Intracluster Light: Illuminating The Universe’s Dark Side


The Universe has a dark side, that sings a bewitching and bewildering Sirens’ song to astronomers who seek to solve its myriad mysteries. Astronomers refer to one of its especially enticing mysteries as the “dark matter”. This is because they are “in the dark” about its true nature. However, many scientists propose that the dark matter is an exotic and abundant ghostly material that cannot be seen, because it does not dance with light or any other form of electromagnetic radiation. Nevertheless, even though the dark stuff is invisible, astronomers generally think that it really does haunt the Universe because it exerts an observable gravitational influence on objects that can be seen–such as fiery stars and the brilliant galaxies that house them. In December 2018, a team of astronomers announced that their new study of Hubble Space Telescope (HST) images could provide an important step toward illuminating this elusive, exotic substance, thus shedding new light on its mysterious and secretive nature. Using HST’s past observations of a half dozen massive galaxy clusters in the Frontier Fields program, the astronomers showed that intracluster light–the gentle and diffuse glow that shimmers between individual galaxies within a cluster–reveals the path that this ghostly, transparent material takes through space, thus illuminating its distribution with more accuracy than other methods that observe X-ray light.

Intracluster light is the result of distruptive interactions between galaxies within a cluster. In the chaos that ensues, individual stars are torn screeching from the gravitational ties that bind them to their host galaxy. These rudely evicted stars then go on to realign themselves with the gravity map of the entire cluster. This also happens to be where most of the cluster’s transparent dark matter lurks in invisible secret. X-ray light shows where groups of galaxies are bumping into one another, but it does not reveal the underlying structure of the cluster itself. This renders X-ray light a less exact tracer of the dark stuff.

“The reason that intracluster light is such an excellent tracer of dark matter in a galaxy cluster is that both the dark matter and these stars forming the intracluster light are free-floating on the gravitational potential of the cluster itself–so they are following exactly the same gravity. We have found a new way to see the location where the dark matter should be, because you are tracing exactly the same gravitational potential. We can illuminate, with a very faint glow, the position of dark matter,” explained Dr. Mireia Montes in a December 20, 2018 Hubblesite Press Release. Dr. Montes, who is of the University of New South Wales in Sydney, Australia, is a co-author of the study.

The discovery and quantification of the diffuse glow of intracluster light, streaming within galaxy clusters, provides a new and valuable tool that astronomers can use to study the history and structure of galactic clusters in greater detail than previously possible. Because the intracluster light emanates from unfortunate orphaned stars inhabiting galaxy clusters, that have been torn gravitationally from their parent-galaxies, it is a product of the dynamical interactions within the cluster. For this reason, the intracluster light has the potential to reveal a great deal of important information concerning the cluster’s accretion history and evolutionary past, as well as the mass distribution of the individual cluster galaxies themselves and the entire cluster as a whole. The morphology, amount, and kinematics of the intracluster light each provide potential valuable information concerning the cluster’s evolution, and processes affecting individual galaxies can be traced using individual streams of intracluster light.

The Dark Side

The mysterious dark matter is believed to consist of exotic non-atomic particles that do not interact with electromagnetic radiation. This exotic material only dances with so-called “ordinary” atomic (baryonic) matter by way of the force of gravity. According to the Standard Model of Cosmology, the Universe is composed of approximately 4.9% “ordinary” atomic matter, 26.8% dark matter, and a whopping 68.3% dark energy. Indeed, the dark energy, which makes up most of the Universe, is an even greater mystery than the dark matter. The most widely accepted explanation for the dark energy proposes that it is a property of space itself, and it is causing the Universe to accelerate in its expansion towards its own “heat death”. As the Universe speeds up in its expansion, it grows ever colder and colder; larger and larger–doomed to become an enormous frigid expanse, as its fires flicker out like a dying candle flame.

The very badly misnamed “ordinary” atomic matter is actually quite extraordinary. Even though atomic matter is clearly the runt of the cosmic litter of three, it accounts for all of the elements listed in the familiar Periodic Table. The atomic elements create the world that we are most familiar with, and that we can experience with our Earth-evolved senses. Even though atomic matter accounts for only a relatively small fraction of the Cosmos, it is what brought life into it. The iron in your blood, the calcium in your bones, the water that you drink, the sand that you walk upon are all composed of so-called “ordinary” atomic matter. Most of the atomic elements were formed within the searing-hot, nuclear-fusing furnaces of the Universe’s myriad stars. The Big Bang birth of the Universe, thought to have occurred almost 14 billion years ago, only manufactured the lightest of atomic elements–hydrogen, helium, and traces of lithium. The stars produced the rest in their incredibly hot cores, starting with the hydrogen and helium produced in the Big Bang, and then creating increasingly heavier and heavier atomic elements all the way up to iron by way of the process of nuclear fusion. However, the heaviest atomic elements of all–such as uranium and gold–were produced when a massive star blew itself up in a supernova explosion. These fiery, brilliant stellar blasts hurl freshly forged heavy atomic elements out into the space between stars–all manufactured in the searing-hot heart of the progenitor massive star, or in its explosive death throes.

According to the Standard Model for the formation of the large-scale structure of the Universe, exotic particles of the non-atomic dark matter at first performed a gravitational ballet with one another, thus constructing a crowded region of space, termed the dark matter halo. Gradually, the invisible primordial halos composed of the dark stuff snatched up clouds of pristine hydrogen gas. Hydrogen is both the most abundant, as well as the lightest, atomic element in the Universe. As a result, galaxies and their population of shimmering stars, emerged out of this primordial darkness.

Vast, swirling, and shapeless clouds of opaque pristine ancient gases gathered together in the primeval darkness. The clouds then somersaulted down into the secretive hearts of the strange halos of the dark matter. As time marched onward in the direction of the universal expansion, the very first generation of baby stars were born. Then, the newly lit fires of the first stars raged brilliantly within the first ancient galaxies that served the important function of being primordial stellar cradles.

Even though dark matter cannot be seen, it is generally thought to exist because of the very important discrepancies that scientists have observed between the mass of large-scale celestial bodies–obtained from their calculated gravitational interactions—-and the mass measured from the visible atomic matter that they host.

The possible existence of dark matter was first proposed by the Dutch astronomer Jan Oort (1900-1932) as a result of his dedicated effort to understand the orbital velocities of our own Milky Way Galaxy’s constituent stars. In 1933, the Swiss-American astronomer Fritz Zwicky (1898-1974), also proposed the existence of an exotic form of abundant and transparent matter. Zwicky reached this conclusion because he realized that some form of invisible “missing mass” traveled ghost-like through the Cosmos–and that this transparent and invisible exotic material influenced the orbital velocities of constituent galaxies inhabiting distant galaxy clusters. In 1939, strong evidence that the bizarre invisible matter really exists in nature was calculated from galaxy rotation curves by the astrophysicist Horace W. Babcock (1912-2016) of the California Institute of Technology (Caltech) in Pasadena. However, Babcock did not realize that his extremely suggestive observations indicated the presence of dark matter.

Finally, half a century ago, the astronomer Vera Rubin (1928-2016) became the first scientist to offer convincing evidence for the existence of the dark stuff. In the 1960s, Rubin–who had studied Zwicky’s work as a graduate student–proposed her new theory that she based on galactic rotation curves. Soon after Rubin’s study was published, a number of important observations were made by other astronomers that also indicated the existence of this exotic, ghostly form of transparent matter. The later studies were based on observations that used gravitational lensing of background objects by foreground galaxy clusters, the distribution and temperature of hot gas located within individual galaxies and galaxy clusters, and (more recently) the observed pattern of anisotropies seen in the Cosmic Microwave Background (CMB) radiation that formed in the newborn Universe at the time of its birth in the Big Bang. Gravitational lensing is a phenomenon proposed by Albert Einstein in his General Theory of Relativity (1915), when he realized that gravity could distort Spacetime–and, for this reason, have lens-like effects.

The galaxies that perform their fantastic dance throughout the entire visible Universe emerged less than a billion years after the Big Bang. In the very ancient Cosmos the transparent, exotic dark matter snared floating clouds of gas that became the primeval nurseries of the first generation of fiery stars to illuminate what was once a dark and featureless expanse.

At last, the swirling floating gas clouds and the ghostly dark matter met up with one another and performed an ancient waltz throughout the Universe. Gradually, they combined to create the familiar structures that now exist in today’s Cosmos.

The theoretical existence of dark matter is an integral part of recent scenarios describing galaxy birth, evolution, and the formation of cosmic structure. In addition, the real existence of this exotic form of matter is important because it provides an explanation for the anisotropies observed in the CMB–the remnant radiation left over from the Universe’s tumultuous birth. All lines of evidence, so far, indicate that galaxies, galaxy clusters, as well as the entire vast Universe as a whole, contain considerably more matter than can be observed by astronomers using electromagnetic radiation.

Evicted Stars Shed New Light On The Darkness

Study co-author Dr. Montes commented that not only is the new method of using intracluster light accurate, it is also more efficient than other methods. This is because it utilizes only deep imaging, instead of the more complex, time-consuming techniques that use spectroscopy. For this reason, more clusters and other objects in space can be observed in less time, and it can provide more evidence of what dark matter is made of and how it behaves.

“This method puts us in the position to characterize, in a statistical way, the ultimate nature of dark matter,” Dr. Montes noted in the December 20, 2018 Hubblesite Press Release.

“The idea for the study was sparked while looking at the pristine Hubble Frontier Fields images. The Hubble Frontier Fields showed intracluster light in unprecented clarity. The images were inspiring,” commented study co-author Dr. Ignacio Trujillo in the same Hubblesite Press Release. Dr. Trujillo is of the Canary Islands Institute of Astronomy in Tenerife, Spain and, with Dr. Montes, has studied intracluster light for many years.

“Still, I did not expect the results to be so precise. The implications for future space-based research are very exciting,” Dr. Trujillo added.

The team of astronomers used the Modified Hausdorff Distance (MHD), which is a metric used in shape matching, in order to measure the similarities between the contours of the intracluster light and the contours of varying mass maps of the clusters, which are part of the data acquired from the Hubble Frontier Fields project. The Hubble Frontier Fields project is kept at the Mikulski Archive for Space Telescopes (MAST). The MHD is a measure of how far two subsets are from one another. The smaller the value of MHD, the more alike the two point sets are. This study revealed that the distribution of intracluster light as seen in the Hubble Frontier Fields images matched the mass distribution of a half dozen galaxy clusters better than did X-ray emission, as derived from Chandra X-ray Observatory’s CCD Imaging Spectrometer (ACIS).

In the future, Drs. Montes and Trujillo expect to see multiple opportunities to expand their study. First, they would like to increase the radius of observation in the original six clusters, in order to discover if the degree of tracing accuracy holds up. A second important test of their method will be the observation and analysis of additional galaxy clusters by more research teams, in order to increase the data set and confirm their findings.

The astronomers also look forward to the application of the same techniques using future powerful space-based telescopes like the James Webb Space Telescope (JWST) and WFIRST, which carry even more sensitive instruments for resolving dim intracluster light in the distant Universe.

Dr. Trujillo would also like to test scaling down observations from massive galaxy clusters to individual, isolated galaxies. “It would be fantastic to do this at galactic scales, for example exploring the stellar halos. In principal, the same idea should work: the stars that surround the galaxy as a result of the merging activity should also be following the gravitational potential of the galaxy, illuminating the location and distribution of dark matter,” he commented in the December 20, 2018 Hubblesite Press Release.

The Hubble Frontier Fields program was a deep imaging initiative created to use the natural magnifying glass of a cluster’s gravity (gravitational lensing) in order to observe the extremely remote galaxies behind them, and in this way gain new insight into the ancient (distant) Universe and the evolution of galaxies since that very ancient time. In astronomy, long ago is the same as far away. The more distant an object is in Space, the more ancient it is in Time (Spacetime).

In the Hubble Frontier Fields program the magnifying glass of a foreground cluster served as the lens, while the more remote galaxy behind the cluster was the magnified object being lensed. For the astronomers of the Hubble Frontier Fields program, the diffuse intracluster light was irritating.This is because it partially obscured the distant galaxies beyond. However, that faint and distant glow of ancient starlight could end up shedding new light on one of astronomy’s most intriguing mysteries–the nature of the exotic dark matter.

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