Silently, secretly, and swathed in a strange, exotic material woven of an invisible form of mysterious matter, it lurks in intergalactic space, bereft of starlight. It is a small dark galaxy that haunts the halo of a much larger, star-blasted galaxy nearly 4 billion light-years away. A dark galaxy is one that hosts no, or very few, brilliant and fiery stars, and these objects got their name because they are barren of the stellar constituents that can illuminate them. In April 2016, a team of astronomers announced that they have detected subtle distortions hidden in an Atacama Large Millimeter/submillimeter Array (ALMA) gravitational wave image that shows tattle-tale signs that a dwarf dark galaxy is haunting the outskirts of its much larger galactic neighbor. ALMA is an astronomical array of radio telescopes in the Atacama desert of northern Chile. This important discovery paves the way for ALMA to find many more similar objects and could help shed new light on important questions concerning the nature of the transparent, mysterious, and exotic non-atomic dark matter.
In 2014, as part of ALMA’s Long Baseline Campaign, astronomers investigated a variety of astronomical denizens of the Cosmos in order to test the telescope’s new, state-of-the-art high-resolution abilities. One of these experimental images proved to be particularly special because it revealed a truly remarkable buried treasure in the enchanting shape of an Einstein ring. The Einstein ring had formed as a result of the gravity of a massive foreground galaxy warping, bending, and distorting the light emitted by another–and much smaller–invisible galaxy 12 billion light-years away.
The phenomenon that formed the Einstein ring is termed gravitational lensing, and it is a prediction of Albert Einstein’s General theory of Relativity (1915). Gravitational lensing is a natural and precious tool that astronomers can use for studying galaxies that are otherwise too remote for them to observe. It also reveals the properties of the nearby lensing galaxy because of the way its gravity warps and focuses light from more distant objects.
The term gravitational lensing refers to the path that light takes when it has been deflected. The light does not need to be exclusively visible light–it can be any form of radiation. As a result of lensing, beams of emitted light, that would normally not have been detectable, are bent in such a way that their paths travel towards the observer. Likewise, light can also be bent in such a way that the emitted beams move away from the observer. There are various forms of gravitational lenses: strong lenses, weak lenses, and microlenses. The differences that exist between these varying forms of gravitational lenses are based on the position of the background object that is emitting the beams of light, the foreground lens that is bending the traveling light, and the position of the observer–as well as the shape and mass of the foreground gravitational lens. The foreground object determines how much light from the background object will be warped, as well as the path that this emitted light will take.
The Universe that we see today is filled with billions upon billions of fiery stars that inhabit the more than 100 billion galaxies situated in that relatively small region that we are able to observe. What may (or may not) exist beyond what we can see, from the small region of the Cosmos that we call the observable, or visible Universe, is lost to us. Indeed, the answer to our origins and very existence may lie hidden in those mysterious regions that we can never reach. We are unable to observe those objects because the light that travels towards us from those incredibly distant domains has not had enough time to reach us since the Big Bang. This is because of the accelerating expansion of the Universe. The speed of light, the universal speed limit, has prevented us from observing beyond the cosmological horizon. As we peer further and further out into Space, we look further and further back in Time. This is because the more distant a luminous object is in Space, the longer it has taken for its traveling light to reach us. The light wandering to us from the most distant regions of the observable Universe has taken billions of years to reach us, and so we observe those remote objects now as they were billions of years ago. In astronomy, long ago is the same as far away. Time is the fourth dimension. The three spatial dimensions of our familiar world are up-and-down, back-and-forth, and side-to-side. It is impossible to locate an object in Space without also locating it in Time–hence, the term Spacetime.
Soon after our Universe was born in the inflationary Big Bang, that occurred almost 14 billion years ago, there was a mysterious era devoid of light, termed the Cosmic Dark Ages. During this ancient era, the Universe was a bizarre expanse of featureless darkness, barren of starlight. This mysterious era came to a dramatic grand finale when the first generation of brilliant baby stars were born to hurl their fabulous light into this domain of perpetual midnight. The first galaxies were dark and opaque clouds of pristine gas, tumbling into, and then collecting, within the strange hearts of halos composed of the exotic and invisible dark matter. The primeval objects then pulled in the very first sparkling batches of bright baby stars.
Strange Dark Objects
Astronomers have long proposed the existence of dark galaxies. Even though these strange, dark objects are bereft of sparkling stars, they may be detectable if they contain large amounts of glowing gas.
The actual size of dark galaxies is not known. This is because they cannot be seen with the normal telescopes that astronomers use. A variety of estimates have been suggested, however, ranging from twice the size of our large barred-spiral Milky Way Galaxy to the size of a small quasi-stellar object (quasar).
Dark galaxies are composed of the dark matter. In addition, dark galaxies are theoretically composed of hydrogen gas and dust. Some astronomers suggest the possibility that dark galaxies really do host stars. Alas, the precise composition of dark galaxies remains mysterious because there is no conclusive way to detect them so far. However, many astronomers estimate that the mass of the gas in these galaxies is about 1 billion times that of our Sun.
A Small Dark Galaxy Hides In Space
In a new paper accepted for publication in the Astrophysical Journal, Dr. Yashar Hezaveh and his team provide a detailed analysis of the ALMA image showing signs of a hidden dwarf dark galaxy lurking within the halo of the more nearby, larger galaxy.
“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,” Dr. Hezaveh explained in an April 14, 2016 National Radio Astronomy Observatory (NRAO) Press Release. In the case of a rain drop, the image distortions are caused by refraction. In the revealing ALMA image, similar distortions are formed as a result of the gravitational influence of dark matter. Dr. Hezaveh is an astronomer at Stanford University in Palo Alto, California. The NRAO is in Charlottesville, Virginia.
The dark matter is theorized to account for 80 percent of the mass of the Universe. Even though dark matter is thought to be composed of strange, unidentified, non-atomic particles that do not interact with light or other forms of electromagnetic radiation, it can be identified because of its gravitational influence on objects that can be seen.
For their study, the team of astronomers used thousands of computers working in parallel for many weeks, including the National Science Foundation’s most powerful supercomputer, Blue Waters. With the help of Blue Waters, the scientists searched for subtle anomalies that displayed consistent and measurable counterpart in each “band” of radio data. From this collection of combined computations, the astronomers were able to attain an unprecedented understanding of the lensing galaxy’s halo, which is a diffuse and almost star-free region surrounding the galaxy–and they discovered a distinctive clump less than one-thousandth of the mass of our Milky Way.
Because of the clump’s estimated mass, its relationship to the larger galaxy, and the lack of an optical counterpart, the astronomers suggest that this gravitational anomaly may be the result of an extremely dim, dark-matter dominated dwarf satellite of the large, lensing galaxy. According to theoretical predictions, most galaxies should be swamped by swarms of similar dwarf galaxies, as well as other companion objects. Spotting these objects, however, has presented a great challenge. Indeed, in respect to our own Galaxy, astronomers are able to identify only about 40 of the thousands of satellite objects that are predicted to be lurking there.
“This discrepancy between observed satellites and predicted abundances has been a major problem in cosmology for nearly two decades, even called a ‘crisis’ by some researchers. If these dwarf objects are dominated by dark matter, this could explain the discrepancy while offering new insights into the true nature of dark matter,” explained Dr. Neal Dalil in the April 14, 2016 NRAO Press Release. Dr. Dalil is of the University of Illinois in Urbana-Champaign, and a member of the discovery team.
Computer models of the evolution of the Universe suggest that by measuring the “clumpiness” of the invisible dark matter, it may be possible to measure its temperature. Therefore, by counting the number of small clumps of dark matter around distant galaxies, astronomers can determine the temperature of dark matter–which has an important influence on the smoothness of the Cosmos.
“If these halo objects are simply not there, then our current dark matter model cannot be correct and we will have to modify what we think we understand about dark matter particles,” commented study co-author Dr. Daniel Marrone in the same NRAO Press Release. Dr. Marrone is of the University of Arizona in Tucson.
These new observations are important because they suggests that most of the dwarf galaxies may simply be in hiding, and cannot be seen because they are primarily made up of the invisible dark matter–and, thus, emit very little (if any) revealing light, to give away their secret presence. “Our current measurements agree with the predictions of cold dark matter. In order to increase our confidence we will need to look at many more lenses,” explained Dr. Gilbert Holder in the April 14, 2016 NRAO Press Release. Dr. Holder is of McGill University in Montreal, Canada.
“This is an amazing demonstration of the power of ALMA. We are now confident that ALMA can efficiently discover these dark galaxies. Our next step is to look for more of them and to have a census of their abundance to figure out if there is any possibility of a warm temperature for dark matter particles,” Dr. Hezaveh explained to the press.