̽��ֱ�� of Cambridge - dwarf galaxy

̽��ֱ��Gaia Sausage: the major collision that changed the Milky Way

sc604 — Wed, 04 Jul 2018 06:59:56 +0000

̽��ֱ��astronomers propose that around eight to 10 billion years ago, an unknown dwarf galaxy smashed into our own Milky Way. ̽��ֱ��dwarf did not survive the impact. It quickly fell apart, and the wreckage is now all around us.

“ ̽��ֱ��collision ripped the dwarf to shreds, leaving its stars moving on very radial orbits, like needles,” said Vasily Belokurov of the ̽��ֱ�� of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City. “These stars’ paths take them very close to the centre of our galaxy. This is a tell-tale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.”

̽��ֱ��salient features of this extraordinary event are outlined in several new papers, some of which were led by Cambridge graduate student GyuChul Myeong. He and colleagues used data from the European Space Agency's Gaia satellite. This spacecraft has been mapping the stellar content of our galaxy, recording the journeys of stars as they travel through the Milky Way. Thanks to Gaia, astronomers now know the positions and trajectories of our celestial neighbours with unprecedented accuracy.

“ ̽��ֱ��paths of the stars from the galactic merger earned the moniker ‘Gaia Sausage’,” said Wyn Evans of Cambridge’s Institute of Astronomy. “We plotted the velocities of the stars, and the sausage shape just jumped out at us. As the smaller galaxy broke up, its stars were thrown out on very radial orbits. These Sausage stars are what's left of the last major merger of the Milky Way.”

There are ongoing mergers taking place right now, such as between the puny Sagittarius dwarf galaxy and the Milky Way. However, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter was more than 10 billion times the mass of our sun. When it crashed into the young Milky Way, it caused a lot of mayhem. ̽��ֱ��Sausage’s piercing trajectory meant that the Milky Way’s disk was probably puffed up or even fractured following the impact, and the Milky Way had to re-grow a new disk. At the same time, the Sausage debris was scattered all around the inner parts of the Milky Way, creating the ‘bulge’ at the galaxy’s centre and the surrounding ‘stellar halo’.

“Numerical simulations of the galactic smash-up can reproduce these features,” said Denis Erkal of the ̽��ֱ�� of Surrey. In simulations ran by Erkal and colleagues, stars from the Sausage galaxy enter stretched out orbits. ̽��ֱ��orbits are further elongated by the growing Milky Way disk, which swells and becomes thicker following the collision.

“Evidence of this galactic remodelling is seen in the paths of stars inherited from the dwarf galaxy,” said Alis Deason of Durham ̽��ֱ��. “ ̽��ֱ��Sausage stars are all turning around at about the same distance from the centre of the Galaxy. These U-turns cause the density in the Milky Way’s stellar halo to drop dramatically where the stars flip directions.” This discovery was especially pleasing for Deason, who predicted this orbital apocentric pile-up almost five years ago.

̽��ֱ��new research also identified at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies do not normally have globular clusters of their own, so the Sausage galaxy was big enough to host its own entourage of clusters.

“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” said Sergey Koposov of Carnegie-Mellon ̽��ֱ��, who has been studying the kinematics of the Sausage stars and globular cluster in detail.

̽��ֱ��head-on collision of the Sausage galaxy was a defining event in the early history of the Milky Way. It created the thick disk and the inner stellar halo. Even though the merger took place at a very remote epoch, the stars in the Sausage galaxy can be picked out today. Memory of this event persists in the kinematics and chemistry of its stars. Thanks to the Gaia satellite, astronomers have miraculous data with which we can peer back into the very distant past and recreate the pre-history of our galactic home.

Reference:
Paper 1, Paper 2, Paper 3, Paper 4, Paper 5

An international team of astronomers has discovered an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed ‘the Sausage galaxy’. ̽��ֱ��cosmic crash was a defining event in the early history of the Milky Way and reshaped the structure of our galaxy, fashioning both the galaxy’s inner bulge and its outer halo, the astronomers report in a series of new papers.

These Sausage stars are what's left of the last major merger of the Milky Way.

Wyn Evans

V. Belokurov (Cambridge, UK) based on an image by ESO/Juan Carlos Muñoz

Artist's impression of a collision between the Milky Way and a massive dwarf

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Fastest stars in the Milky Way are ‘runaways’ from another galaxy

sc604 — Tue, 04 Jul 2017 23:09:10 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, used data from the Sloan Digital Sky Survey and computer simulations to demonstrate that these stellar sprinters originated in the Large Magellanic Cloud (LMC), a dwarf galaxy in orbit around the Milky Way.

These fast-moving stars, known as hypervelocity stars, were able to escape their original home when the explosion of one star in a binary system caused the other to fly off with such speed that it was able to escape the gravity of the LMC and get absorbed into the Milky Way. ̽��ֱ��results are published in the Monthly Notices of the Royal Astronomical Society, and will be presented today (5 July) at the National Astronomy Meeting in Hull.

Astronomers first thought that the hypervelocity stars, which are large blue stars, may have been expelled from the centre of the Milky Way by a supermassive black hole. Other scenarios involving disintegrating dwarf galaxies or chaotic star clusters can also account for the speeds of these stars but all three mechanisms fail to explain why they are only found in a certain part of the sky.

To date, roughly 20 hypervelocity stars have been observed, mostly in the northern hemisphere, although it’s possible that there are many more that can only be observed in the southern hemisphere.

“Earlier explanations for the origin of hypervelocity stars did not satisfy me,” said Douglas Boubert, a PhD student at Cambridge’s Institute of Astronomy and the paper’s lead author. “ ̽��ֱ��hypervelocity stars are mostly found in the Leo and Sextans constellations – we wondered why that is the case.”

An alternative explanation to the origin of hypervelocity stars is that they are runaways from a binary system. In binary star systems, the closer the two stars are, the faster they orbit one another. If one star explodes as a supernova, it can break up the binary and the remaining star flies off at the speed it was orbiting. ̽��ֱ��escaping star is known as a runaway. Runaway stars originating in the Milky Way are not fast enough to be hypervelocity because blue stars can’t orbit close enough without the two stars merging. But a fast-moving galaxy could give rise to these speedy stars.

̽��ֱ��LMC is the largest and fastest of the dozens of dwarf galaxies in orbit around the Milky Way. It only has 10% of the mass of the Milky Way, and so the fastest runaways born in this dwarf galaxy can easily escape its gravity. ̽��ֱ��LMC flies around the Milky Way at 400 kilometres per second and, like a bullet fired from a moving train, the speed of these runaway stars is the velocity they were ejected at plus the velocity of the LMC. This is fast enough for them to be the hypervelocity stars.

“These stars have just jumped from an express train – no wonder they’re fast,” said co-author Rob Izzard, a Rutherford fellow at the Institute of Astronomy. “This also explains their position in the sky, because the fastest runaways are ejected along the orbit of the LMC towards the constellations of Leo and Sextans.”

̽��ֱ��researchers used a combination of data from the Sloan Digital Sky Survey and computer simulations to model how hypervelocity stars might escape the LMC and end up in the Milky Way. ̽��ֱ��researchers simulated the birth and death of stars in the LMC over the past two billion years, and noted down every runaway star. ̽��ֱ��orbit of the runaway stars after they were kicked out of the LMC was then followed in a second simulation that included the gravity of the LMC and the Milky Way. These simulations allow the researchers to predict where on the sky we would expect to find runaway stars from the LMC.

“We are the first to simulate the ejection of runaway stars from the LMC – we predict that there are 10,000 runaways spread across the sky,” said Boubert. Half of the simulated stars which escape the LMC are fast enough to escape the gravity of the Milky Way, making them hypervelocity stars. If the previously known hypervelocity stars are runaway stars it would also explain their position in the sky.

Massive blue stars end their lives by collapsing to a neutron star or black hole after hundreds of millions of years and runaway stars are no different. Most of the runaway stars in the simulation died ‘in flight’ after being kicked out of the LMC. ̽��ֱ��neutron stars and black holes that are left behind just continue on their way and so, along with the 10,000 runaway stars, the researchers also predict a million runaway neutron stars and black holes flying through the Milky Way.

“We’ll know soon enough whether we’re right,” said Boubert. “ ̽��ֱ��European Space Agency’s Gaia satellite will report data on billions of stars next year, and there should be a trail of hypervelocity stars across the sky between the Leo and Sextans constellations in the North and the LMC in the South.”

Reference
D. Boubert, D. Erkal, N. W. Evans and R. G. Izzard. ‘Hypervelocity runaways from the Large Magellanic Cloud.’ Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stex848.

A group of astronomers have shown that the fastest-moving stars in our galaxy – which are travelling so fast that they can escape the Milky Way – are in fact runaways from a much smaller galaxy in orbit around our own.

These stars have just jumped from an express train – no wonder they’re fast.

Rob Izzard

Amanda Smith, Institute of Astronomy

Artist’s impression of a runaway star

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Massive holes ‘punched’ through a trail of stars likely caused by dark matter

sc604 — Wed, 07 Sep 2016 08:00:50 +0000

Researchers have detected two massive holes which have been ‘punched’ through a stream of stars just outside the Milky Way, and found that they were likely caused by clumps of dark matter, the invisible substance which holds galaxies together and makes up a quarter of all matter and energy in the universe.

̽��ֱ��scientists, from the ̽��ֱ�� of Cambridge, found the holes by studying the distribution of stars in the Milky Way. While the clumps of dark matter that likely made the holes are gigantic in comparison to our Solar System – with a mass between one million and 100 million times that of the Sun – they are actually the tiniest clumps of dark matter detected to date.

̽��ֱ��results, which have been submitted to the Monthly Notices of the Royal Astronomical Society, could help researchers understand the properties of dark matter, by inferring what type of particle this mysterious substance could be made of. According to their calculations and simulations, dark matter is likely made up of particles more massive and more sluggish than previously thought, although such a particle has yet to be discovered.

“While we do not yet understand what dark matter is formed of, we know that it is everywhere,” said Dr Denis Erkal from Cambridge’s Institute of Astronomy, the paper’s lead author. “It permeates the universe and acts as scaffolding around which astrophysical objects made of ordinary matter – such as galaxies – are assembled.”

Current theory on how the universe was formed predicts that many of these dark matter building blocks have been left unused, and there are possibly tens of thousands of small clumps of dark matter swarming in and around the Milky Way. These small clumps, known as dark matter sub-haloes, are completely dark, and don’t contain any stars, gas or dust.

Dark matter cannot be directly measured, and so its existence is usually inferred by the gravitational pull it exerts on other objects, such as by observing the movement of stars in a galaxy. But since sub-haloes don’t contain any ordinary matter, researchers need to develop alternative techniques in order to observe them.

̽��ֱ��technique the Cambridge researchers developed was to essentially look for giant holes punched through a stream of stars. These streams are the remnants of small satellites, either dwarf galaxies or globular clusters, which were once in orbit around our own galaxy, but the strong tidal forces of the Milky Way have torn them apart. ̽��ֱ��remnants of these former satellites are often stretched out into long and narrow tails of stars, known as stellar streams.

“Stellar streams are actually simple and fragile structures,” said co-author Dr Sergey Koposov. “ ̽��ֱ��stars in a stellar stream closely follow one another since their orbits all started from the same place. But they don’t actually feel each other’s presence, and so the apparent coherence of the stream can be fractured if a massive body passes nearby. If a dark matter sub-halo passes through a stellar stream, the result will be a gap in the stream which is proportional to the mass of the body that created it.”

̽��ֱ��researchers used data from the stellar streams in the Palomar 5 globular cluster to look for evidence of a sub-halo fly-by. Using a new modelling technique, they were able to observe the stream with greater precision than ever before. What they found was a pair of wrinkled tidal tails, with two gaps of different widths.

By running thousands of computer simulations, the researchers determined that the gaps were consistent with a fly-by of a dark matter sub-halo. If confirmed, these would be the smallest dark matter clumps detected to date.

“If dark matter can exist in clumps smaller than the smallest dwarf galaxy, then it also tells us something about the nature of the particles which dark matter is made of – namely that it must be made of very massive particles,” said co-author Dr Vasily Belokurov. “This would be a breakthrough in our understanding of dark matter.”

̽��ֱ��reason that researchers can make this connection is that the mass of the smallest clump of dark matter is closely linked to the mass of the yet unknown particle that dark matter is composed of. More precisely, the smaller the clumps of dark matter, the higher the mass of the particle.

Since we do not yet know what dark matter is made of, the simplest way to characterise the particles is to assign them a particular energy or mass. If the particles are very light, then they can move and disperse into very large clumps. But if the particles are very massive, then they can’t move very fast, causing them to condense – in the first instance – into very small clumps.

“Mass is related to how fast these particles can move, and how fast they can move tells you about their size,” said Belokurov. “So that’s why it’s so interesting to detect very small clumps of dark matter, because it tells you that the dark matter particle itself must be very massive.”

“If our technique works as predicted, in the near future we will be able to use it to discover even smaller clumps of dark matter,” said Erkal. “It’s like putting dark matter goggles on and seeing thousands of dark clumps each more massive than a million suns whizzing around.”

Reference:
Denis Erkal et al. ‘A sharper view of Pal 5’s tails: Discovery of stream perturbations with a novel non-parametric technique.’ arXiv:1609.01282

̽��ֱ��discovery of two massive holes punched through a stream of stars could help answer questions about the nature of dark matter, the mysterious substance holding galaxies together.

While we do not yet understand what dark matter is formed of, we know that it is everywhere.

Denis Erkal

V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge). Photo: Colour image of M31 from Adam Evans.

Artist's impression of dark matter clumps around a Milky Way-like galaxy

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Welcome to the neighbourhood: new dwarf galaxies discovered in orbit around the Milky Way

sc604 — Tue, 10 Mar 2015 12:00:00 +0000

A team of astronomers from the ̽��ֱ�� of Cambridge have identified nine new dwarf satellites orbiting the Milky Way, the largest number ever discovered at once. ̽��ֱ��findings, from newly-released imaging data taken from the Dark Energy Survey, may help unravel the mysteries behind dark matter, the invisible substance holding galaxies together.

̽��ֱ��new results also mark the first discovery of dwarf galaxies – small celestial objects that orbit larger galaxies – in a decade, after dozens were found in 2005 and 2006 in the skies above the northern hemisphere. ̽��ֱ��new satellites were found in the southern hemisphere near the Large and Small Magellanic Cloud, the largest and most well-known dwarf galaxies in the Milky Way’s orbit.

̽��ֱ��Cambridge findings are being jointly released today with the results of a separate survey by astronomers with the Dark Energy Survey, headquartered at the US Department of Energy’s Fermi National Accelerator Laboratory. Both teams used the publicly available data taken during the first year of the Dark Energy Survey to carry out their analysis.

̽��ֱ��newly discovered objects are a billion times dimmer than the Milky Way, and a million times less massive. ̽��ֱ��closest is about 95,000 light years away, while the most distant is more than a million light years away.

According to the Cambridge team, three of the discovered objects are definite dwarf galaxies, while others could be either dwarf galaxies or globular clusters – objects with similar visible properties to dwarf galaxies, but not held together with dark matter.

“ ̽��ֱ��discovery of so many satellites in such a small area of the sky was completely unexpected,” said Dr Sergey Koposov of Cambridge’s Institute of Astronomy, the study’s lead author. “I could not believe my eyes.”

Dwarf galaxies are the smallest galaxy structures observed, the faintest of which contain just 5000 stars – the Milky Way, in contrast, contains hundreds of billions of stars. Standard cosmological models of the universe predict the existence of hundreds of dwarf galaxies in orbit around the Milky Way, but their dimness and small size makes them incredibly difficult to find, even in our own ‘backyard’.

“ ̽��ֱ��large dark matter content of Milky Way satellite galaxies makes this a significant result for both astronomy and physics,” said Alex Drlica-Wagner of Fermilab, one of the leaders of the Dark Energy Survey analysis.

Since they contain up to 99 percent dark matter and just one percent observable matter, dwarf galaxies are ideal for testing whether existing dark matter models are correct. Dark matter – which makes up 25 percent of all matter and energy in our universe – is invisible, and only makes its presence known through its gravitational pull.

“Dwarf satellites are the final frontier for testing our theories of dark matter,” said Dr Vasily Belokurov of the Institute of Astronomy, one of the study’s co-authors. “We need to find them to determine whether our cosmological picture makes sense. Finding such a large group of satellites near the Magellanic Clouds was surprising, though, as earlier surveys of the southern sky found very little, so we were not expecting to stumble on such treasure.”

̽��ֱ��closest of these pieces of ‘treasure’ is 97,000 light years away, about halfway to the Magellanic Clouds, and is located in the constellation of Reticulum, or the Reticle. Due to the massive tidal forces of the Milky Way, it is in the process of being torn apart.

̽��ֱ��most distant and most luminous of these objects is 1.2 million light years away in the constellation of Eridanus, or the River. It is right on the fringes of the Milky Way, and is about to get pulled in. According to the Cambridge team, it looks to have a small globular cluster of stars, which would make it the faintest galaxy to possess one.

“These results are very puzzling,” said co-author Wyn Evans, also of the Institute of Astronomy. “Perhaps they were once satellites that orbited the Magellanic Clouds and have been thrown out by the interaction of the Small and Large Magellanic Cloud. Perhaps they were once part of a gigantic group of galaxies that – along with the Magellanic Clouds – are falling into our Milky Way galaxy.”

̽��ֱ��Dark Energy Survey is a five-year effort to photograph a large portion of the southern sky in unprecedented detail. Its primary tool is the Dark Energy Camera, which – at 570 megapixels – is the most powerful digital camera in the world, able to see galaxies up to eight billion light years from Earth. Built and tested at Fermilab, the camera is now mounted on the four-metre Victor M Blanco telescope at the Cerro Tololo Inter-American Observatory in the Andes Mountains in Chile. ̽��ֱ��camera includes five precisely shaped lenses, the largest nearly a yard across, designed and fabricated at ̽��ֱ�� College London (UCL) and funded by the UK Science and Technology Facilities Council (STFC).

̽��ֱ��Dark Energy Survey is supported by funding from the STFC, the US Department of Energy Office of Science; the National Science Foundation; funding agencies in Spain, Brazil, Germany and Switzerland; and the participating institutions.

̽��ֱ��Cambridge research, funded by the European Research Council, will be published in ̽��ֱ��Astrophysical Journal.

Inset image: ̽��ֱ��Magellanic Clouds and the Auxiliary Telescopes at the Paranal Observatory in the Atacama Desert in Chile. Only 6 of the 9 newly discovered satellites are present in this image. ̽��ֱ��other three are just outside the field of view. ̽��ֱ��insets show images of the three most visible objects (Eridanus 1, Horologium 1 and Pictoris 1) and are 13x13 arcminutes on the sky (or 3000x3000 DECam pixels). Credit: V. Belokurov, S. Koposov (IoA, Cambridge). Photo: Y. Beletsky (Carnegie Observatories)

Astronomers have discovered a ‘treasure trove’ of rare dwarf satellite galaxies orbiting our own Milky Way. ̽��ֱ��discoveries could hold the key to understanding dark matter, the mysterious substance which holds our galaxy together.

Earlier surveys of the southern sky found very little, so we were not expecting to stumble on such treasure

Vasily Belokurov

European Southern Observatory

̽��ֱ��dwarf galaxies are located near the Large and Small Magellanic Clouds, at the centre of the image.

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