̽��ֱ�� of Cambridge - Vasily Belokurov

Gaia spots a ‘ghost’ galaxy next door

sc604 — Tue, 13 Nov 2018 00:00:00 +0000

An international team of astronomers, including from the ̽��ֱ�� of Cambridge, discovered the massive object when trawling through data from the European Space Agency’s Gaia satellite. ̽��ֱ��object, named Antlia 2 (or Ant 2), has avoided detection until now thanks to its extremely low density as well as a perfectly-chosen hiding place, behind the shroud of the Milky Way’s disc. ̽��ֱ��researchers have published their results online today.

Ant 2 is known as a dwarf galaxy. As structures emerged in the early Universe, dwarfs were the first galaxies to form, and so most of their stars are old, low-mass and metal-poor. But compared to the other known dwarf satellites of our Galaxy, Ant 2 is immense: it is as big as the Large Magellanic Cloud (LMC), and a third the size of the Milky Way itself.

What makes Ant 2 even more unusual is how little light it gives out. Compared to the LMC, another satellite of the Milky Way, Ant 2 is 10,000 times fainter. In other words, it is either far too large for its luminosity or far too dim for its size.

“This is a ghost of a galaxy,” said Gabriel Torrealba, the paper’s lead author. “Objects as diffuse as Ant 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data.”

̽��ֱ��ESA’s Gaia mission has produced the richest star catalogue to date, including high-precision measurements of nearly 1.7 billion stars and revealing previously unseen details of our home Galaxy. Earlier this year, Gaia’s second data release made new details of stars in the Milky Way available to scientists worldwide.

̽��ֱ��researchers behind the current study – from Taiwan, the UK, the US, Australia and Germany – searched the new Gaia data for Milky Way satellites by using RR Lyrae stars. These stars are old and metal-poor, typical of those found in a dwarf galaxy. RR Lyrae change their brightness with a period of half a day and can be located thanks to these well-defined pulses.

“RR Lyrae had been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disc, we weren’t totally surprised,” said co-author Vasily Belokurov from Cambridge’s Institute of Astronomy. “But when we looked closer at their location on the sky it turned out we found something new, as no previously identified object came up in any of the databases we searched through.”

̽��ֱ��team contacted colleagues at the Anglo-Australian Telescope (AAT) in Australia, but when they checked the coordinates for Ant 2, they realised they had a limited window of opportunity to get follow-up data. They were able to measure the spectra of more than 100 red giant stars just before the Earth’s motion around the Sun rendered Ant 2 unobservable for months.

̽��ֱ��spectra enabled the team to confirm that the ghostly object they spotted was real: all the stars were moving together. Ant 2 never comes too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years) away. ̽��ֱ��researchers were also able to obtain the galaxy’s mass, which was much lower than expected for an object of its size.

“ ̽��ֱ��simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the Galactic tides of the Milky Way,” said co-author Sergey Koposov from Carnegie Mellon ̽��ֱ��. “What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow.”

If it is impossible to puff the dwarf up by removing matter from it, then Ant 2 had to have been born huge. ̽��ֱ��team has yet to figure out the exact process that made Ant 2 so extended. While objects of this size and luminosity have not been predicted by current models of galaxy formation, recently it has been speculated that some dwarfs could be inflated by vigorous star formation. Stellar winds and supernova explosions would push away the unused gas, weakening the gravity that binds the galaxy and allowing the dark matter to drift outward as well.

“Even if star formation could re-shape the dark matter distribution in Ant 2 as it was put together, it must have acted with unprecedented efficiency,” said co-author Jason Sanders, also from Cambridge.

Alternatively, Ant 2’s low density could mean that a modification to the dark matter properties is needed. ̽��ֱ��currently favoured theory predicts dark matter to pack tightly in the centres of galaxies. Given how fluffy the new dwarf appears to be, a dark matter particle which is less keen to cluster may be required.

“Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball,” said co-author Matthew Walker, also from Carnegie Mellon ̽��ֱ��. “We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one.”

̽��ֱ��gap between Ant 2 and the rest of the Galactic dwarfs is so wide that this may well be an indication that some important physics is missing in the models of dwarf galaxy formation. Solving the Ant 2 puzzle may help researchers understand how the first structures in the early Universe emerged. Finding more objects like Ant 2 will show just how common such ghostly galaxies are, and the team is busy looking for other similar galaxies in the Gaia data.

Reference:
G. Torrealba et al. ‘ ̽��ֱ��hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2.’ arXiv: 1811.04082

̽��ֱ��Gaia satellite has spotted an enormous ‘ghost’ galaxy lurking on the outskirts of the Milky Way.

When we looked closer, it turned out we found something new

Vasily Belokurov

V. Belokurov based on the images by Marcus and Gail Davies and Robert Gendler

L-R: Large Magellanic Cloud, the Milky Way, Antlia 2

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

̽��ֱ��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

Yes

A bridge of stars connects two dwarf galaxies

ps748 — Wed, 08 Feb 2017 12:36:40 +0000

For the past 15 years, scientists have been eagerly anticipating the data from Gaia. ̽��ֱ��first portion of information from the satellite was released three months ago and is freely accessible to everyone. This dataset of unprecedented quality is a catalogue of the positions and brightness of a billion stars in our Milky Way galaxy and its environs.

What Gaia has sent to Earth is unique. ̽��ֱ��satellite’s angular resolution is similar to that of the Hubble Space Telescope, but given its greater field of view, it can cover the entire sky rather than a small portion of it. In fact, Gaia uses the largest number of pixels to take digital images of the sky for any space-borne instrument. Better still, the Observatory has not just one telescope but two, sharing the one metre wide focal plane.

Unlike typical telescopes, Gaia does not just point and stare: it constantly spins around its axis, sweeping the entire sky in less than a month. Therefore, it not only measures the instantaneous properties of the stars, but also tracks their changes over time. This provides a perfect opportunity for finding a variety of objects, for example stars that pulsate or explode - even if this is not what the satellite was primarily designed for.

̽��ֱ��Cambridge team concentrated on the area around the Magellanic Clouds and used the Gaia data to pick out pulsating stars of a particular type: the so-called RR Lyrae, very old and chemically un-evolved. As these stars have been around since the earliest days of the Clouds’ existence, they offer an insight into the pair’s history. Studying the Large and Small Magellanic Clouds (LMC and SMC respectively) has always been difficult as they sprawl out over a large area. But with Gaia’s all-sky view, this has become a much easier task.

Around the Milky Way, the clouds are the brightest, and largest, examples of dwarf satellite galaxies. Known to humanity since the dawn of history (and to Europeans since their first voyages to the Southern hemisphere) the Magellanic Clouds have remained an enigma to date. Even though the clouds have been a constant fixture of the heavens, astronomers have only recently had the chance to study them in any detail.

̽��ֱ��Magellanic Clouds can be seen just above the horizon and below the arc of the Milky Way - D Erkal

Whether the clouds fit the conventional theory of galaxy formation or not depends critically on their mass and the time of their first approach to the Milky Way. ̽��ֱ��researchers at Cambridge’s Institute of Astronomy found clues that could help answer both of these questions.

Firstly, the RR Lyrae stars detected by Gaia were used to trace the extent of the Large Magellanic Cloud. ̽��ֱ��LMC was found to possess a fuzzy low-luminosity ‘halo’ stretching as far as 20 degrees from its centre. ̽��ֱ��LMC would only be able to hold on to the stars at such large distances if it was substantially bigger than previously thought, totalling perhaps as much as a tenth of the mass of the entire Milky Way.

An accurate timing of the clouds’ arrival to the galaxy is impossible without knowledge of their orbits. Unfortunately, satellite orbits are difficult to measure: at large distances, the object’s motion in the sky is so minute that it is simply unobservable over a human lifespan. In the absence of an orbit, Dr Vasily Belokurov and colleagues found the next best thing: a stellar stream.

Streams of stars form when a satellite - a dwarf galaxy or a star cluster - starts to feel the tidal force of the body around which it orbits. ̽��ֱ��tides stretch the satellite in two directions: towards and away from the host. As a result, on the periphery of the satellite, two openings form: small regions where the gravitational pull of the satellite is balanced by the pull of the host. Satellite stars that enter these regions find it easy to leave the satellite altogether and start orbiting the host. Slowly, star after star abandons the satellite, leaving a luminous trace on the sky, and thus revealing the satellite’s orbit.

“Stellar streams around the Clouds were predicted but never observed,” explains Dr Belokurov. “Having marked the locations of the Gaia RR Lyrae on the sky, we were surprised to see a narrow bridge-like structure connecting the two clouds. We believe that at least in part this ‘bridge’ is composed of stars stripped from the Small Cloud by the Large. ̽��ֱ��rest may actually be the LMC stars pulled from it by the Milky Way.”

̽��ֱ��researchers believe the RR Lyrae bridge will help to clarify the history of the interaction between the clouds and our galaxy.

"We have compared the shape and the exact position of the Gaia stellar bridge to the computer simulations of the Magellanic Clouds as they approach the Milky Way”, explains Dr Denis Erkal, a co-author of the study. "Many of the stars in the bridge appear to have been removed from the SMC in the most recent interaction, some 200 million years ago, when the dwarf galaxies passed relatively close by each other. “We believe that as a result of that fly-by, not only the stars but also hydrogen gas was removed from the SMC. By measuring the offset between the RR Lyrae and hydrogen bridges, we can put constraints on the density of the gaseous Galactic corona.”

Composed of ionised gas at very low density, the hot Galactic corona is notoriously difficult to study. Nevertheless, it has been the subject of intense scrutiny because scientists believe it may contain most of the missing baryonic - or ordinary - matter. Astronomers are trying to estimate where this missing matter (the atoms and ions that make up stars, planets, dust and gas) is. It’s thought that most, or even all, of these missing baryons are in the corona. By measuring the coronal density at large distances they hope to solve this conundrum.

During the previous encounter between the Small and Large Magellanic Cloud, both stars and gas were ripped out of the Small Cloud, forming a tidal stream. Initially, the gas and stars were moving at the same speed. However, as the Clouds approached our Galaxy, the Milky Way’s corona exerted a drag force on both of them. ̽��ֱ��stars, being relatively small and dense, punched through the corona with no change in their speed. However, the more tenuous neutral hydrogen gas slowed down substantially in the corona. By comparing the current location of the stars and the gas, taking into account the density of the gas and how long the Clouds have spent in the corona, the team estimated the density of the corona. Dr. Erkal concludes, “Our estimate showed that the corona could make up a significant fraction of the missing baryons, in agreement with previous independent techniques. With the missing baryon problem seemingly alleviated, the current model of galaxy formation is holding up well to the increased scrutiny possible with Gaia.”

Reference
Vasily Belokurov et al. “Clouds, Streams and Bridges. Redrawing the blueprint of the Magellanic System with Gaia DR1”. Monthly Notices of the Royal Astronomical Society; 8th Feb. 2017; DOI:10.1093/mnras/stw3357

̽��ֱ��Magellanic Clouds, the two largest satellite galaxies of the Milky Way, appear to be connected by a bridge stretching across 43,000 light years, according to an international team of astronomers led by researchers from the ̽��ֱ�� of Cambridge. ̽��ֱ��discovery is reported in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) and is based on the Galactic stellar census being conducted by the European Space Observatory, Gaia.

We believe that at least in part this 'bridge' is composed of stars stripped from the Small Cloud by the Large

Vasily Belokurov

V Belokurov, D Erkal, A Mellinger

Pale white veils and the narrow bridge between the clouds represent the distribution of the RR Lyrae stars

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

Yes

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.

Yes

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.

̽��ֱ��text in this work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page. For image rights, please see the credits associated with each individual image.

Yes

Licence type:

Attribution