellipticals, but the value of classifying all of the roughly one million galaxies in SDSS with human eyes quickly became apparent. Of course, going through a million galaxies by myself wasn’t possible.</p>
<p>A short time later, a group of collaborators and I launched <a href="galaxyzoo.org">galaxyzoo.org</a> and invited members of the public – citizen scientists – to participate in astrophysics research. When you logged on to Galaxy Zoo, you’d be shown an image of a galaxy and a set of buttons corresponding to possible classifications, and a tutorial to help you recognize the different classes. </p>
<p>By the time we stopped recording classifications from a quarter-million people, each of the one million galaxies on Galaxy Zoo had been classified over 70 times, giving me <a href="http://doi.org/10.1111/j.1365-2966.2008.13689.x">reliable, human classifications of galaxy shape</a>, including a measure of uncertainty. Did 65 out of 70 citizen scientists agree that this galaxy is an elliptical? Good! If there’s no agreement at all, that’s information too.</p>
<p>Tapping into the “wisdom of the crowd” effect coupled with the unparalleled human ability for pattern recognition helped sort through a million galaxies and unearthed many of the less common blue ellipticals and red spirals for us to study.</p>
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<a href="https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The galaxy color-mass diagram. Blue, star-forming galaxies are at the bottom, in the blue cloud. Red, quiescent galaxies are at the top, in the red sequence. The ‘green valley’ is the transition zone in between.</span>
<span class="attribution"><span class="source">Schawinski+14</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<h2>Unwittingly living in the green valley?</h2>
<p>The crossroads of galaxy evolution is a place called the “<a href="https://en.wikipedia.org/wiki/Galaxy_color%E2%80%93magnitude_diagram">green valley</a>.” This may sound scenic, but refers to the population between the blue star-forming galaxies (the “blue cloud”) and the red, passively evolving galaxies (the “red sequence”). Galaxies with “green” or intermediate colors should be those galaxies in which star formation is in the process of turning off, but which still have some ongoing star formation – indicating the process only shut down a short while ago, perhaps a few hundred million years.</p>
<p>As a curious aside, the origin of the term “green valley” may actually go back to a talk given at the University of Arizona on galaxy evolution where, when the speaker described the galaxy color-mass diagram, a member of the audience called out: “the green valley, where galaxies go to die!” Green Valley, Arizona, is a retirement community just outside of the university’s hometown, Tucson.</p>
<p>For our project, the really exciting moment came when we looked at the <a href="http://doi.org/10.1093/mnras/stu327">rate at which various galaxies were dying</a>. We found the slowly dying ones are the spirals and the rapidly dying ones are the ellipticals. There must be two fundamentally different evolutionary pathways that lead to quenching in galaxies. When we explored these two scenarios – dying slowly, and dying quickly – it became obvious that these two pathways have to be tied to the gas supply that fuels star formation in the first place.</p>
<p>Imagine a spiral galaxy like our own Milky Way merrily converting gas to stars as new gas keeps flowing in. Then something happens that turns off that supply of fresh outside gas: perhaps the galaxy fell into a massive cluster of galaxies where the hot intra-cluster gas cuts off fresh gas from the outside, or perhaps the dark matter halo of the galaxy grew so much that gas falling into it gets shock heated to such a high temperature that it cannot cool down within the age of the universe. In any case, the spiral galaxy is now left with just the gas it has in its reservoir. </p>
<p>Since these reservoirs can be enormous, and the conversion of gas to stars is a very slow process, our spiral galaxy could go on for quite a while looking “alive” with new stars, while the actual rate of star formation declines over several billion years. The glacial slowness of using up the remaining gas reservoir means that by the time we realize that a galaxy is in terminal decline, the “trigger moment” occurred billions of years ago.</p>
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<a href="https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=192&fit=crop&dpr=1 600w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=192&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=192&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=241&fit=crop&dpr=1 754w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=241&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=241&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A Hubble image of part of the Andromeda galaxy, which like our Milky Way may be a galactic zombie.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/content/goddard/hubble-s-high-definition-panoramic-view-of-the-andromeda-galaxy">NASA, ESA, J. Dalcanton, B.F. Williams and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The Andromeda galaxy, our nearest massive spiral galaxy, is in the green valley and likely began its decline eons ago: it is a zombie galaxy, according to our latest research. It’s dead, but keeps on moving, still producing stars, but at a diminished rate compared to what it should if it were still a normal star-forming galaxy. Working out whether the Milky Way is in the green valley – in the process of shutting down – is much more challenging, as we are in the Milky Way and cannot easily measure <a href="http://doi.org/10.1088/0004-637X/736/2/84">its integrated properties</a> the way we can for distant galaxies.</p>
<p>Even with the more uncertain data, it looks like the Milky Way is just at the edge, ready to tumble into the green valley. It’s entirely possible that the Milky Way galaxy is a zombie, having died a billion years ago.</p><img src="https://counter.theconversation.com/content/52732/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kevin Schawinski receives funding from the Swiss National Fund. He is the co-founder of Galaxy Zoo. </span></em></p>Extragalactic astrophysicists want to know how and why galaxies stop forming stars, change their shape and fade away. With help from citizen scientists, they’re figuring it out.Kevin Schawinski, Assistant Professor of Galaxy & Black Hole Astrophysics, Swiss Federal Institute of Technology ZurichLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/272712014-05-28T06:26:05Z2014-05-28T06:26:05ZHeavens above! What made the cosmic flash that lit Earth today?<figure><img src="https://images.theconversation.com/files/49623/original/wmt6h7h5-1401256863.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The origin of today's burst of energy has astronomers puzzled.</span> <span class="attribution"><a class="source" href="http://www.flickr.com/photos/ap-photographie/8514447719">AP Photographie /Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>A titanic eruption in our neighbouring galaxy, <a href="http://www.nasa.gov/mission_pages/chandra/multimedia/bonanza_image.html">Andromeda</a>, has sent shockwaves through the astronomical community here on Earth. </p>
<p>NASA’s <a href="http://swift.gsfc.nasa.gov/">Swift</a> satellite detected a flood of gamma rays at 21:15 UTC yesterday (7:15 AEST today), triggering telescopes across the globe within minutes to hunt for the afterglow of the explosion. Not long after, the news had raced around the world thanks to <a href="http://www.sciencealert.com.au/images/stories/PicMonkey_Collageastronomy.jpg.jpg">Twitter</a>. </p>
<p>While the cause of the explosion is a mystery at the moment, its implications could be huge.</p>
<p>There are many potential causes of such a flood of gamma rays, but the one that tops astronomers’ wish-list is a short-duration <a href="https://theconversation.com/death-of-a-star-how-radio-waves-can-capture-a-cosmic-obituary-822">gamma ray burst</a>. The cause is still unknown but current models suggest it’s an enormous collision (or inspiral) of two <a href="http://astronomy.swin.edu.au/cosmos/N/Neutron+Star">neutron stars</a>. These are the dead cores of massive stars with as much mass as the sun crushed into a region no larger than a city. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=425&fit=crop&dpr=1 754w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=425&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/49617/original/9q379c4j-1401254915.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=425&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A neutron star compared to the island of Manhattan.</span>
<span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>During their inspiral and ultimate collision into a black hole, an enormous amount of energy is blasted out which we see as light of all wavelengths (from gamma rays, X-rays, visible/optical light and even into the radio).</p>
<p>The energies are so large that were such an explosion to occur in our galaxy it might lead to <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">mass extinctions on Earth</a>. Don’t worry though – the event in Andromeda is 2.5 million <a href="https://theconversation.com/explainer-light-years-and-units-for-the-stars-16995">light years</a> away making it harmless to us (and that this “breaking” story happened while humanity was barely in the Stone Age).</p>
<p>Such events will also set spacetime itself rippling, offering the hope of directly detecting Einstein’s final prediction of <a href="https://theconversation.com/topics/gravitational-waves">gravitational waves</a>.</p>
<figure>
<iframe src="https://player.vimeo.com/video/28966581" width="500" height="281" frameborder="0" webkitallowfullscreen="" mozallowfullscreen="" allowfullscreen=""></iframe>
</figure>
<p></p><p><a href="http://vimeo.com/28966581"></a><a href="</a"></a><a href="</a"></a></p><p></p><a href="</a">
<h2>The (un)usual suspects</h2>
<p>The most discussed cause for the explosion is a gamma ray burst, a collision between neutron stars. These are rare, with perhaps only one every million years expected in a galaxy such as ours or Andromeda.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/V2_kVIGdNRE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The first 500 gamma ray bursts seen by NASA’s Swift satellite.</span></figcaption>
</figure>
</a><p><a href="</a">Other suggested causes could be a “belch” from a feeding black hole in what’s called a </a><a href="http://imagine.gsfc.nasa.gov/docs/science/know_l1/binary_stars.html">Low-Mass X-ray Binary</a>. This is unlikely as the energies are hundreds of time greater than what can normally be produced. </p>
<p>A similar idea, given a catch-all title of Ultraluminous X-ray (<a href="http://www.nasa.gov/mission_pages/swift/bursts/Ultraluminous-Xrays.html">ULX</a>) object, is that it’s a much larger black hole feeding messily, although likely we should have seen this before now.</p>
<p>Another possibility, and the one hardest to discriminate right now, is a flare from a <a href="https://theconversation.com/a-rare-magnetic-star-is-born-with-a-push-in-the-right-direction-26510">magnetar</a>, similar to the eruptions from our sun that cause the Northern and Southern Lights on Earth, but hugely more energetic. </p>
<p>These magnetars are super-magnetised neutron stars, with a magnetic field strong enough to wipe credit cards from half a million kilometres away. While plausible, the gamma ray signal comes from a clump of ancient stars, called a <a href="http://ned.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html">Globular Cluster</a>, which would be unlikely to house a still powered-up magnetar.</p>
<h2>A new sense for humanity</h2>
<p>Everything we know about the world around us is via light or electromagnetic waves. Even when we touch objects the atoms never meet – instead we are stopped by electromagnetic fields between our hand and the object. </p>
<p>Yet if this explosion is a collision between two neutron stars then it would have opened up a fundamentally new sense for humanity, that of gravitational waves, entirely separate to electromagnetic waves. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=422&fit=crop&dpr=1 600w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=422&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=422&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=530&fit=crop&dpr=1 754w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=530&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/49619/original/qsj5dkqt-1401256050.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=530&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The LIGO facility in California.</span>
<span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Although indirect detections have been <a href="https://theconversation.com/first-hints-of-gravitational-waves-in-the-big-bangs-afterglow-24475">claimed</a>, today could have been the first direct confirmation in the lab. </p>
<p>Such a detection would be a potential Nobel Prize winning discovery, which is why it’s particularly unlucky that the Laser Interferometer Gravitational Wave Observatory (<a href="http://www.ligo.org/science/GW-Inspiral.php">LIGO</a>) facility built to detect these gamma ray bursts is currently shutdown for an upgrade.</p>
<h2>After the excitement</h2>
<p>Over the next few days the manner in which the afterglow fades will tell us if it’s the hoped-for gamma ray burst or less extreme but no less exotic magnetar flares or feeding black holes. </p>
<p>In addition to telescopes, particle detectors such as the <a href="http://icecube.wisc.edu/">IceCube</a> facility in Antarctica will be searching for any hints of ghost-like neutrino particles that are often associated with high energy events in the sky, although recent efforts show gamma ray bursts unusually <a href="https://theconversation.com/an-extragalactic-mystery-where-do-high-energy-cosmic-rays-come-from-6623">might be an exception</a>. </p>
<p>Finally, operating gravitational wave detectors, such as <a href="http://www.geo600.org/">GEO600</a>, will analyse their data for a tell-tale high frequency “chirp” as the two neutron stars orbited each other ever faster until finally colliding.</p>
<p>Whatever the cause of the explosion ultimately is, the events of today have shown that even in a science that measures time in billions of years, things can still move fast in astronomy.</p>
<hr>
<p><em>Update: an announcement from the NASA Swift team was that this completely unexpected signal from Andromeda was a <a href="http://gcn.gsfc.nasa.gov/gcn3/16336.gcn3">false alert</a> after all.</em></p>
<p><em>The signal from an existing X-ray source, added to the normal random extra X-rays from distant objects, pushed the NASA satellite over its threshold and triggered an automatic email to astronomers worldwide. Some excellent reanalysis by the team discovered the mistake within a day.</em></p>
<p><em>In science, just because you want a result doesn’t mean nature will oblige, but one thing is for sure – with a cosmic explosion once a day it’s only a matter of time before we get to see the most violent event in the universe close up.</em></p><img src="https://counter.theconversation.com/content/27271/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alan Duffy does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A titanic eruption in our neighbouring galaxy, Andromeda, has sent shockwaves through the astronomical community here on Earth. NASA’s Swift satellite detected a flood of gamma rays at 21:15 UTC yesterday…Alan Duffy, Research Fellow, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/115812013-01-14T03:52:12Z2013-01-14T03:52:12ZCosmic dance challenges our understanding of the universe<figure><img src="https://images.theconversation.com/files/19186/original/hcfn598w-1358132144.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Andromeda galaxy and its companions is challenging the very foundations of cosmology.</span> <span class="attribution"><span class="source">Adam Evans/Wikimedia Commons</span></span></figcaption></figure><p>Deep images of the sky reveal that the universe contains billions of galaxies. Some, such as our own Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, being much smaller and with only a few billion stars.</p>
<p>Modern cosmology has proved to be amazingly accurate in predicting how galaxies are scattered through the universe. Instead of being randomly thrown about, galaxies seem to live together, some in clusters of a thousand individual systems, but most in groups of tens or hundreds.</p>
<p>But new research on dwarf galaxies by my colleagues and me – <a href="http://www.nature.com/nature/journal/v493/n7430/fig_tab/nature11717_ft.html">published in the journal Nature</a> – seems to present a major challenge to ideas of how the universe actually works.</p>
<p>After more than a decade of study, we’ve discovered that small galaxies that accompany the nearest spiral galaxy to our own – the <a href="http://en.wikipedia.org/wiki/Andromeda_Galaxy">Andromeda Galaxy</a> – are dancing together in a vast plane.</p>
<p>To understand why this is significant, we need to begin with what we know about the universe and our own cosmic backyard.</p>
<h2>Local galaxies</h2>
<p>The Milky Way is located in the <a href="http://messier.seds.org/more/local.html">“Local Group”</a>, a small patch of the universe it inhabits with the similar-sized Andromeda Galaxy, and a smaller spiral known as the <a href="http://messier.seds.org/m/m033.html">Triangulum Galaxy</a>.</p>
<p>Accompanying these larger systems are almost 100 dwarf galaxies. Those of us lucky enough to be living in the Southern Hemisphere can clearly see two of these dwarf galaxies in the night sky, namely the <a href="http://messier.seds.org/xtra/ngc/lmc.html">Large</a> and <a href="http://messier.seds.org/xtra/ngc/smc.html">Small</a> Clouds of Magellan.</p>
<p>These many Local Group dwarfs tend to be grouped around the larger galaxies, precisely as predicted by our understanding of the expansion of the universe and the growth of <a href="http://arxiv.org/abs/0910.3211">cosmic structure</a>. But it is precisely these dwarf galaxies that are proving to be a serious headache for the cosmological models they appear to be supporting.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/PPES4lY2IVQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<h2>Where are all the galaxies?</h2>
<p>The first problem, known as the <a href="http://arxiv.org/abs/1009.4505">“missing satellite problem”</a>, has been known for more than a decade. While the <a href="http://arxiv.org/abs/1204.1562">almost-100</a> galaxies in the Local Group may sound impressive, this is far fewer than the several thousand predicted by <a href="http://arxiv.org/abs/1208.5931">cosmological theories</a>.</p>
<p>Some believe that our diminutive dwarf galaxy population is a crippling blow to the prevailing <a href="http://www.scientificamerican.com/podcast/episode.cfm?id=A2B71EFB-ABFA-C6D7-0A728C56892215F8">“cold dark matter” model</a> of galaxy formation – that is, the idea that dominant component of mass in the universe is invisible to us, the scary-sounding <a href="https://theconversation.com/topics/dark-matter">dark matter</a> which shepherds atoms into the stars and galaxies that we see.</p>
<p>Others, however, think <a href="http://arxiv.org/abs/0706.0516">there is no crisis</a>, suggesting the dwarfs are out there as starless dark matter halos, having lost their gas – the raw material for forming new stars – due to the explosions of super-stars in the very early universe.</p>
<p>This neatly brings us to our result in the Nature paper.</p>
<h2>PAndAS</h2>
<p>For more than ten years I have been part of a collaboration that has been trying to map out the extensive stellar halo of Andromeda.</p>
<p>This tenuous distribution of stars, which extends hundreds of thousands of light years away from the Andromeda Galaxy, is made from the remnants of small galaxies that have strayed too close and have been cannibalised by the larger galaxy.</p>
<p>Over the years, our collaboration has used large telescopes and sensitive instruments to map out the immense portion of the sky that encompasses Andromeda’s halo. Since 2008, we have used the 3.6m <a href="http://www.cfht.hawaii.edu/">Canada-France-Hawaii Telescope</a> to undertake the Pan-Andromeda Archaeological Survey (or, more cutely, PAndAS).</p>
<p>Now that the data-taking and processing is complete, the scientific results from PAndAS are starting to flow. As well as the stellar halo, there are large, extended shreds of stars, the ongoing cannibalisation of other systems and lots of globular clusters – small balls of a million stars living together.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19185/original/v4dfg42z-1358131829.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The dwarf galaxies were found to be rotating in a giant plane around Andromeda.</span>
<span class="attribution"><span class="source">Nature</span></span>
</figcaption>
</figure>
<p>Within PAndAS we found almost 30 dwarf galaxies orbiting Andromeda, most of which were identified only in the past few years. With the quality of the PAndAS data, PhD student Anthony Conn was able to accurately measure the distances to each of the dwarf galaxies and, for the first time, we knew the three-dimensional distribution of dwarf galaxies surrounding Andromeda.</p>
<h2>Randomly distributed?</h2>
<p>What do our theoretical models for the structure and evolution of galaxies tell us about the expected distribution of dwarf galaxies around large galaxies? While we know there are not as many dwarfs as predicted, our models tell us that the ones we do see should be buzzing around randomly like a swarm of angry bees.</p>
<p>So, what do we see with the dwarfs orbiting Andromeda? On the face of it, it seems that the theoretical predictions are borne out, with the dwarfs seemingly distributed at random.</p>
<p>But we decided to look a little deeper and see if there was any underlying structure in the dwarf galaxy population. Instead of considering the entire dwarf population, we instead looked for structure subsamples of galaxies, comparing the actual distributions to many thousands of randomly generated samples.</p>
<p>What we found surprised us, with 13 dwarf galaxies lying in an extremely thin plane, 45,000 light years thick, but immense in size – 1.2 million light years in diameter. Such a configuration would appear extremely rarely in a randomly distributed population of dwarfs.</p>
<p>Rather strangely, the edge of this plane points towards the Milky Way; the plane could have been oriented in any direction, but is this mysterious alignment telling us something about the planes origin?</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=597&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=597&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=597&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=750&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=750&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19187/original/cn4m5xxc-1358133450.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=750&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Numerical simulation of the formation of a filament containing hundreds of galaxies.</span>
<span class="attribution"><span class="source">Stefan Gottlöber, Arman Khalatyan & Anatoly Klypin</span></span>
</figcaption>
</figure>
<h2>Only the beginning</h2>
<p>But there were more surprises to come. As the dwarfs were initially discovered, they became the targets of the world’s largest telescopes, using the <a href="http://www.cfht.hawaii.edu/">Doppler shift of light</a> to measure the velocities of the individual galaxies. What we found stopped us in our tracks. </p>
<p>The dwarfs north of Andromeda were moving towards us, while those in the south were moving away from us. That is, this vast plane of dwarfs is rotating!</p>
<p>Remember, nothing like this plane is predicted in our cosmological models.</p>
<p>The mystery deepens when we look closer to home where there has been growing evidence that the Milky Way possesses its own plane of dwarf galaxies, known as <a href="http://arxiv.org/abs/1204.5176">Vast Polar Structure (VPoS)</a>.</p>
<p>Perversely, studying our own galaxy’s stellar halo is more challenging than that of Andromeda as we need to image the entire sky, but the evidence is growing stronger that the Milky Way and Andromeda possess unexplained planes of dwarf galaxies.</p>
<p>What are they doing there?</p>
<h2>Tidal dwarfs?</h2>
<p><a href="http://www.scilogs.com/the-dark-matter-crisis/2013/01/03/andromedas-satellites-behave-as-expected-if-they-are-tidal-dwarf-galaxies/">Some have suggested</a> that what we see are not normal dwarf galaxies, but are actually “tidal dwarfs”, small agglomerations of stars that form from the debris when a large galaxy tears apart a smaller one.</p>
<p>But such dwarfs are transitory and quickly disperse, so such planes should be rare and we would be extremely lucky to see them in both the Milky Way and Andromeda at the same time. </p>
<p>Furthermore, if what we are seeing are truly tidal dwarfs then we are missing even more of the satellite galaxies predicted by our theories for the growth of structure in the universe.</p>
<p>Hence we are faced with a serious cosmological conundrum, a problem that our current theoretical models have to explain if we are to have faith that they are an accurate description of the universe around us.</p>
<p>And if they cannot, well, we mightn’t have to go right back to the drawing-board, but we will need to question our underlying assumptions on things like the nature of dark matter.</p>
<p><a href="http://arxiv.org/abs/1204.2546">Some already</a> think the existence of these planes is telling us that we should be questioning the more fundamental properties of the universe, including the very existence of dark matter and even the action of gravity.</p>
<p>So, what is the solution? Honestly, I do not know, but it is going to be fun trying to find out.</p><img src="https://counter.theconversation.com/content/11581/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Geraint Lewis receives funding from the Australian Research Council, including Discovery Projects and a Future Fellowship.</span></em></p>Deep images of the sky reveal that the universe contains billions of galaxies. Some, such as our own Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs…Geraint Lewis, Professor of Astrophysics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.