��Ī - Wyn Evans

Dark energy could be measured by studying the galaxy next door

sc604 — Mon, 14 Aug 2023 14:31:03 +0000

The researchers, from the ��Ī, found that it may be possible to detect and measure dark energy by studying Andromeda, our galactic next-door neighbour that is on a slow-motion collision course with the Milky Way.

Since it was first identified in the late 1990s, scientists have used very distant galaxies to study dark energy but have yet to directly detect it. However, the ��Ī researchers found that by studying how Andromeda and the Milky Way are moving toward each other given their collective mass, they could place an upper limit on the value of the cosmological constant, which is the simplest model of dark energy. The upper limit they found is five times higher than the value of the cosmological constant that can be detected from the early universe.

Although the technique is still early in its development, the researchers say that it could be possible to detect dark energy by studying our own cosmic neighbourhood. The results are reported in The Astrophysical Journal Letters.

Everything we can see in our world and in the skies – from tiny insects to massive galaxies – makes up just five percent of the observable universe. The rest is dark: scientists believe that about 27% of the universe is made of dark matter, which holds objects together, while 68% is dark energy, which pushes objects apart.

“Dark energy is a general name for a family of models you could add to Einstein’s theory of gravity,” said first author Dr David Benisty from the Department of Applied Mathematics and Theoretical Physics. “The simplest version of this is known as the cosmological constant: a constant energy density that pushes galaxies away from each other.”

The cosmological constant was temporarily added by Einstein to his theory of general relativity. From the 1930s to the 1990s, the cosmological constant was set at zero, until it was discovered that an unknown force – dark energy – was causing the expansion of the universe to accelerate. There are at least two big problems with dark energy, however: we don’t know exactly what it is, and we haven’t directly detected it.

Since it was first identified, astronomers have developed a variety of methods to detect dark energy, most of which involve studying objects from the early universe and measuring how quickly they are moving away from us. Unpacking the effects of dark energy from billions of years ago is not easy: since it is a weak force between galaxies, dark energy is easily overcome by the much stronger forces inside galaxies.

However, there is one region of the universe that is surprisingly sensitive to dark energy, and it’s in our own cosmic backyard. The Andromeda galaxy is the closest to our own Milky Way, and the two galaxies are on a collision course. As they draw closer, the two galaxies will start to orbit each other – very slowly. A single orbit will take 20 billion years. However, due to the massive gravitational forces, well before a single orbit is complete, about five billion years from now, the two galaxies will start merging and falling into each other.

“Andromeda is the only galaxy that isn’t running away from us, so by studying its mass and movement, we may be able to make some determinations about the cosmological constant and dark energy,” said Benisty, who is also a Research Associate at Queens’ College.

Using a series of simulations based on the best available estimates of the mass of both galaxies, Benisty and his co-authors – Professor Anne Davis from DAMTP and Professor Wyn Evans from the Institute of Astronomy – found that dark energy is affecting how Andromeda and the Milky Way are orbiting each other.

“Dark energy affects every pair of galaxies: gravity wants to pull galaxies together, while dark energy pushes them apart,” said Benisty. “In our model, if we change the value of the cosmological constant, we can see how that changes the orbit of the two galaxies. Based on their mass, we can place an upper bound on the cosmological constant, which is about five times higher than we can measure from the rest of the universe.”

The researchers say that while the technique could prove immensely valuable, it is not yet a direct detection of dark energy. Data from the James Webb Telescope (JWST) will provide far more accurate measurements of Andromeda’s mass and motion, which could help reduce the upper bounds of the cosmological constant.

In addition, by studying other pairs of galaxies, it could be possible to further refine the technique and determine how dark energy affects our universe. “Dark energy is one of the biggest puzzles in cosmology,” said Benisty. “It could be that its effects vary over distance and time, but we hope this technique could help unravel the mystery.”

Reference:
David Benisty, Anne-Christine Davis, and N. Wyn Evans. ‘Constraining Dark Energy from the Local Group Dynamics.’ The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/ace90b

Researchers have found a new way to measure dark energy – the mysterious force that makes up more than two-thirds of the universe and is responsible for its accelerating expansion – in our own cosmic backyard.

Andromeda is the only galaxy that isn’t running away from us, so by studying its mass and movement, we may be able to make some determinations about dark energy

David Benisty

NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

Artist's impression of the predicted collision between the Milky Way and Andromeda

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

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

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

“The collision ripped the dwarf to shreds, leaving its stars moving on very radial orbits, like needles,” said Vasily Belokurov of the ��Ī 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.”

The salient features of this extraordinary event are outlined in several new papers, some of which were led by ��Ī 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.

“The paths of the stars from the galactic merger earned the moniker ‘Gaia Sausage’,” said Wyn Evans of ��Ī’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. The 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 University of Surrey. In simulations ran by Erkal and colleagues, stars from the Sausage galaxy enter stretched out orbits. The 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 University. “The 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.

The 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 University, who has been studying the kinematics of the Sausage stars and globular cluster in detail.

The 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’. The 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 (��Ī, 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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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 ��Ī have identified nine new dwarf satellites orbiting the Milky Way, the largest number ever discovered at once. The 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.

The 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. The 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.

The ��Ī 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.

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

According to the ��Ī 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.

“The discovery of so many satellites in such a small area of the sky was completely unexpected,” said Dr Sergey Koposov of ��Ī’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’.

“The 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.”

The 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.

The 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 ��Ī 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.”

The 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. The camera includes five precisely shaped lenses, the largest nearly a yard across, designed and fabricated at University College London (UCL) and funded by the UK Science and Technology Facilities Council (STFC).

The 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.

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

Inset image: The 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. The other three are just outside the field of view. The 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, ��Ī). Photo: Y. Beletsky (Carnegie Observatories)

Astronomers have discovered a ‘treasure trove’ of rare dwarf satellite galaxies orbiting our own Milky Way. The 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

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

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�������������Ī - Wyn Evans

Dark energy could be measured by studying the galaxy next door

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

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

��Ī - Wyn Evans