Exotic dark matter theories. Gravitational waves. Observatories in space. Giant black holes. Colliding galaxies. Lasers. If you’re a fan of all the awesomest stuff in the universe, then this article is for you.
Since the 1960s, astrophysicists have postulated that in addition to all the matter that we can see, the Universe is also filled with a mysterious, invisible mass. Known as “Dark Matter”, it’s existence was proposed to explain the “missing mass” of the Universe, and is now considered a fundamental part of it. Not only is it theorized to make up about 80% of the Universe’s mass, it is also believed to have played a vital role in the formation and evolution of galaxies.
However, a recent finding may throw this entire cosmological perspective sideways. Based on observations made using the NASA/ESA Hubble Space Telescope and other observatories around the world, astronomers have found a nearby galaxy (NGC 1052-DF2) that does not appear to have any dark matter. This object is unique among galaxies studied so far, and could force a reevaluation of our predominant cosmological models.
The study which details their findings, titled “A galaxy lacking dark matter“, recently appeared in the journal Nature. Led by Pieter van Dokkum of Yale University, the study also included members from the Max Planck Institute for Astronomy, San Jose State University, the University of California Observatories, the University of Toronto, and the Harvard-Smithsonian Center for Astrophysics.
For the sake of their study, the team consulted data from the Dragonfly Telephoto Array (DFA), which was used to identify NGC 1052-DF2. Based on data from Hubble, the team was able to determined its distance – 65 million light-years from the Solar System – as well as its size and brightness. In addition, the team discovered that NGC 1052-DF52 is larger than the Milky Way but contains about 250 times fewer stars, which makes it an ultra diffuse galaxy.
As van Dokkum explained, NGC 1052-DF2 is so diffuse that it’s essentially transparent. “I spent an hour just staring at this image,” he said. “This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy.”
Using data from the Sloan Digital Sky Survey (SDSS), the Gemini Observatory, and the Keck Observatory, the team studied the galaxy in more detail. By measuring the dynamical properties of ten globular clusters orbiting the galaxy, the team was able to infer an independent value of the galaxy’s mass – which is comparable to the mass of the stars in the galaxy.
This led the team to conclude that either NGC 1052-DF2 contains at least 400 times less dark matter than is predicted for a galaxy of its mass, or none at all. Such a finding is unprecedented in the history of modern astronomy and defied all predictions. As Allison Merritt – an astronomer from Yale University, the Max Planck Institute for Astronomy and a co-author on the paper – explained:
“Dark matter is conventionally believed to be an integral part of all galaxies — the glue that holds them together and the underlying scaffolding upon which they are built… There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown.”
“This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work,” added van Dokkum.
However, it is important to note that the discovery of a galaxy without dark matter does not disprove the theory that dark matter exists. In truth, it merely demonstrates that dark matter and galaxies are capable of being separate, which could mean that dark matter is bound to ordinary matter through no force other than gravity. As such, it could actually help scientists refine their theories of dark matter and its role in galaxy formation and evolution.
In the meantime, the researchers already have some ideas as to why dark matter is missing from NGC 1052-DF2. On the one hand, it could have been the result of a cataclysmic event, where the birth of a multitude of massive stars swept out all the gas and dark matter. On the other hand, the growth of the nearby massive elliptical galaxy (NGC 1052) billions of years ago could have played a role in this deficiency.
However, these theories do not explain how the galaxy formed. To address this, the team is analyzing images that Hubble took of 23 other ultra-diffuse galaxies for more dark-matter deficient galaxies. Already, they have found three that appear to be similar to NGC 1052-DF2, which could indicate that dark-matter deficient galaxies could be a relatively common occurrence.
If these latest findings demonstrate anything, it is that the Universe is like an onion. Just when you think you have it figured out, you peal back an additional layer and find a whole new set of mysteries. They also demonstrate that after 28 years of faithful service, the Hubble Space Telescope is still capable of teaching us new things. Good thing too, seeing as the launch of its successor has been delayed until 2020!
Further Reading: Hubble Space Telescope
The first results of the IllustrisTNG Project have been published in three separate studies, and they’re shedding new light on how black holes shape the cosmos, and how galaxies form and grow. The IllustrisTNG Project bills itself as “The next generation of cosmological hydrodynamical simulations.” The Project is an ongoing series of massive hydrodynamic simulations of our Universe. Its goal is to understand the physical processes that drive the formation of galaxies.
At the heart of IllustriousTNG is a state of the art numerical model of the Universe, running on one of the most powerful supercomputers in the world: the Hazel Hen machine at the High-Performance Computing Center in Stuttgart, Germany. Hazel Hen is Germany’s fastest computer, and the 19th fastest in the world.
Our current cosmological model suggests that the mass-energy density of the Universe is dominated by dark matter and dark energy. Since we can’t observe either of those things, the only way to test this model is to be able to make precise predictions about the structure of the things we can see, such as stars, diffuse gas, and accreting black holes. These visible things are organized into a cosmic web of sheets, filaments, and voids. Inside these are galaxies, which are the basic units of cosmic structure. To test our ideas about galactic structure, we have to make detailed and realistic simulated galaxies, then compare them to what’s real.
Astrophysicists in the USA and Germany used IllustrisTNG to create their own universe, which could then be studied in detail. IllustrisTNG correlates very strongly with observations of the real Universe, but allows scientists to look at things that are obscured in our own Universe. This has led to some very interesting results so far, and is helping to answer some big questions in cosmology and astrophysics.
How Do Black Holes Affect Galaxies?
Ever since we’ve learned that galaxies host supermassive black holes (SMBHs) at their centers, it’s been widely believed that they have a profound influence on the evolution of galaxies, and possibly on their formation. That’s led to the obvious question: How do these SMBHs influence the galaxies that host them? Illustrious TNG set out to answer this, and the paper by Dr. Dylan Nelson at the Max Planck Institute for Astrophysics shows that “the primary driver of galaxy color transition is supermassive blackhole feedback in its low-accretion state.”
“The only physical entity capable of extinguishing the star formation in our large elliptical galaxies are the supermassive black holes at their centers.” – Dr. Dylan Nelson, Max Planck Institute for Astrophysics,
Galaxies that are still in their star-forming phase shine brightly in the blue light of their young stars. Then something changes and the star formation ends. After that, the galaxy is dominated by older, red stars, and the galaxy joins a graveyard full of “red and dead” galaxies. As Nelson explains, “The only physical entity capable of extinguishing the star formation in our large elliptical galaxies are the supermassive black holes at their centers.” But how do they do that?
Nelson and his colleagues attribute it to supermassive black hole feedback in its low-accretion state. What that means is that as a black hole feeds, it creates a wind, or shock wave, that blows star-forming gas and dust out of the galaxy. This limits the future formation of stars. The existing stars age and turn red, and few new blue stars form.
How Do Galaxies Form and How Does Their Structure Develop?
It’s long been thought that large galaxies form when smaller galaxies join up. As the galaxy grows larger, its gravity draws more smaller galaxies into it. During these collisions, galaxies are torn apart. Some stars will be scattered, and will take up residence in a halo around the new, larger galaxy. This should give the newly-created galaxy a faint background glow of stellar light. But this is a prediction, and these pale glows are very hard to observe.
“Our predictions can now be systematically checked by observers.” – Dr. Annalisa Pillepich (Max Planck Institute for Astrophysics)
IllustrisTNG was able to predict more accurately what this glow should look like. This gives astronomers a better idea of what to look for when they try to observe this pale stellar glow in the real Universe. “Our predictions can now be systematically checked by observers,” Dr. Annalisa Pillepich (MPIA) points out, who led a further IllustrisTNG study. “This yields a critical test for the theoretical model of hierarchical galaxy formation.”
IllustrisTNG is an on-going series of simulations. So far, there have been three IllustrisTNG runs, each one creating a larger simulation than the previous one. They are TNG 50, TNG 100, and TNG 300. TNG300 is much larger than TNG50 and allows a larger area to be studied which reveals clues about large-scale structure. Though TNG50 is much smaller, it has much more precise detail. It gives us a more detailed look at the structural properties of galaxies and the detailed structure of gas around galaxies. TNG100 is somewhere in the middle.
IllustrisTNG is not the first cosmological hydrodynamical simulation. Others include Eagle, Horizon-AGN, and IllustrisTNG’s predecessor, Illustris. They have shown how powerful these predictive theoretical models can be. As our computers grow more powerful and our understanding of physics and cosmology grow along with them, these types of simulations will yield greater and more detailed results.
Since it was first proposed in the 1960s to account for all the “missing mass” in the Universe, scientists have been trying to find evidence of dark matter. This mysterious, invisible mass theoretically accounts for 26.8% of the baryonic matter (aka. visible matter) out there. And yet, despite almost fifty years of ongoing research and exploration, scientists have not found any direct evidence of this missing mass.
However, according to two new research papers that were recently published in the journal Physical Review Letters, we may have gotten our first glimpse of dark matter thanks to an experiment aboard the International Space Station. Known as the Alpha Magnetic Spectrometer (AMS-02), this a state-of-the-art particle physics detector has been recording cosmic rays since 2011 – which some theorize are produced by the annihilation of dark matter particles.
Like its predecessor (the AMS), the AMS-02 is the result of collaborative work and testing by an international team composed of 56 institutes from 16 countries. With sponsorship from the US Department of Energy (DOE) and overseen by the Johnson Space Center’s AMS Project Office, the AMS-02 was delivered to the ISS aboard the Space Shuttle Endeavour on May 16th, 2011.
Ostensibly, the AMS-02 is designed to monitor cosmic rays to see how much in the way of antiprotons are falling to Earth. But for the sake of their research, the two science teams also been consulted the data it has been collecting to test theories about dark matter. To break it down, the WIMPs theory of dark matter states that it is made up of Weakly-Interacted Massive Particles (WIMPS), protons and antiprotons are the result of WIMPs colliding.
By monitoring the number of antiprotons that interact with the AMS-02, two science teams (who were working independently of each other) hoped to infer whether or not any of the antiprotons being detected could be caused by WIMP collisions. The difficulty in this, however, is knowing what would constitute an indication, as cosmic rays have many sources and the properties of WIMPs are not entirely defined.
To do this, the two teams developed mathematical models to predict the cosmic ray background, and thus isolate the number of antiprotons that AMS-02 would detect. They further incorporated fine-tuned estimates of the expected mass of the WIMPs, until it fit with the AMS-02 data. One team, led by Alessandro Cuoco, was made up of researchers from the Institute for Theoretical Particle Physics and Cosmology.
Using computer simulations, Cuoco and his colleagues examined the AMS-02 data based on two scenarios – one which accounted for dark matter and one which did not. As they indicate in their study, they not only concluded that the presence of antiprotons created by WIMP collisions better fit the data, but they were also able to constrain the mass of dark matter to about 80 GeV (about 85 times the mass of a single proton or antiproton).
As they state in their paper:
“[T]he very accurate recent measurement of the CR antiproton flux by the AMS-02 experiment allows [us] to achieve unprecedented sensitivity to possible DM signals, a factor ~4 stronger than the limits from gamma-ray observations of dwarf galaxies. Further, we find an intriguing indication for a DM signal in the antiproton flux, compatible with the DM interpretation of the Galactic center gamma-ray excess.”
The other team was made up of researchers from the Chinese Academy of Sciences, Nanjing University, the University of Science and Technology of China, and the National Center for Theoretical Sciences. Led by Ming-Yang Cui of Nanjing University, this team made estimates of the background parameters for cosmic rays by using prior data from previous boron-to-carbon ratio and proton measurements.
These measurements, which determine the rate at which boron decays into carbon, can be used to guage the distance that boron molecules travel through space. In this case, they were combined with proton measurements to determine background levels for cosmic rays. They incorporated this data into a Bayesian Analysis framework (i.e. a statistical model used to determine probabilities) to see how many antiprotons could be attributed to WIMP collisions.
The results, as they state it in their paper were quite favorable and produced similar mass estimates to the study led by Cuoco’s team. “Compared with the astrophysical background only hypothesis, we find that a dark matter signal is favored,” they write. “The rest mass of the dark matter particles is ?20 – 80 GeV.”
What’s more, both scientific teams obtained similar estimates when it came to cross-section measurements of dark matter – i.e. the likelihood of collisions happening based on how densely dark matter is distributed. For example, Cuoco’s team obtained a cross-section estimate of 3 x 10-26 per cm³ while Cui’s team obtained an estimate that ranged from 0.2 – 5 × 10-26 per cm³.
The fact that two scientific teams, which were operating independently of each other, came to very similar conclusions based on the same data is highly encouraging. While it is not definitive proof of dark matter, it is certainly a step in the right direction. At best, it shows that we are getting closer to creating a detailed picture of what dark matter looks like.
And in the meantime, both teams acknowledge that further work is necessary. Cuoco and his team also suggest what further steps should be taken. “Confirmation of the signal will require a more accurate study of the systematic uncertainties,” they write, “i.e., the antiproton production cross-section, and the modeling of the effect of solar modulation.”
While scientists have attempted to find evidence of dark matter by monitoring cosmic rays in the past, the AMS-02 stands apart because of its extreme sensitivity. As of May 8th, the spectrometer has conducted measurements on 100 billion particles. As of the penning of this article, that number has increased to over 100,523,550,000!
Dark matter is mysterious stuff, because we can’t really “see” it. But that hasn’t stopped scientists from researching it, and from theorizing about it. One theory says that there should be filament structures of dark matter connecting galaxies. Scientists from the University of Waterloo have now imaged one of those dark matter filaments for the first time.
Theory predicts that filaments of dark matter connect galaxies together, by reaching from the dark matter halo of one galaxy to the same halo in another galaxy. Other researchers have found dark matter filaments connecting entire galaxy clusters, but this is the first time that filaments have been imaged between individual galaxies.
“This image moves us beyond predictions to something we can see and measure.” – Mike Hudson, University of Waterloo
“For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together,” said Mike Hudson, a professor of astronomy at the University of Waterloo. “This image moves us beyond predictions to something we can see and measure.”
Dark matter makes up about 25% of the Universe. But it doesn’t shine, reflect, or interact with light in any way, so it’s difficult to study. The only way we can really study it is by observing gravity. In this study, the pair of astronomers used the weak gravitational lensing technique.
Weak gravitational lensing relies on the effect that mass has on light. Enough concentrated mass in the foreground—dark matter in this case—will warp light from distant sources in the background.
When dealing with something as large as a super-massive Black Hole, gravitational lensing is quite pronounced. But galaxy-to-galaxy filaments of dark matter are much less dense than a black hole, so their individual effect is minimal. What the astronomers needed was the combined data from multiple galaxy pairs in order to detect the weak gravitational lensing.
Key to this study is the Canada-France-Hawaii Telescope. It performed a multi-year sky survey that laid the groundwork for this study. The researchers combined lensing images of over 23,000 pairs of galaxies 4.5 billion light years away. The resulting composite image revealed the filament bridge between the two galaxies.
“By using this technique, we’re not only able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together.” – Seth D. Epps, University of Waterloo
We still don’t know what dark matter is, but the fact that scientists were able to predict these filaments, and then actually find them, shows that we’re making progress understanding it.
We’ve known about the large scale structure of the Universe for some time, and we know that dark matter is a big part of it. Galaxies tend to cluster together, under the influence of dark matter’s gravitational pull. Finding a dark matter bridge between galaxies is an intriguing discovery. It at least takes a little of the mystery out of dark matter.
Dark matter remains largely mysterious, but astrophysicists keep trying to crack open that mystery. Last year’s discovery of gravity waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) may have opened up a new window into the dark matter mystery. Enter what are known as ‘primordial black holes.’
Theorists have predicted the existence of particles called Weakly Interacting Massive Particles (WIMPS). These WIMPs could be what dark matter is made of. But the problem is, there’s no experimental evidence to back it up. The mystery of dark matter is still an open case file.
When LIGO detected gravitational waves last year, it renewed interest in another theory attempting to explain dark matter. That theory says that dark matter could actually be in the form of Primordial Black Holes (PBHs), not the aforementioned WIMPS.
Primordial black holes are different than the black holes you’re probably thinking of. Those are called stellar black holes, and they form when a large enough star collapses in on itself at the end of its life. The size of these stellar black holes is limited by the size and evolution of the stars that they form from.
Unlike stellar black holes, primordial black holes originated in high density fluctuations of matter during the first moments of the Universe. They can be much larger, or smaller, than stellar black holes. PBHs could be as small as asteroids or as large as 30 solar masses, even larger. They could also be more abundant, because they don’t require a large mass star to form.
When two of these PBHs larger than about 30 solar masses merge together, they would create the gravitational waves detected by LIGO. The theory says that these primordial black holes would be found in the halos of galaxies.
If there are enough of these intermediate sized PBHs in galactic halos, they would have an effect on light from distant quasars as it passes through the halo. This effect is called ‘micro-lensing’. The micro-lensing would concentrate the light and make the quasars appear brighter.
The effect of this micro-lensing would be stronger the more mass a PBH has, or the more abundant the PBHs are in the galactic halo. We can’t see the black holes themselves, of course, but we can see the increased brightness of the quasars.
Working with this assumption, a team of astronomers at the Instituto de Astrofísica de Canarias examined the micro-lensing effect on quasars to estimate the numbers of primordial black holes of intermediate mass in galaxies.
“The black holes whose merging was detected by LIGO were probably formed by the collapse of stars, and were not primordial black holes.” -Evencio Mediavilla
The study looked at 24 quasars that are gravitationally lensed, and the results show that it is normal stars like our Sun that cause the micro-lensing effect on distant quasars. That rules out the existence of a large population of PBHs in the galactic halo. “This study implies “says Evencio Mediavilla, “that it is not at all probable that black holes with masses between 10 and 100 times the mass of the Sun make up a significant fraction of the dark matter”. For that reason the black holes whose merging was detected by LIGO were probably formed by the collapse of stars, and were not primordial black holes”.
Depending on you perspective, that either answers some of our questions about dark matter, or only deepens the mystery.
We may have to wait a long time before we know exactly what dark matter is. But the new telescopes being built around the world, like the European Extremely Large Telescope, the Giant Magellan Telescope, and the Large Synoptic Survey Telescope, promise to deepen our understanding of how dark matter behaves, and how it shapes the Universe.
It’s only a matter of time before the mystery of dark matter is solved.
Erik Verlinde explains his new view of gravity
Let’s be honest. Dark matter’s a pain in the butt. Astronomers have gone to great lengths to explain why is must exist and exist in huge quantities, yet it remains hidden. Unknown. Emitting no visible energy yet apparently strong enough to keep galaxies in clusters from busting free like wild horses, it’s everywhere in vast quantities. What is the stuff – axions, WIMPS, gravitinos, Kaluza Klein particles?
It’s estimated that 27% of all the matter in the universe is invisible, while everything from PB&J sandwiches to quasars accounts for just 4.9%. But a new theory of gravity proposed by theoretical physicist Erik Verlinde of the University of Amsterdam found out a way to dispense with the pesky stuff.
Unlike the traditional view of gravity as a fundamental force of nature, Verlinde sees it as an emergent property of space. Emergence is a process where nature builds something large using small, simple pieces such that the final creation exhibits properties that the smaller bits don’t. Take a snowflake. The complex symmetry of a snowflake begins when a water droplet freezes onto a tiny dust particle. As the growing flake falls, water vapor freezes onto this original crystal, naturally arranging itself into a hexagonal (six-sided) structure of great beauty. The sensation of temperature is another emergent phenomenon, arising from the motion of molecules and atoms.
So too with gravity, which according to Verlinde, emerges from entropy. We all know about entropy and messy bedrooms, but it’s a bit more subtle than that. Entropy is a measure of disorder in a system or put another way, the number of different microscopic states a system can be in. One of the coolest descriptions of entropy I’ve heard has to do with the heat our bodies radiate. As that energy dissipates in the air, it creates a more disordered state around us while at the same time decreasing our own personal entropy to ensure our survival. If we didn’t get rid of body heat, we would eventually become disorganized (overheat!) and die.
Emergent or entropic gravity, as the new theory is called, predicts the exact same deviation in the rotation rates of stars in galaxies currently attributed to dark matter. Gravity emerges in Verlinde’s view from changes in fundamental bits of information stored in the structure of space-time, that four-dimensional continuum revealed by Einstein’s general theory of relativity. In a word, gravity is a consequence of entropy and not a fundamental force.
Space-time, comprised of the three familiar dimensions in addition to time, is flexible. Mass warps the 4-D fabric into hills and valleys that direct the motion of smaller objects nearby. The Sun doesn’t so much “pull” on the Earth as envisaged by Isaac Newton but creates a great pucker in space-time that Earth rolls around in.
In a 2010 article, Verlinde showed how Newton’s law of gravity, which describes everything from how apples fall from trees to little galaxies orbiting big galaxies, derives from these underlying microscopic building blocks.
His latest paper, titled Emergent Gravity and the Dark Universe, delves into dark energy’s contribution to the mix. The entropy associated with dark energy, a still-unknown form of energy responsible for the accelerating expansion of the universe, turns the geometry of spacetime into an elastic medium.
“We find that the elastic response of this ‘dark energy’ medium takes the form of an extra ‘dark’ gravitational force that appears to be due to ‘dark matter’,” writes Verlinde. “So the observed dark matter phenomena is a remnant, a memory effect, of the emergence of spacetime together with the ordinary matter in it.”
I’ll be the first one to say how complex Verlinde’s concept is, wrapped in arcane entanglement entropy, tensor fields and the holographic principal, but the basic idea, that gravity is not a fundamental force, makes for a fascinating new way to look at an old face.
Physicists have tried for decades to reconcile gravity with quantum physics with little success. And while Verlinde’s theory should be rightly be taken with a grain of salt, he may offer a way to combine the two disciplines into a single narrative that describes how everything from falling apples to black holes are connected in one coherent theory.
For almost a century, astronomers and cosmologists have postulated that space is filled with an invisible mass known as “dark matter”. Accounting for 27% of the mass and energy in the observable universe, the existence of this matter was intended to explain all the “missing” baryonic matter in cosmological models. Unfortunately, the concept of dark matter has solved one cosmological problem, only to create another.
If this matter does exist, what is it made of? So far, theories have ranged from saying that it is made up of cold, warm or hot matter, with the most widely-accepted theory being the Lambda Cold Dark Matter (Lambda-CDM) model. However, a new study produced by a team of European astronomer suggests that the Warm Dark Matter (WDM) model may be able to explain the latest observations made of the early Universe.
But first, some explanations are in order. The different theories on dark matter (cold, warm, hot) refer not to the temperatures of the matter itself, but the size of the particles themselves with respect to the size of a protogalaxy – an early Universe formation, from which dwarf galaxies would later form.
The size of these particles determines how fast they can travel, which determines their thermodynamic properties, and indicates how far they could have traveled – aka. their “free streaming length” (FSL) – before being slowed by cosmic expansion. Whereas hot dark matter would be made up of very light particles with high FSLs, cold dark matter is believed to be made up of massive particles that have a low FSL.
Cold dark matter has been speculated to take the form of Massive Compact Halo Objects (MACHOs) like black holes, or a class of undiscovered heavy particles – i.e. Weakly-Interacting Massive Particles (WIMPs), and axions. The widely-accepted Lambda-CDM model is based in part of the theory that dark matter is “cold”.
As cosmological explanations go, it is the most simple and can account for the formation of galaxies or galaxy cluster formations. However, there remains some holes in this theory, the biggest of which is that it predicts that there should be many more small, dwarf galaxies in the early Universe than we can account for.
In short, the existence of dark matter as massive particles that have low FSL would result in small fluctuations in the density of matter in the early Universe – which would lead to large amounts of low-mass galaxies to be found as satellites of galactic halos, and with large concentrations of dark matter in their centers.
Naturally, the absence of these galaxies might lead one to speculate that we simply haven’t spotted these galaxies yet, and that IR surveys like the Two-Micron All Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE) missions might find them in time.
But as the international research team – which includes astronomers from the Astronomical Observatory of Rome (INAF), the Italian Space Agency Science Data Center and the Paris Observatory – another possibility is that dark matter is neither hot nor cold, but “warm” – i.e. consisting of middle-mass particles (also undiscovered) with FSLs that are roughly the same as objects big as galaxies.
As Dr. Nicola Menci – a researcher with the INAF and the lead author of the study – told Universe Today via email:
“The Cold Dark Matter particles are characterized by low root mean square velocities, due to their large masses (usually assumed of the order of >~ 100 GeV, a hundred times the mass of a proton). Such low thermal velocities allow for the clumping of CDM even on very small scales. Conversely, lighter dark matter particles with masses of the order of keV (around 1/500 the mass of the electron) would be characterized by larger thermal velocities, inhibiting the clumping of DM on mass scales of dwarf galaxies. This would suppress the abundance of dwarf galaxies (and of satellite galaxies) and produce shallow inner density profiles in such objects, naturally matching the observations without the need for a strong feedback from stellar populations.”
In other words, they found that the WDM could better account for the early Universe as we are seeing it today. Whereas the Lambda-CDM model would result in perturbations in densities in the early Universe, the longer FSL of warm dark matter particles would smooth these perturbations out, thus resembling what we see when we look deep into the cosmos to see the Universe during the epoch of galaxy formation.
For the sake of their study, which appeared recently in the July 1st issue of The Astrophysical Journal Letters, the research team relied on data obtained from the Hubble Frontier Fields (HFF) program. Taking advantage of improvements made in recent years, they were able to examine the magnitude of particularly faint and distant galaxies.
As Menci explained, this is a relatively new ability which the Hubble Space Telescope would not have been able to do a few years ago:
“Since galaxy formation is deeply affected by the nature of DM on the scale of dwarf galaxies, a powerful tool to constraint DM models is to measure the abundance of low-mass galaxies at early cosmic times (high redshifts z=6-8), the epoch of their formation. This is a challenging task since it implies finding extremely faint objects (absolute magnitudes M_UV=-12 to -13) at very large distances (12-13 billion of light years) even for the Hubble Space Telescope.
“However, the Hubble Frontier Field program exploits the gravitational lensing produced by foreground galaxy clusters to amplify the light from distant galaxies. Since the formation of dwarf galaxies is suppressed in WDM models – and the strength of the suppression is larger for lighter DM particles – the high measured abundance of high-redshift dwarf galaxies (~ 3 galaxies per cube Mpc) can provide a lower limit for the WDM particle mass, which is completely independent of the stellar properties of galaxies.”
The results they obtained provided strict constraints on dark matter and early galaxy formation, and were thus consistent with what HFF has been seeing. These results could indicate that our failure to detect dark matter so far may have been the result of looking for the wrong kind of particles. But of course, these results are just one step in a larger effort, and will require further testing and confirmation.
Looking ahead, Menci and his colleagues hope to obtain further information from the HFF program, and hopes that future missions will allow them to see if their findings hold up. As already noted, these include infrared astronomy missions, which are expected to “see” more of the early Universe by looking beyond the visible spectrum.
For almost a century, dark matter has been a pervasive and elusive mystery, always receding away the moment we think we about to figure it out. But the deeper we look into the known Universe (and the farther back in time) the more we are able to learn about the its evolution, and thus see if they accord with our theories.
A rare object called an Einstein Ring has been discovered by a team in the Stellar Populations group at the Instituto de Astrofísica de Canarias (IAC) in Spain. An Einstein Ring is a specific type of gravitational lensing.
Einstein’s Theory of General Relativity predicted the phenomena of gravitational lensing. Gravitational lensing tells us that instead of travelling in a straight line, light from a source can be bent by a massive object, like a black hole or a galaxy, which itself bends space time.
Einstein’s General Relativity was published in 1915, but a few years before that, in 1912, Einstein predicted the bending of light. Russian physicist Orest Chwolson was the first to mention the ring effect in scientific literature in 1924, which is why the rings are also called Einstein-Chwolson rings.
Gravitational lensing is fairly well-known, and many gravitational lenses have been observed. Einstein rings are rarer, because the observer, source, and lens all have to be aligned. Einstein himself thought that one would never be observed at all. “Of course, there is no hope of observing this phenomenon directly,” Einstein wrote in 1936.
The team behind the recent discovery was led by PhD student Margherita Bettinelli at the University of La Laguna, and Antonio Aparicio and Sebastian Hidalgo of the Stellar Populations group at the Instituto de Astrofísica de Canarias (IAC) in Spain. Because of the rarity of these objects, and the strong scientific interest in them, this one was given a name: The Canarias Einstein Ring.
There are three components to an Einstein Ring. The first is the observer, which in this case means telescopes here on Earth. The second is the lens galaxy, a massive galaxy with enormous gravity. This gravity warps space-time so that not only are objects drawn to it, but light itself is forced to travel along a curved path. The lens lies between Earth and the third component, the source galaxy. The light from the source galaxy is bent into a ring form by the power of the lens galaxy.
When all three components are aligned precisely, which is very rare, the light from the source galaxy is formed into a circle with the lens galaxy right in the centre. The circle won’t be perfect; it will have irregularities that reflect irregularities in the gravitational force of the lens galaxy.
The objects are more than just pretty artifacts of nature. They can tell scientists things about the nature of the lens galaxy. Antonio Aparicio, one of the IAC astrophysicists involved in the research said, “Studying these phenomena gives us especially relevant information about the composition of the source galaxy, and also about the structure of the gravitational field and of the dark matter in the lens galaxy.”
Looking at these objects is like looking back in time, too. The source galaxy is 10 billion light years from Earth. Expansion of the Universe means that the light has taken 8.5 billion light years to reach us. That’s why the ring is blue; that long ago, the source galaxy was young, full of hot blue stars.
The lens itself is much closer to us, but still very distant. It’s 6 billion light years away. Star formation in that galaxy likely came to a halt, and its stellar population is now old.
The discovery of the Canarias Einstein Ring was a happy accident. Bettinelli was pouring over data from what’s known as the Dark Energy Camera (DECam) of the 4m Blanco Telescope at the Cerro Tololo Observatory, in Chile. She was studying the stellar population of the Sculptor dwarf galaxy for her PhD when the Einstein Ring caught her attention. Other members of the Stellar Population Group then used OSIRIS spectrograph on the Gran Telescopio CANARIAS (GTC) to observe and analyze it further.
Everybody knows that galaxies are enormous collections of stars. A single galaxy can contain hundreds of billions of them. But there is a type of galaxy that has no stars. That’s right: zero stars.
These galaxies are called Dark Galaxies, or Dark Matter Galaxies. And rather than consisting of stars, they consist mostly of Dark Matter. Theory predicts that there should be many of these Dwarf Dark Galaxies in the halo around ‘regular’ galaxies, but finding them has been difficult.
Now, in a new paper to be published in the Astrophysical Journal, Yashar Hezaveh at Stanford University in California, and his team of colleagues, announce the discovery of one such object. The team used enhanced capabilities of the Atacamas Large Millimeter Array to examine an Einstein ring, so named because Einstein’s Theory of General Relativity predicted the phenomenon long before one was observed.
An Einstein Ring is when the massive gravity of a close object distorts the light from a much more distant object. They operate much like the lens in a telescope, or even a pair of eye-glasses. The mass of the glass in the lens directs incoming light in such a way that distant objects are enlarged.
Einstein Rings and gravitational lensing allow astronomers to study extremely distant objects, by looking at them through a lens of gravity. But they also allow astronomers to learn more about the galaxy that is acting as the lens, which is what happened in this case.
If a glass lens had tiny water spots on it, those spots would add a tiny amount of distortion to the image. That’s what happened in this case, except rather than microscopic water drops on a lens, the distortions were caused by tiny Dwarf Galaxies consisting of Dark Matter. “We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects,” explained Hezaveh. The difference is that water distorts light by refraction, whereas matter distorts light by gravity.
As the ALMA facility increased its resolution, astronomers studied different astronomical objects to test its capabilities. One of these objects was SDP81, the gravitational lens in the above image. As they examined the more distant galaxy being lensed by SDP81, they discovered smaller distortions in the ring of the distant galaxy. Hezaveh and his team conclude that these distortions signal the presence of a Dwarf Dark Galaxy.
But why does this all matter? Because there is a problem in the Universe, or at least in our understanding of it; a problem of missing mass.
Our understanding of the formation of the structure of the Universe is pretty solid, at least in the larger scale. Predictions based on this model agree with observations of the Cosmic Microwave Background (CMB) and galaxy clustering. But our understanding breaks down somewhat when it comes to the smaller scale structure of the Universe.
One example of our lack of understanding in this area is what’s known as the Missing Satellite Problem. Theory predicts that there should be a large population of what are called sub-halo objects in the halo of dark matter surrounding galaxies. These objects can range from things as large as the Magellanic Clouds down to much smaller objects. In observations of the Local Group, there is a pronounced deficit of these objects, to the tune of a factor of 10, when compared to theoretical predictions.
Because we haven’t found them, one of two things needs to happen: either we get better at finding them, or we modify our theory. But it seems a little too soon to modify our theories of the structure of the Universe because we haven’t found something that, by its very nature, is hard to find. That’s why this announcement is so important.
The observation and identification of one of these Dwarf Dark Galaxies should open the door to more. Once more are found, we can start to build a model of their population and distribution. So if in the future more of these Dwarf Dark Galaxies are found, it will gradually confirm our over-arching understanding of the formation and structure of the Universe. And it’ll mean we’re on the right track when it comes to understanding Dark Matter’s role in the Universe. If we can’t find them, and the one bound to the halo of SDP81 turns out to be an anomaly, then it’s back to the drawing board, theoretically.
It took a lot of horsepower to detect the Dwarf Dark Galaxy bound to SDP81. Einstein Rings like SDP81 have to have enormous mass in order to exert a gravitational lensing effect, while Dwarf Dark Galaxies are tiny in comparison. It’s a classic ‘needle in a haystack’ problem, and Hezaveh and his team needed massive computing power to analyze the data from ALMA.
ALMA, and the methodology developed by Hezaveh and team will hopefully shed more light on Dwarf Dark Galaxies in the future. The team thinks that ALMA has great potential to discover more of these halo objects, which should in turn improve our understanding of the structure of the Universe. As they say in the conclusion of their paper, “… ALMA observations have the potential to significantly advance our understanding of the abundance of dark matter substructure.”