Dark matter has been found to be (unexpectedly) evenly distributed across dwarf galaxies, rather than clumping together in the centre - in the way we that we had expected of 'cold' dark matter.
Dark matter – there’s a growing feeling that we are getting closer to finding out the true nature of this elusive stuff. At least we are running a number of experiments that seem (on theoretical grounds) to have the capacity to identify it – and if they don’t… well, maybe it’s time for a rethink of the whole ball game.
There are two arguably quite separate requirements for dark matter to make sense of our current dataset and our theoretical schema for the universe. Firstly, the Standard Model of cosmology (Lambda-Cold Dark Matter) requires that 96% of the universe is composed of stuff of an unknown nature that cannot be directly observed.
This artist's conception shows a dwarf galaxy seen from the surface of a hypothetical exoplanet. A new study finds that the dark matter in dwarf galaxies is distributed smoothly rather than being clumped at their centers. This contradicts simulations using the standard cosmological model known as lambda-CDM. Credit: David A. Aguilar (CfA)
[/caption]
Dark matter… If it can’t be seen, then how do we know it’s there? If it wasn’t for the effects of gravity, we wouldn’t. We’d have a galaxy filled with runaway stars and no galaxy would exist for long. But how it behaves and how it is distributed in one of the biggest cosmic cryptograms of all. Even with new research, there seems to be more questions than answers!
“After completing this study, we know less about dark matter than we did before,” said lead author Matt Walker, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.
It is generally accepted that our Universe is predominately composed of dark matter and dark energy. Of the former, it is considered to be “cold”, stately exotic particles which coalesce through gravitation. As they evolve, these dark matter “clumps” then attract “normal” matter which forms present day galaxy structures. Through computer modeling, astronomers have simulated this growth process which concludes that galactic centers should be dense with dark matter. However, these models aren’t consistent with findings. By measuring two dwarf galaxies, scientists have found a even distribution instead.
“Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data,” Walker stated.
Why study a dwarf instead of a spiral? In this case, the dwarf galaxy is a perfect candidate because of its composition – 99% dark matter and 1% stars. Walker and his co-author Jorge Penarrubia (University of Cambridge, UK) chose two nearby representatives – the Fornax and Sculptor dwarfs – for their study. In comparison to the Milky Way’s estimated 400 billion stars, this pair averages around 10 million instead. This allowed the team to take a comprehensive sample of around 1500 to 2500 stars for location, speed and basic chemical composition. But even at a reduced amount, this type of stellar accounting isn’t exactly easy picking.
“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” explained Penarrubia. “That makes it much more challenging to determine the distribution of dark matter.”
What the team found was somewhat surprising. According to the modeling techniques, dark matter should have clumped at the core. Instead they found it evenly distributed over a distance measuring several hundred light years across.
“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit’ at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia.
It is hypothesized that interactions between normal and dark matter might be responsible for the distribution, but the computer simulations say it shouldn’t happen to a dwarf. New queries to new findings? Yes. This revelation may suggest that dark matter isn’t always “cold” and that it could be impacted by normal matter in unexpected ways.
This image of galaxy cluster MACS J1206.2-0847 is part of a broad survey with NASA's Hubble Space Telescope. Credit: NASA, ESA, M. Postman (STScI), and the CLASH Team
[/caption]
Way off in the constellation of Virgo, galaxy cluster MACS J1206.2-0847 -or MACS 1206 for short – is making news as the forerunner of a brand new Hubble Space Telescope survey. What’s new for the aging telescope? Now astronomers are able to assemble a highly detailed dark matter map… one that involves more galaxy clusters than ever before.
These “dark matter” maps are proving their worth by allowing astronomers to test some theories. In this case, it’s some unusual findings which suggest dark matter is more densely packed inside galaxy clusters than some models predict. If this holds true, it may point to evidence that galaxy clusters pulled together sooner than once predicted. The multiwavelength survey, called the Cluster Lensing And Supernova survey with Hubble (CLASH), takes an unprecedented look at the distribution of dark matter in 25 massive clusters of galaxies.
“The era when the first clusters formed is not precisely known, but is estimated to be at least 9 billion years ago and possibly as far back as 12 billion years ago.” says the Hubble team. “If most of the clusters in the CLASH survey are found to have excessively high accumulations of dark matter in their central cores, then it may yield new clues to the early stages in the origin of structure in the universe.”
To date, the CLASH team has finished their observations of six of the 25 clusters. Of these, MACS 1206 has a distance of about 4.5 billion light-years and was photographed with Hubble’s Advanced Camera for Surveys and the Wide Field Camera 3 in April 2011 through July 2011. What are the strange shapes? These “distortions” are where the light is bent by the extreme gravitation of dark matter.
“Lensing effects can also produce multiple images of the same distant object, as evident in this Hubble picture. In particular, the apparent numbers and shapes of distant galaxies far beyond a galaxy cluster become distorted as the light passes through, yielding a visible measurement of how much mass is in the intervening cluster and how it is distributed.” says the team. “The substantial lensing distortions seen are proof that the dominant component of clusters is dark matter. The distortions would be far weaker if the clusters’ gravity came only from the visible galaxies in the clusters.”
In this video Saul Perlmutter, one of the three winners of the 2011 Nobel Prize in Physics, explains how dark energy, which makes up 70 percent of the universe, is causing our universe to expand.
A cluster of galaxies as seen from the Hubble Space Telescope
[/caption]
Hey. We’re all aware of Einstein’s theories and how gravity affects light. We know it was proved during a total solar eclipse, but what we’ve never realized in observational astronomy is that light just might get bent by other gravitational influences. If it can happen from something as small as a star, then what might occur if you had a huge group of stars? Like a galaxy… Or a group of galaxies!
What’s new in the world of light? Astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have now gone around the bend and came up with a method of measuring how outgoing light is affected by the gravity of galaxy clusters. Not only does each individual star and each individual galaxy possess its own gravity, but a galaxy group is held together by gravitational attraction as well. Sure, it stands to reason that gravity is affecting what we see – but there’s even more to it. Redshift…
“It is really wonderful. We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift”, says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen.
Together with team members Steen Hansen and Jens Hjorth, Wojtak has been collecting light data and measurements from 8,000 galaxy clusters. Their studies have included calculations from mid-placed members to calibrations on those that reside at the periphery.
“We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge”, explains Radek Wojtak.
Until now, the gravitational redshift has only been tested with experiments and observations in relation to distances her on Earth and in relation to the solar system. With the new research the theory has been tested on a cosmological scale for the first time by analyzing galaxies in galaxy clusters in the distant universe. It is a grotesquely large scale, which is a factor 1,022 times greater (ten thousand billion billion times larger than the laboratory test). The observed data confirms Einstein’s general theory of relativity. Credit: Dark Cosmology Centre, Niels Bohr Institute
The next step in the equation is to measure the entire galaxy cluster’s total mass to arrive at its gravitational potential. Then, using the general theory of relativity, the gravitational redshift could be determined by galaxy location.
“It turned out that the theoretical calculations of the gravitational redshift based on the general theory of relativity was in complete agreement with the astronomical observations.” explains Wojtak. “Our analysis of observations of galaxy clusters show that the redshift of the light is proportionally offset in relation to the gravitational influence from the galaxy cluster’s gravity. In that way our observations confirm the theory of relativity.”
Of course, this kind of revelation also has other implications… theoretical dark matter just might play a role in gravitational redshift, too. And don’t forget dark energy. All these hypothetical models need to be taken into account. But, for now, we’re looking at the big picture in a different way.
“Now the general theory of relativity has been tested on a cosmological scale and this confirms that the general theory of relativity works and that means that there is a strong indication for the presence of dark energy”, explains Radek Wojtak.
As Walt Whitman once said, “I open the scuttle at night and see the far-sprinkled systems, And all I see multiplied as high as I can cypher edge but the rim of the farther systems. Wider and wider they spread, expanding, always expanding,Outward and outward and forever outward.”
Princeton and New York University researchers have simulated the effect of a primordial black hole passing through a star. Primordial black holes are among the objects hypothesized to make up dark matter -- the invisible substance thought to constitute much of the universe -- and astronomers could use the researchers' model to finally observe the elusive black holes. This image illustrates the resulting vibration waves as a primordial black hole (white dots) passes through the center of a star. The different colors correspond to the density of the primordial black hole and strength of the vibration. (Image by Tim Sandstrom)
[/caption]
Well, we’re off to see the Wizard again, my friends. This time it’s to explore the possibilities of primordial black holes colliding with stars and all the implications therein. If this theory is correct, then we should be able to observe the effects of dark matter first hand – proof that it really does exist – and deeper understand the very core of the Universe.
Are primordial black holes blueprints for dark matter? Postdoctoral researchers Shravan Hanasoge of Princeton’s Department of Geosciences and Michael Kesden of NYU’s Center for Cosmology and Particle Physics have utilized computer modeling to visualize a primordial black hole passing through a star. “Stars are transparent to the passage of primordial black holes (PBHs) and serve as seismic detectors for such objects.” says Kesden. “The gravitational field of a PBH squeezes a star and causes it to ring acoustically.”
If primordial black holes do exist, then chances are great that these type of collisions happen within our own galaxy – and frequently. With ever more telescopes and satellites observing the stellar neighborhoods, it only stands to reason that sooner or later we’re going to catch one of these events. But, the most important thing is simply understanding what we’re looking for. The computer model developed by Hanasoge and Kesden can be used with these current solar-observation techniques to offer a more precise method for detecting primordial black holes than existing tools.
“If astronomers were just looking at the Sun, the chances of observing a primordial black hole are not likely, but people are now looking at thousands of stars,” Hanasoge said.”There’s a larger question of what constitutes dark matter, and if a primordial black hole were found it would fit all the parameters — they have mass and force so they directly influence other objects in the Universe, and they don’t interact with light. Identifying one would have profound implications for our understanding of the early Universe and dark matter.”
Sure. We haven’t seen DM, but what we can see are galaxies that are hypothesized to have extended dark-matter halos and to study the effects the gravity has on their materials – like gaseous regions and stellar members. If these new models are correct, primordial black holes should be heavier than existing dark matter and when they collide with a star, should cause a rippling effect.
“If you imagine poking a water balloon and watching the water ripple inside, that’s similar to how a star’s surface appears,” Kesden said. “By looking at how a star’s surface moves, you can figure out what’s going on inside. If a black hole goes through, you can see the surface vibrate.”
Using the Sun as a model, Kesden and Hanasoge calculated the effects a PBH might have and then gave the data to NASA’s Tim Sandstrom. Using the Pleiades supercomputer at the agency’s Ames Research Center in California, the team was then able to create a video simulation of the collisional effect. Below is the clip which shows the vibrations of the Sun’s surface as a primordial black hole — represented by a white trail — passes through its interior.
“It’s been known that as a primordial black hole went by a star, it would have an effect, but this is the first time we have calculations that are numerically precise,” comments Marc Kamionkowski, a professor of physics and astronomy at Johns Hopkins University. “This is a clever idea that takes advantage of observations and measurements already made by solar physics. It’s like someone calling you to say there might be a million dollars under your front doormat. If it turns out to not be true, it cost you nothing to look. In this case, there might be dark matter in the data sets astronomers already have, so why not look?”
PAMELA team and detector in Rome before launch. Photo courtesy of the PAMELA Experiment
[/caption]
When it comes to planets with rings, we know the answer: Jupiter, Saturn, Uranus, and Neptune. But new findings from the PAMELA experiment show that Earth has a ring system, too… One made up of geomagnetically trapped cosmic ray antiprotons.
“The existence of a significant flux of antiprotons confined to Earth’s magnetosphere has been considered in several theoretical works.” says team leader, O. Adriani of the University of Florence Department of Physics. “These antiparticles are produced in nuclear interactions of energetic cosmic rays with the terrestrial atmosphere and accumulate in the geomagnetic field at altitudes of several hundred kilometers.”
The PAMELA experiment – short for Payload for Antimatter Exploration and Light-nuclei Astrophysics – is based on an international collaboration involving about 100 physicists. Its state-of-the-art equipment was designed to investigate the nature of dark matter, the apparent absence of cosmological antimatter and the origin and evolution of matter in the galaxy. Utilizing a permanent magnet spectrometer with a variety of specialized detectors, PAMELA whips around Earth on a highly inclined orbit.
“The satellite orbit (70 degree inclination and 350–610 km altitude) allows PAMELA to perform a very detailed measurement of the cosmic radiation in different regions of Earth’s magnetosphere, providing information about the nature and energy spectra of sub-cutoff particles.” says Adriani. “The satellite orbit passes through the South Atlantic Anomaly (SAA), allowing the study of geomagnetically trapped particles in the inner radiation belt.”
From its subdetectors, PAMELA dished up a serving of antiprotons, but it wasn’t an easy job. “Antiprotons in the selected energy range are likely to annihilate inside the calorimeter, thus leaving a clear signature.” says the team. “The longitudinal and transverse segmentation of the calorimeter is exploited to allow the shower development to be characterized. These selections are combined with dE/dx measurements from individual strips in the silicon detector planes to allow electromagnetic showers to be identified with very high accuracy.”
For 850 days, the detectors collected data and compared it against simulations. The trapped antiprotons were highly dependent on angular collection, directional response function on the satellite orbital position and on its orientation relative to the geomagnetic field. “All the identified antiprotons, characterized by a pitch angle near 90 deg, were found to spiral around field lines, bounce between mirror points, and also perform a slow longitudinal drift around the Earth, for a total path length amounting to several Earth radii.” said the team. “PAMELA results allow CR transport models to be tested in the terrestrial atmosphere and significantly constrain predictions from trapped antiproton models, reducing uncertainties concerning the antiproton production spectrum in Earth’s magnetosphere.”
Original Story Source: Astrophysical Journal Newsletters.
This is the portion of sky in which astronomers found the Segue 1 dwarf galaxy. Can you see it? Credit: Marla Geha
[/caption]
Two years ago, Marla Geha, a Yale University astronomer, Joshua Simon from the Carnegie Institution of Washington, and their colleagues discovered something unusual while studying with the Keck II telescope and information for the Sloan Digital Sky Survey. Their observations turned up a contrasting group of stars which all appeared to be moving in unison – not just a moving cluster of similar stars which could have been torn away from the nearby Sagittarius dwarf galaxy. The team knew they were on to something, but a competing group of astronomers at Cambridge University was skeptical. Too bad… there was a dark treasure right there before their eyes.
Not to be dissuaded, Simon, Geha and their group returned to Keck and turned the photographic eye of the telescope’s Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) towards their target area. Even though it was only about 1,000 small, dim stars, they wanted to know how they migrated both in respect to the Milky Way and to each other. Named Segue 1, the target the team was looking at could possibly have 3,400 times more mass than can be accounted for by its visible stars… a galaxy dominated by dark matter and salted with a handful of ancient suns. If the 1,000 or so stars were all there was to Segue 1, with just a touch of dark matter, the stars would all move at about the same speed, said Simon. But the Keck data show they do not. Instead of moving at a steady 209 km/sec relative to the Milky Way, some of the Segue 1 stars are moving at rates as slow as 194 kilometers per second while others are going as fast as 224 kilometers per second.
Using the DEIMOS instrument on the Keck II telescope, astronomers could identify which stars were moving together as a group. They are circled here in green Credit: Marla Geha
“That tells you Segue 1 must have much more mass to accelerate the stars to those velocities,” Geha explained. The paper confirming Segue 1’s dark nature appeared in the May 2011 issue of The Astrophysical Journal. “The mass required to cause the different star velocities seen in Segue 1 has been calculated at 600,000 solar masses. But there are only about 1,000 stars in Segue 1, and they are all close to the mass of our Sun,” Simon said. “Virtually all of the remainder of the mass must be dark matter.”
But the information from DEIMOS didn’t stop there… It also revealed an eclectic collection of nearly primordial metal-poor stars. The researchers managed to gather iron data on six stars in Segue 1 with the Keck II telescope, and a seventh Segue 1 star was measured by an Australian team using the Very Large Telescope. Of those seven, three proved to have less than one 2,500th as much iron as the Sun. “That suggests these are some of the oldest and least evolved stars that are known,” said Simon. This is fascinating data considering investigations for stars of this type out of the Milky Way’s billions have produced less than 30. “In Segue 1 we already have 10 percent of the total in the Milky Way,” Geha said. “For studying these most primitive stars, dwarf galaxies are going to be very important.”
By subtracting out all the other objects in the image and leaving the Segue I member stars, the “darkest galaxy” emerges. Credit: Marla Geha
By confirming Segue 1’s massive concentration of dark matter, other types of research into this dark galaxy’s lifestyle now become more dedicated. The space-based Fermi Gamma Ray Telescope has also been looking its way in hopes of catching a gamma-ray event created by the collision and annihilation of pairs of dark matter particles. So far the Fermi telescope has not detected anything of the sort, which isn’t entirely surprising and doesn’t mean the dark matter isn’t there, said Simon.
“The current predictions are that the Fermi telescope is just barely strong enough or perhaps not quite strong enough to see these gamma rays from Segue 1,” Simon explained. So there are hopes that Fermi will detect at least the hint of a collision. “A detection would be spectacular,” said Simon. “People have been trying to learn about dark matter for 35 years and not made much progress. Even a faint glow of the predicted gamma rays would be a powerful confirmation of theoretical predictions about the nature of dark matter.”
A new study combining data from ESO’s Very Large Telescope and ESA’s XMM-Newton X-ray space observatory has turned up a surprise. Most of the huge black holes in the centres of galaxies in the past 11 billion years were not turned on by mergers between galaxies, as had been previously thought. Credit: CFHT/IAP/Terapix/CNRS/ESO
[/caption]
ESO’s Very Large Telescope and ESA’s XMM-Newton X-ray Space Observatory has just opened our eyes once again. While we thought that the massive black holes that lurk at the center of large galaxies (and they always lurk, don’t they? they never just lay about, lallygag, or loiter…) for the last 11 billion years were turned on by mergers, we’re finding out it just might not be so.
For all astronomers, we’re aware that galactic structure involves a mostly quiescent central black hole. But as we reach further out into the Universe, we’re finding that early, brighter galaxies have a middle monster – one which appears to be noshing on a material that emits intense radiation. So if a galaxy merger isn’t responsible, then where does the material originate to ignite a quiet black hole into an active galactic nucleus? Maybe the omni-present dark matter…
Viola Allevato (Max-Planck-Institut für Plasmaphysik; Excellence Cluster Universe, Garching, Germany) and an international team of scientists from the COSMOS collaboration have studied 600 active galaxies in an intensively mapped region called the COSMOS field. Spanning an area consisting of about five degrees of celestial real estate in the constellation of Sextans, the COSMOS field has been richly observed by multiple telescopes at multiple wavelengths. This gives astronomers a great “picture” from which to draw data.
What they found was pretty much what they had expected – most of the active galaxies in the past 11 billion years were only moderately bright. But what they weren’t prepared to understand is why the majority of these more common, less bright active galaxies weren’t triggered by mergers. It’s a problematic situation that had previously been tackled by the Hubble Space Telescope, but COSMOS is looking back even further in time and with greater detail – a three-dimensional map showing where the active galaxies reside. “It took more than five years, but we were able to provide one of the largest and most complete inventories of active galaxies in the X-ray sky,” said Marcella Brusa, one of the authors of the study.
These new charts could help further our understanding of distribution as the universe aged and further refine modeling techniques. The new information also points to active galactic nuclei being hosted in large galaxies with abundances of dark matter… against popular theory. “These new results give us a new insight into how supermassive black holes start their meals,” said Viola Allevato, who is lead author on the new paper. “They indicate that black holes are usually fed by processes within the galaxy itself, such as disc instabilities and starbursts, as opposed to galaxy collisions.”
Alexis Finoguenov, who supervised the work, concludes: “Even in the distant past, up to almost 11 billion years ago, galaxy collisions can only account for a small percentage of the moderately bright active galaxies. At that time galaxies were closer together so mergers were expected to be more frequent than in the more recent past, so the new results are all the more surprising.”
Abell 2744, shown above in a composite of images from the Hubble Space Telescope, the ESO’s Very Large Telescope and NASA’s Chandra X-ray Observatory, is one of the most complex and dramatic collisions ever seen between galaxy clusters.
X-ray image of Abell 2744
Dubbed “Pandora’s Cluster”, this is a region 5.9 million light-years across located 3.5 billion light-years away. Many different kinds of structures are found here, shown in the image as different colors. Data from Chandra are colored red, showing gas with temperatures in the millions of degrees. Dark matter is shown in blue based on data from Hubble, the European Southern Observatory’s VLT array and Japan’s Subaru telescope. Finally the optical images showing the individual galaxies have been added.
Even though there are many bright galaxies visible in the image, most of the mass in Pandora’s Cluster comes from the vast areas of dark matter and extremely hot gas. Researchers made the normally invisible dark matter “visible” by identifying its gravitational effects on light from distant galaxies. By carefully measuring the distortions in the light a map of the dark matter’s mass could be created.
Galaxy clusters are the largest known gravitationally-bound structures in the Universe, and Abell 2744 is where at least four clusters have collided together. The vast collision seems to have separated the gas from the dark matter and the galaxies themselves, creating strange effects which have never been seen together before. By studying the history of events like this astronomers hope to learn more about how dark matter behaves and how the different structures that make up the Universe interact with each other.
Check out this HD video tour of Pandora’s Cluster from the team at Chandra:
Jason Major is a graphic designer, photo enthusiast and space blogger. Visit his website Lights in the Dark and follow him on Twitter @JPMajor or on Facebook for the most up-to-date astronomy awesomeness!
