The outer reaches of the Milky Way galaxy are a different place. Stars are much harder to come by, with most of this “galactic halo” being made up of empty space. But scientists theorize that there is an abundance of one particular thing in this desolate area – dark matter. Now, a team from Harvard and the University of Arizona (UA) spent some time studying and modeling one of the galaxy’s nearest neighbors to try to tease out more information about that dark matter, and as a result came up with an all new way to look at the halo itself.
An artist's impression of the four tails of the Sagittarius Dwarf Galaxy (the orange clump on the left of the image) orbiting the Milky Way. The bright yellow circle to the right of the galaxy's center is our Sun (not to scale). Image credit: Amanda Smith (University of Cambridge)
Around the Milky Way, there are literally dozens of dwarf galaxies that continue to be slowly absorbed into our own. These galaxies are a major source of interest for astronomers because they can teach us a great deal about cosmic evolution, like how smaller galaxies merged over time to create larger structures. Since they are thought to be relics of the very first galaxies in the Universe, they are also akin to “galactic fossils.”
Recently, a team of astrophysicists from the Massachusetts Institute of Technology (MIT) observed one of the most ancient of these galaxies (Tucana II) and noticed something unexpected. At the edge of the galaxy, they observed stars in a configuration that suggest that Tucana II has an extended Dark Matter halo. These findings imply that the most ancient galaxies in the Universe had more Dark Matter than previously thought.
Image of the large-scale structure of the Universe, showing filaments and voids within the cosmic structure. Who knows how many other civilizations might be out there? Credit: Millennium Simulation Project
Matter in the Universe is not distributed equally. It’s dominated by super-clusters and the filaments of matter that string them together, surrounded by huge voids. Galaxy super-clusters are at the top of the hierarchy. Inside those is everything else: galaxy groups and clusters, individual galaxies, and solar systems. This hierarchical structure is called the “Cosmic Web.”
Hubble Space Telescope image of Messier 77 spiral galaxy. A version of this image won second place in the Hubble’s Hidden Treasures Image Processing Competition. Image: NASA, ESA & A. van der Hoeven, 2013
It’s a difficult thing to wrap your head around sometimes. Though it might feel stationary, planet Earth is actually moving at an average velocity of 29.78 km/s (107,200 km/h; 66600 mph). And yet, our planet has nothing on the Sun itself, which travels around the center of our galaxy at a velocity of 220 km/s (792,000 km/h; 492,000 mph).
But as is so often the case with our Universe, things only get more staggering the farther you look. According to a new study by an international team of astronomers, the most massive “super spiral” galaxies in the Universe rotate twice as fast as the Milky Way. The cause, they argue, is the massive clouds (or halos) of Dark Matter that surround these galaxies.
The LSST, or Vera Rubin Survey Telescope, under construction at Cerro Pachon, Chile. Image Credit: LSST
The U.S. House of Representatives have passed a bill to change the name of the Large Synoptic Survey Telescope (LSST.) Instead of that explanatory yet cumbersome name, it will be named after American astronomer Vera Rubin. Rubin is well-known for her pioneering work in discovering dark matter.
A section of the virtual universe, a billion light years across, showing how dark matter is distributed in space, with dark matter halos the yellow clumps, interconnected by dark filaments. Cosmic void, shown as the white areas, are the lowest density regions in the Universe. Credit: Joachim Stadel, UZH
Understanding the Universe and how it has evolved over the course of billions of years is a rather daunting task. On the one hand, it involves painstakingly looking billions of light years into deep space (and thus, billions of years back in time) to see how its large-scale structure changed over time. Then, massive amounts of computing power are needed to simulate what it should look like (based on known physics) and seeing if they match up.
That is what a team of astrophysicists from the University of Zurich (UZH) did using the “Piz Daint” supercomputer. With this sophisticated machine, they simulated the formation of our entire Universe and produced a catalog of about 25 billion virtual galaxies. This catalog will be launched aboard the ESA’s Euclid mission in 2020, which will spend six years probing the Universe for the sake of investigating dark matter.
The team’s work was detailed in a study that appeared recently in the journal Computational Astrophysics and Cosmology. Led by Douglas Potter, the team spent the past three years developing an optimized code to describe (with unprecedented accuracy) the dynamics of dark matter as well as the formation of large-scale structures in the Universe.
The code, known as PKDGRAV3, was specifically designed to optimally use the available memory and processing power of modern super-computing architectures. After being executed on the “Piz Daint” supercomputer – located at the Swiss National Computing Center (CSCS) – for a period of only 80 hours, it managed to generate a virtual Universe of two trillion macro-particles, from which a catalogue of 25 billion virtual galaxies was extracted.
Intrinsic to their calculations was the way in which dark matter fluid would have evolved under its own gravity, thus leading to the formation of small concentrations known as “dark matter halos”. It is within these halos – a theoretical component that is thought to extend well beyond the visible extent of a galaxy – that galaxies like the Milky Way are believed to have formed.
Naturally, this presented quite the challenge. It required not only a precise calculation of how the structure of dark matter evolves, but also required that they consider how this would influence every other part of the Universe. As Joachim Stadel, a professor with the Center for Theoretical Astrophysics and Cosmology at UZH and a co-author on the paper, told Universe Today via email:
“We simulated 2 trillion such dark matter “pieces”, the largest calculation of this type that has ever been performed. To do this we had to use a computation technique known as the “fast multipole method” and use one of the fastest computers in the world, “Piz Daint” at the Swiss National Supercomputing Centre, which among other things has very fast graphics processing units (GPUs) which allow an enormous speed-up of the floating point calculations needed in the simulation. The dark matter clusters into dark matter “halos” which in turn harbor the galaxies. Our calculation accurately produces the distribution and properties of the dark matter, including the halos, but the galaxies, with all of their properties, must be placed within these halos using a model. This part of the task was performed by our colleagues at Barcelona under the direction of Pablo Fossalba and Francisco Castander. These galaxies then have the expected colors, spatial distribution and the emission lines (important for the spectra observed by Euclid) and can be used to test and calibrate various systematics and random errors within the entire instrument pipeline of Euclid.”
Artist impression of the Euclid probe, which is set to launch in 2020. Credit: ESA
Thanks to the high precision of their calculations, the team was able to turn out a catalog that met the requirements of the European Space Agency’s Euclid mission, whose main objective is to explore the “dark universe”. This kind of research is essential to understanding the Universe on the largest of scales, mainly because the vast majority of the Universe is dark.
Between the 23% of the Universe which is made up of dark matter and the 72% that consists of dark energy, only one-twentieth of the Universe is actually made up of matter that we can see with normal instruments (aka. “luminous” or baryonic matter). Despite being proposed during the 1960s and 1990s respectively, dark matter and dark energy remain two of the greatest cosmological mysteries.
Given that their existence is required in order for our current cosmological models to work, their existence has only ever been inferred through indirect observation. This is precisely what the Euclid mission will do over the course of its six year mission, which will consist of it capturing light from billions of galaxies and measuring it for subtle distortions caused by the presence of mass in the foreground.
Much in the same way that measuring background light can be distorted by the presence of a gravitational field between it and the observer (i.e. a time-honored test for General Relativity), the presence of dark matter will exert a gravitational influence on the light. As Stadel explained, their simulated Universe will play an important role in this Euclid mission – providing a framework that will be used during and after the mission.
Diagram showing the Lambda-CBR universe, from the Big Bang to the the current era. Credit: Alex Mittelmann/Coldcreation
“In order to forecast how well the current components will be able to make a given measurement, a Universe populated with galaxies as close as possible to the real observed Universe must be created,” he said. “This ‘mock’ catalogue of galaxies is what was generated from the simulation and will be now used in this way. However, in the future when Euclid begins taking data, we will also need to use simulations like this to solve the inverse problem. We will then need to be able to take the observed Universe and determine the fundamental parameters of cosmology; a connection which currently can only be made at a sufficient precision by large simulations like the one we have just performed. This is a second important aspect of how such simulation work [and] is central to the Euclid mission.”
From the Euclid data, researchers hope to obtain new information on the nature of dark matter, but also to discover new physics that goes beyond the Standard Model of particle physics – i.e. a modified version of general relativity or a new type of particle. As Stadel explained, the best outcome for the mission would be one in which the results do not conform to expectations.
“While it will certainly make the most accurate measurements of fundamental cosmological parameters (such as the amount of dark matter and energy in the Universe) far more exciting would be to measure something that conflicts or, at the very least, is in tension with the current ‘standard lambda cold dark matter‘ (LCDM) model,” he said. “One of the biggest questions is whether the so called ‘dark energy’ of this model is actually a form of energy, or whether it is more correctly described by a modification to Einstein’s general theory of relativity. While we may just begin to scratch the surface of such questions, they are very important and have the potential to change physics at a very fundamental level.”
In the future, Stadel and his colleagues hope to be running simulations on cosmic evolution that take into account both dark matter and dark energy. Someday, these exotic aspects of nature could form the pillars of a new cosmology, one which reaches beyond the physics of the Standard Model. In the meantime, astrophysicists from around the world will likely be waiting for the first batch of results from the Euclid mission with baited breath.
Euclid is one of several missions that is currently engaged in the hunt for dark matter and the study of how it shaped our Universe. Others include the Alpha Magnetic Spectrometer (AMS-02) experiment aboard the ISS, the ESO’s Kilo Degree Survey (KiDS), and CERN’s Large Hardon Collider. With luck, these experiments will reveal pieces to the cosmological puzzle that have remained elusive for decades.
The image on the left shows a portion of our sky, called the Boötes field, in infrared light, while the image on the right shows a mysterious, background infrared glow captured by NASA’s Spitzer Space Telescope in the same region of sky.Credit: NASA/JPL-Caltech
What causes the mysterious glow of radiation seen across the entire sky by infrared telescopes? The answer may lie in a combination of concepts that are relatively new to the field of astronomy, and are somewhat controversial, too. Rogue stars that have been kicked out of galaxies may be embedded in dark matter halos that have been theorized to surround galaxies. While these dark matter halos have previously only been detected indirectly by observing their gravitational effects, they may also hold the source of the enigmatic background glow of radiation.
“The infrared background glow in our sky has been a huge mystery,” said Asantha Cooray of the University of California at Irvine, lead author of the new research published today in the journal Nature. “We have new evidence this light is from the stars that linger between galaxies. Individually, the stars are too faint to be seen, but we think we are seeing their collective glow.”
The collective glow is from the “interhalo” of dark matter halos that pervade the Universe, and may answer the big question of why the amount of light observed exceeds the amount of light emitted from known galaxies.
“Galaxies exist in dark matter halos that are much bigger than the galaxies; when galaxies form and merge together, the dark matter halo gets larger and the stars and gas sink to the middle of the halo,” said Edward L. (Ned) Wright from UCLA and a member of the team that used the Spitzer Space Telescope to seek out the source of the infrared light. “What we’re saying is one star in a thousand does not do that and instead gets distributed like dark matter. You can’t see the dark matter very well, but we are proposing that it actually has a few stars in it — only one-tenth of 1 percent of the number of stars in the bright part of the galaxy. One star in a thousand gets stripped out of the visible galaxy and gets distributed like the dark matter.”
The dark matter halo is not totally dark, Wright said. “A tiny fraction, one-tenth of a percent, of the stars in the central galaxy has been spread out into the halo, and this can produce the fluctuations that we see.”
In large clusters of galaxies, astronomers have found much higher percentages of intra-halo light, as large as 20 percent, Wright said.
For this study, Cooray, Wright and colleagues used the Spitzer Space Telescope to produce an infrared map of a region of the sky in the constellation Boötes. The light has been travelling to us for 10 billion years.
“Presumably this light in halos occurs everywhere in the sky and just has not been measured anywhere else,” said Wright, who is also principal investigator of NASA’s Wide-field Infrared Survey Explorer (WISE) mission.
“If we can really understand the origin of the infrared background, we can understand when all of the light in the universe was produced and how much was produced,” Wright said. “The history of all the production of light in the universe is encoded in this background. We’re saying the fluctuations can be produced by the fuzzy edges of galaxies that existed at the same time that most of the stars were created, about 10 billion years ago.”
The light appears at a blotchy pattern in the Spitzer images.
The new finding are at odds with a study that came out this summer. Alexander “Sasha” Kashlinsky of NASA’s Goddard Space Flight Center and his team looked at this same patch of sky with Spitzer and proposed the light making the unusual pattern was coming from the very first stars and galaxies.
In the new study, Cooray and colleagues looked at data from a larger portion of the sky, called the Bootes field, covering an arc equivalent to 50 full Earth moons. These observations were not as sensitive as those from the Kashlinsky group’s studies, but the larger scale allowed researchers to analyze better the pattern of the background infrared light.
“We looked at the Bootes field with Spitzer for 250 hours,” said co-author Daniel Stern of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Studying the faint infrared background was one of the core goals of our survey, and we carefully designed the observations in order to directly address the important, challenging question of what causes the background glow.”
The team concluded the light pattern of the infrared glow is not consistent with theories and computer simulations of the first stars and galaxies. Researchers say the glow is too bright to be from the first galaxies, which are thought not to have been as large or as numerous as the galaxies we see around us today. Instead, the scientists propose a new theory to explain the blotchy light, based on theories of “intracluster” or “intrahalo” starlight.
The team said more research is needed to confirm these findings, adding that the James Webb Space Telescope should help.
“The keen infrared vision of the James Webb Telescope will be able to see some of the earliest stars and galaxies directly, as well as the stray stars lurking between the outskirts of nearby galaxies,” said Eric Smith, JWST’s deputy program manager at NASA Headquarters in Washington. “The mystery objects making up the background infrared light may finally be exposed.”
The circled cluster of stars is the dwarf galaxy Andromeda 29, which University of Michigan astronomers have discovered. The bright star within the circle is a foreground star within our own Milky Way galaxy. This image was obtained with the Gemini Multi-Object Spectrograph at the Gemini North telescope in Hawaii. Credit: Gemini Observstory/AURA/Eric Bell
[/caption]
Yep. It’s that time of year again. Time to enjoy the Andromeda Galaxy at almost every observing opportunity. But now, rather than just look at the nearest spiral to the Milky Way and sneaking a peak at satellites M32 and M110, we can think about something more when we peer M31’s way. There are two newly discovered dwarf galaxies that appear to be companions of Andromeda!
Eric Bell, an associate professor in astronomy, and Colin Slater, an astronomy Ph.D. student, found Andromeda 28 and Andromeda 29 by utilizing the Sloan Digital Sky Survey and a recently developed star counting technique. To back up their observations, the team employed data from the Gemini North Telescope in Hawaii. Located at 1.1 million and 600,000 light-years respectively, Andromeda XXVIII and Andromeda XXIX have the distinction of being the two furthest satellite galaxies ever detected away from the host – M31. Can they be spotted with amateur equipment? Not hardly. This pair comes in about 100,000 fainter than Andromeda itself and can barely be discerned with some of the world’s largest telescopes. They’re so faint, they haven’t even been classified yet.
“With presently available imaging we are unable to determine whether there is ongoing or recent star formation, which prevents us from classifying it as a dwarf spheroidal or a dwarf irregular.” explains Bell.
The dwarf galaxy Andromeda 29, which University of Michigan astronomers have discovered, is clustered toward the middle of this image, obtained with the Gemini North telescope in Hawaii. Credit: Gemini Observstory/AURA/Eric Bell
In their work – published in a recent edition of the edition of the Astrophysical Journal Letters – the team of Bell and Slater explains how they were searching for dwarf galaxies around Andromeda to help them understand how physical matter relates to theoretical dark matter. While we can’t see it, hear it, touch it or smell it, we know it’s there because of its gravitational influence. And when it comes to gravity, many astronomers are convinced that dark matter plays a role in organizing galaxy structure.
“These faint, dwarf, relatively nearby galaxies are a real battleground in trying to understand how dark matter acts at small scales,” Bell said. “The stakes are high.”
Right now, current consensus has all galaxies embedded in surrounding dark matter… and each “bed” of dark matter should have a galaxy. Considering the volume of the Universe, these predictions are pretty much spot on – if we take only large galaxies into account.
“But it seems to break down when we get to smaller galaxies,” Slater said. “The models predict far more dark matter halos than we observe galaxies. We don’t know if it’s because we’re not seeing all of the galaxies or because our predictions are wrong.”
“The exciting answer,” Bell said, “would be that there just aren’t that many dark matter halos.” Bell said. “This is part of the grand effort to test that paradigm.”
Right or wrong… pondering dark matter and dwarf galaxies while observing Andromeda will add a whole new dimension to your observations!
For Further Reading: Andromeda XXVIII: A Dwarf Galaxy more than 350 kpc from Andromeda and Andromeda XXIX: A New Dwarf Spheroidal Galaxy 200 kpc from Andromeda.
