A Ghostly Galaxy Has A Dark Secret

By Judith E Braffman-Miller

Long ago, soon after the Universe was born, there was an ancient era before the first generation of stars caught fire. In this black swath of featureless space, that was the primordial Universe, opaque clouds of mostly hydrogen gas collected along enormous, heavy, and invisible filaments composed of a mysterious non-atomic substance that astronomers call the dark matter. It is generally believed that the first galaxies were dark, amorphous clouds of gas that pooled at the hearts of dark matter halos, and the relentless gravitational pull of this invisible substance served as the "seeds" from which the billions upon billions of galaxies were born in the primordial Cosmos. In March 2018, a team of astronomers announced their new discovery of a bizarre, unique galaxy--the first of its kind--that seems to contain no dark matter at all. This mysterious and exceptional beast inhabiting the galactic zoo is unique because galaxies are commonly thought to contain more invisible dark matter than the so-called "ordinary" atomic matter that composes a galaxy's stars, gas, and dust.

The first galaxies were born from concentrations of the invisible dark matter, and this exotic material gravitationally snared "ordinary" (baryonic) matter. Clouds composed mostly of hydrogen gas ultimately floated down into the secretive, invisible ghostly halo of the mysterious dark substance.

In the modern Universe, there are some galaxies that keep themselves well-hidden from the prying eyes of curious astronomers. They are big--like our own enormous starlit spiral Milky Way--but, unlike our own dazzling pin-wheel whirling in space, they host so few stars for their size that they are dim and diffuse. Indeed, they are so diffuse that some scientists have compared them to "giant cotton balls".

However, new observations derived from NASA's Hubble Space Telescope (HST) of the strange galaxy whose dark matter apparently has gone missing, have revealed this oddity that sets it apart from any other known galaxy in the Universe--even the diffuse "giant cotton balls". This invisible form of non-atomic matter serves as the underlying scaffolding from which galaxies form--and this galaxy doesn't seem to have any. Dark matter composes the bulk of the Universe's matter content, and it is the invisible glue that binds visible, "ordinary" matter together. Dark matter interacts with atomic matter only through the force of gravity, and its gravitational effects on objects that can be seen reveals its ghostly presence.

Dubbed NGC 1052-DF2, this phantom-like galaxy contains at most 1/400th the amount of dark matter that astronomers expect to see. How it formed is, therefore, a complete mystery. This non-conformist galaxy apparently travels to the beat of a different drum than do others of its kind. NGC 1052-DF2 is by no means small--it is as big as our Milky Way. However, this galaxy has managed to elude discovery because it hosts only 1/200th the number of stellar constituents as our Galaxy.

Based on the colors of its attendant globular clusters, NGC 1052-DF2 is approximately 10 billion years old. It is situated about 65 million light years from Earth. A globular cluster is a compact spherical cluster composed primarily of old stars. Globulars are located in the outer region of a galaxy.

"We thought that every galaxy had dark matter and that dark matter is how a galaxy begins. This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is unexpected. It challenges the standard idea of how we think galaxies work, and it shows that dark matter is real: it has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy," explained Dr. Pieter van Dokkum in a March 28, 2018 Hubblesite Press Release. Dr. van Dokkum is of Yale University in New Haven, Connecticut, and lead researcher of the HST observations.

Given NGC 1052-DF2's large size and faint appearance, astronomers classify it as an ultra-diffuse galaxy. A 2015 survey of the Coma galaxy cluster revealed that these faint, large galaxies are surprisingly abundant. Nevertheless, no other ultra-diffuse galaxy known has been found to be as depleted of dark matter as NGC 1052-DF2.

A Dark And Ancient Mystery

Dark matter is thought to be an exotic form of matter that cannot dance with light or any other form of electromagnetic radiation and, as such, is invisible--or transparent. According to the Standard Model of Cosmology, the Universe is composed of about 4.9% "ordinary" atomic (baryonic) matter, 26.8% dark matter, and 68.3% dark energy. The dark energy, which accounts for most of the Universe, is even more of a puzzle than the dark matter. The most widely accepted theory explaining dark energy is that it is a property of space itself, and it is causing the Universe to accelerate in its expansion in the direction of its own doom. As the Universe speeds up in its expansion it grows ever larger and larger; colder and colder--destined to become an enormous, frigid expanse, as its fires go out like the light of a candle, leaving literally everything in a nightmare of emptiness.

The badly misnamed "ordinary" atomic matter is extraordinary. This runt of the cosmic litter of three accounts for literally all of the elements listed in the familiar Periodic Table, and these atomic elements create the familiar world that we can experience with our senses. It is this relatively small percentage of atomic matter that created life in the Cosmos. The calcium in your bones, the oxygen that you breathe, the iron in your blood, the water that you drink, the sand that you walk upon are all composed of so-called "ordinary" matter. Most of these atomic elements were manufactured in the incredibly hot nuclear-fusing furnaces of the Universe's stars. The Big Bang birth of the Universe, thought to have occurred about 13.8 billion years ago, only produced the lightest of atomic elements--hydrogen, helium, and small quantities of lithium. The stars cooked up the rest--all the way up to iron. But, the heaviest atomic elements of all--such as gold and uranium--formed when a massive star perished in the fireworks of supernova blast. These brilliant stellar explosions hurled into interstellar space the dead progenitor star's freshly forged batch of heavy atomic elements.

According to the Standard Model of the formation of large-scale cosmic structure, particles of the mysterious and exotic dark matter at first danced together gravitationally to construct a crowded region of space--the dark matter halo. Eventually, the dark primordial halos gravitationally snared floating clouds of pristine hydrogen gas. Hydrogen is both the lightest and most abundant atomic element in the visible Universe. As a result of this, galaxies and their constituent stars emerged out of the primordial darkness.

Billowing, amorphous, dark and opaque, the pristine clouds of ancient gases gathered together, and then tumbled down into the secret hearts of the bizarre dark matter halos. As time passed, the first generation of baby stars were born, and their newly-lit raging fires sparkled within the first galaxies that served as primordial stellar cradles.

Even though dark matter is transparent and, as such, invisible, it is generally believed to exist because of the extremely important differences astronomers have observed between the mass of enormous celestial objects--calculated from their gravitational interactions--and the mass determined from the visible atomic matter that they host--such as stars, dust, and gas.

The existence of the dark matter was first suggested by the Dutch astronomer Jan Oort (1900-1932), in his effort to make sense of the orbital velocities of stars dwelling within our own Milky Way. In 1933, the Swiss-American astronomer Fritz Zwicky (1898-1974), also proposed the real existence in nature of a weird form of abundant and invisible matter. Zwicky had come to the conclusion that some form of invisible "missing mass" floated phantom-like in the Universe--and that this invisible matter influenced the orbital velocities of galaxies inhabiting distant galactic clusters. Convincing evidence for the existence of the dark stuff was derived from galactic rotation curves by the Caltech astrophysicist Horace W. Babcock (1912-2003) in 1939. Babcock, however, did not attribute his highly suggestive observations to dark matter.

At last, in the 1960s, the great astronomer Vera Rubin (1928-2016) became the first to offer convincing evidence that dark matter really exists in nature. Rubin, who was familiar with Zwicky's work, based her theory on galactic rotation curves. Soon after Rubin's study was released, a number of important observations were conducted by other scientists that suggested the existence of the ghostly and invisible dark stuff. The new discoveries were based on observations that took into account the gravitational lensing of background objects by foreground galaxy clusters such as the Bullet Cluster, the temperature and distribution of searing-hot gas lurking within galaxies and galactic clusters, and--more recently--the pattern of anisotropies observed in the Cosmic Microwave Background (CMB) radiation that formed in the newborn Universe at the time of the Big Bang. Gravitational lensing is a phenomenon suggested by Albert Einstein in his Theory of General Relativity (1915), when he came to the realization that gravity could warp Spacetime--and, as a result, have lens-like effects.

The galaxies of the visible Universe switched on less than a billion years after the Big Bang. In the primordial Cosmos, the invisible and bizarre dark matter snatched at clouds of primarily hydrogen gas, and hoisted them in. These pristine clouds of gas were destined to evolve into the ancient nurseries of the first generation of newborn stars.

Eventually, the whirling, somersaulting, floating gas clouds, along with the ghostly, non-atomic dark matter, danced together throughout the ancient Cosmos. Gradually, they combined to create the structures seen in the modern Universe.

The theoretical existence of the dark stuff is an important component of recent models explaining cosmic structure formation and galaxy birth and evolution--as well as offering an explanation for the anisotropies seen in the CMB--the relic radiation left over from the Big Bang itself. All of the lines of evidence, collected so far, suggest that galaxies, galactic clusters, and the entire Universe contain much more matter than what can be observed with electromagnetic radiation.

Giant Cotton Ball

Dr. van Dokkum and his team discovered NGC 1052-DF2 using the Dragonfly Telephoto Array, which is a custom-built telescope located in New Mexico, that they had designed to detect these giant ghostly galactic "cotton balls". They then made use of the W.M. Keck Observatory in Hawaii to measure the movement of 10 giant stellar groupings called globular clusters inhabiting the galaxy. Keck revealed to the prying eyes of the astronomers that the globulars were traveling at relatively slow speeds--less than 23,000 miles-per-hour. Stars and clusters dwelling in the outskirts of galaxies hosting dark matter travel at least three times faster. From those measurements, the astronomers were able to calculate the galaxy's mass. "If there is any dark matter at all, it's very little. The stars in the galaxy can account for all the mass, and there doesn't seem to be any room for dark matter," Dr. van Dokkum explained in the March 28, 2018 Hubblesite Press Release.

The astronomers then used NASA's HST and the Gemini Observatory in Hawaii to discover new details about this strange one-of-a-kind galaxy. Gemini revealed that the galaxy does not show signs of an interaction with another galaxy. HST also helped to further identify the globulars and measure a precise distance to the galaxy.

In addition, the HST images unveiled the galaxy's rather unusual appearance. "I spent an hour just staring at the Hubble image. It's so rare, particularly these days after so many years of Hubble, that you get an image of something and you say, 'I've never seen that before.' This thing is astonishing, a gigantic blob that you can look through. It's so sparse that you see all the galaxies behind it. It is literally a see-through galaxy," Dr. van Dokkum continued to explain in the Hubblesite Press Release. This extreme giant cotton ball galaxy doesn't seem have a central region, or even spiral arms and a disk, that are typical features of a large galaxy, such as our own. However, NGC 1052-DF2 doesn't look like an elliptical galaxy, either. The giant cotton ball also shows no evidence of a central black hole haunting its heart. Most--and probably all--large galaxies, including our Milky Way, are thought to house a central black hole. These very heavy supermassive black holes can weigh-in at millions to billions of times solar mass. Indeed, even NGC 1052-DF2's  globular clusters are unusual. This is because they are twice as large as typical stellar groupings observed in other galaxies.

"It's like you take a galaxy and you only have the stellar halo and globular clusters, and it somehow forgot to make everything else. There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown," Dr. van Dokkum continued to explain in the March 28, 2018 Hubblesite Press Release.

Nevertheless, the astronomers have come up with some possible explanations. NGC 1052-DF2 is located in a group of galaxies dominated by an enormous elliptical dubbed NGC 1052. Galaxy formation is both a violent and turbulent event. Because of this commotion, Dr. van Dokkum proposes that the growth of the young NGC 1052 billions of years ago possibly played a major role in the mystery of NGC 1052-DF2's missing dark matter.

Another possibility is that gas traveling toward the giant elliptical NGC 1052 may have fallen apart, and in this way created the weird NGC 1052-DF2. The formation of NGC 1052-DF2 may have been aided by strong winds flowing out from the young supermassive black hole lurking in the heart of NGC 1052. These possible scenarios are still speculative, however, and they fail to explain all of the characteristics of the dark matter-depleted galaxy, the astronomers told the press.

The team is already searching for more dark-matter-depleted galaxies, and they are currently analyzing HST images of 23 other cotton ball galaxies. A trio of these diffuse galaxies appear to be similar to NGC 1052-DF2.

"Every galaxy we knew about before has dark matter, and they all fall in familiar categories like spiral or elliptical galaxies. But what would you get if there were no dark matter at all? Maybe this is what you would get," Dr. van Dokkum commented to the press.

The team's results appear in the March 29, 2018 issue of the journal Nature.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.