Thousands Of Galaxies Are In The Dark

Strange objects lurk in our weird Universe, hiding secretively in bewitching darkness, making themselves difficult to see. Dark galaxies are just such mysterious objects, that can hide themselves very well in our Universe’s most secretive places because they have no, or very few, stars to shed light on their shadowy, ghostly presence. However, they still may reveal themselves if they host large quantities of glaring gas. Indeed, since 2015, astronomers have managed to discover literally thousands of these very faint, phantom-like systems concealing themselves within and around several clusters of galaxies. How these strange systems are born remains a tantalizing puzzle to be solved. In November 2016, a team of researchers proposed that intense periods of star-birth, as well as blast waves created in the wake of supernova explosions, could be the culprits that made these dim galaxies switch off their stellar lights.

Not all galaxies are bright with the sparkling flames of a myriad of stars. Indeed, astronomers have recently observed many extremely faint, diffuse galaxies, with too few stars to light their magnificent galactic fires. However, how these faint galaxies came to be is not precisely known. They could be an entirely new and surprising type of galaxy that challenges current theories about how galaxies are born–or, alternatively, they might be galactic nonconformists that started out exactly like the ho-hum herd of galaxies we are familiar with, but experienced a sea-change as a result of being shaped by their environment into systems that travel to the beat of a different drum. Once astronomers reported their observations of the first collection of dark galaxies early in 2015 which–like footprints in the snow–pointed them in the direction of where to hunt, they started discovering dark galactic denizens in a large number of nearby galaxy clusters. In only a little over a year, astronomers went from knowing of none, to knowing of over a thousand of these strange beasts, inhabiting the galactic zoo.

The newly discovered treasure trove of dark galactic denizens of the Cosmos presents an intriguing mystery to be solved. Any galaxy as large as our own Milky Way should be able to readily give birth to a host of lovely stellar babies. However, it is still not known how massive the dark galaxies really are. Perhaps these ghostly systems are galactic failures–just as massive as our own Galaxy–that were mysteriously deprived of the ability to create lovely star-babies. Alternatively, the dark galaxies may be light-weights that have been stretched thin, like a blob of tugged-on taffy, by either external or internal events. In either case, with so few stars, dark galaxies must possess immense amounts of invisible matter in order to resist being ripped apart by the merciless gravity exerted by other nearby galaxies.

Expect The Unexpected

Mysteries are seductive. Once bitten by the bug of trying to solve one, the hapless victim of her own curiosity cannot rest until she finally figures the bewitching, bewildering, and bothersome thing out.The dim galaxies are just as faint as dwarf galaxies, but are spread out over an area that is just as large as our own large spiral Milky Way Galaxy. It is a mystery how galaxies so very dim–hosting up to 1000 times fewer stars than our own Galaxy–could still be equally large. The new research, conducted by scientists from the Niels Bohr Institute in Copenhagen, Denmark, has provided a plausible solution to this mystery. Their new research shows that if a large number of supernovae explode during the star-birthing process within a galaxy, it can result in both the stars and the dark matter being launched unceremoniously outwards. This would cause the galaxy to expand. The new research is published in the scientific journal, Monthly Notices of the Royal Astronomical Society.

There is nothing quite like a good scientific mystery to lure a scientist into a trap woven of the twisted threads of bequiling imagination. As a result of the many detections of these galactic nonconformists, the desire became intense to figure out exactly how many of these baffling celestial beasts there really are–and where they might be hiding.

Telescopes constructed to spot dim objects revealed the presence of a plentiful treasure of large but empty galaxies, dubbed ultra-diffuse galaxies (UDGs). The surging flood of new discoveries started in New Mexico, with a telescope poised in a park approximately 110 kilometers from Roswell, a city that sings a terrible siren’s song to those who are tantalized by tales of UFOs. Named the Dragonfly telescope, this remarkable, but relatively tiny, example of technology revealed dim galaxies that had been missed by other observatories. Bigger telescopes are generally considered to be better than their smaller kin. This is because a comparatively large lens or mirror can snare more light–thus observing dimmer objects. However, even the big guys have a limit–unwanted light. This unwanted, scattered light shows itself in the form of faint “ghosts”–blobs that can veil faint detail in images of space or even mimic very dim galaxies.

Alas, large dark galaxies look a lot like these ghosts–which is why they had been missed. But Dragonfly was designed to keep those interfering blobs of light in check and, as a result, scientific detectives pointed Dragonfly at the Coma Cluster of galaxies. The Coma Cluster is populated by thousands of galaxies, and it is located approximately 340 million light-years from Earth. This is considered to be close to Earth–by cosmological standards. Coma is densely packed with constituent galaxies, and is a favorite hunting ground for those who seek to find rare beasts in the celestial zoo.

A team of astrophysicists led by Dr. Roberto Abraham (University of Toronto, Canada) and Dr. Pieter van Dokkum (Yale University, New Haven, Connecticut) searched the outer limits of Coma Cluster galaxies for evicted stellar streams and stars, that might have been left behind to tell the tragic tale of galactic wreckage, resulting from smaller galaxies bumping into, and then merging together, to construct larger ones.

In astronomy, it is often best to expect the unexpected. The astronomers were not expecting to find the treasure trove of dark galaxies that their observations revealed–literally dozens of these strange systems hiding in plain sight. Because astronomers had been observing the Coma Cluster for 80 years, it was generally thought that there was nothing new to find there. However, this proved not to be the case at all, because dispersed throughout the entire Coma Cluster, were 47 dark galaxies–revealing at last their secretive presence. Many of this batch of almost 50 galaxies were as large as our Milky Way, being tens of thousands to hundreds of thousands of light-years across. This observation was puzzling. This is because a galaxy that big should not have a problem giving birth to a multitude of brilliant, glaring, fiery stars. The study describing this baffling discovery is published in the September 2016 issue of Astrophysical Journal Letters.

To complicate matters further, the Coma Cluster is heavily populated by galactic bullies–which should have made it difficult for this hidden treasure of 47 dark galaxies to survive there. In order for the dark galaxies to be able to survive, they needed to have sufficient mass in the form of gas, stars, and other celestial forms of matter. Therefore, in a cluster like Coma, a badly bullied galaxy needs to be either compact or fairly massive. However, with so few stars, and little observable mass–smeared out over a relatively large swath of Space–dark galaxies should have long ago been shredded beyond repair by their bullying cousins.

Later, when searching through old images of Coma that had been obtained by the Subaru Telescope in Hawaii, a team of astrophysicists led by Dr. James Koda (Stony Brook University, New York) confirmed that those 47 galaxies really do exist. However, the team then went on to discover a whopping 854 dark galaxies in the Coma Cluster—332 of which appear to be approximately the same size as our Milky Way. The team calculated that Coma could house more than 1,000 dark galaxies in an assortment of sizes–comparable to its number of already-known galaxies.

In The Dark

Dark matter is a mysterious substance that is generally thought to be composed of exotic, non-atomic particles that cannot interact with light or any other form of electromagnetic radiation–which is why it is invisible. However, dark matter does interact with the force of gravity, and this is the reason why many scientists think that it is really there. It is currently thought that the Universe is composed of about 68% dark energy, 27% dark matter, and 5% ordinary atomic matter (baryonic matter). The portion of the Universe that we are most familiar with is composed of so-called “ordinary” atomic matter, which is the runt of the Cosmic litter of three. The mysterious dark matter is much more abundant than atomic matter, and the dark energy is even more abundant and more mysterious than the dark matter. Dark energy, which accounts for the lion’s share of the Universe, is thought to be causing the Universe to accelerate in its expansion–and it is frequently considered to be a property of Space itself.

Galaxies are enormous collections of stars, gas and dark matter. Even the smallest galaxies host a few million stars. However, the largest galaxies may contain several hundred billion fiery stellar inhabitants. The first generation of stars came into being only about 200 million years after the Big Bang which occurred about 13.8 billion years ago. The first stars were born from the very light gases hydrogen and helium that were formed in the Big Bang (Big Bang nucleosynthesis). All of the atomic elements heavier than helium are called metals by astronomers, and these heavier elements were created by the stars in their searing-hot nuclear fusing hearts (stellar nucleosynthesis)–or in the fiery supernova blasts that serve as the final death throes of massive stars. These gigantic pristine clouds of hydrogen, helium, and dust contract, and eventually the gas becomes so compact that the resulting pressure heats up the material. The resulting enormous roiling, fiery balls of seething-hot gas evolve into newborn baby stars. In the primordial Universe, the early galaxies hoisted in the first stars with the powerful grip of their gravity.

Long before there were living creatures on Earth, with eyes that could see, the Universe contained a swirling sea composed of pristine hydrogen and helium gases, and the ghostly non-atomic dark matter.

According to the Standard Model of the formation of large-scale structure, the transparent dark matter particles initially merged together gravitationally to form a crowded area–termed a dark matter halo. As time passed, these invisible halos hoisted in–with the powerful grip of their gravity–billowing, floating clouds of pristine primarily hydrogen gas. The stars and galaxies were born as a result.

The favored theory of galactic formation–the bottom up theory–proposes that the neonatal protogalaxies gradually grew larger and more massive as a result of collisions and mergers with other protogalactic blobs, amd by constantly giving birth to more and more sparkling baby stars.

All of the myriad galaxies, that do their fantastic whirling ballet throughout the observable Universe, are filled with dark matter–which can unveil itself only through its gravitational interactions with brilliant stars and luminous clouds of gas.

Thousands Of Galaxies Are In The Dark

The newly discovered batch of dark galaxies proved to be difficult to classify. This is because one group of astronomers thought that these diffuse galaxies were just large spiral galaxies gifted with an unusually large amount of dark matter, while a second group thought that the dark galaxies were just ordinary dwarf galaxies.

However, a project led by a Niels Bohr Institute researcher recreated the attributes of the mysterious myriad of dim galaxies, that have been observed, by using advanced computer simulations.

“By recreating almost 100 virtual galaxies we have shown that when there are a lot of supernovae during the star formation process, it can result in the stars and the dark matter in the galaxy to be pushed outwards, causing the extent of the galaxy to expand. When there is a small number of stars in an expanded area, it means that the galaxy becomes faint and diffuse and is therefore difficult to observe with telescopes,” explained Dr. Arianna Di Cintio in a November 29, 2016 Niels Bohr Institute Press Release. Dr. Di Cintio is an astrophysicist in the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen, who is the lead scientist on the project.

Dr. Di Cintio continued to explain that the mechanism that forces the stars to travel away from the center of a galaxy is the same one that creates regions with a lower density of dark matter. The large number of supernovae explosions are so extremely powerful that they blast the gas outwards in the galaxy. Because of this, both the dark matter and the stars travel outwards so that the area of the galaxy expands. Because the galaxy is spread out over a larger area, it becomes more diffuse and difficult to observe.

“If we can recreate ultra-diffuse galaxies with computer simulations, it proves that we are on track with our cosmological model. We therefore predict that there are ultra-diffuse galaxies everywhere–not only in galaxy clusters. They are dominated by dark matter and only a small percentage of their content is comprised of gas and stars and the most important thing is that they are dwarf galaxies with a mass of only about 10 to 60 times less than a large spiral galaxy, which is to say significantly smaller than a large galaxy like the Milky Way,” Dr. Di Cintio explained in the Niels Bohr Institute Press Release.

The astronomers also expect that among the most isolated UDGs, the largest should contain the most gas. As a result, they are beginning close collaborations with research teams carrying out observations of very remote regions of the sky with especially powerful telescopes, in order to confirm these theories.

Dr. Di Cintio continued to explain that “It will open a whole new window into galaxy formation–there may be thousands of ultra-faint galaxies that are just waiting to be discovered.” She also added that she is looking forward to finding out the number of stars that the dark galaxies contain, their content of elements, and how the ultra-diffuse galaxies manage to survive in galaxy clusters.

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