How A Vast, Mysterious Cloud Got Its Glow
By Judith E Braffman-Miller
The Universe beckons us with its tantalizing abundance of haunting, captivating mysteries that beg to be solved–and a newly discovered weird mammoth of a beautiful glowing cloud, termed an Enormous Lyman-alpha Nebula (ELAN) provides the scientific detectives among us with just such an irresistible mystery. ELAN’s are enormous concentrations of gas that are some of the largest known individual objects inhabiting the Cosmos. In February 2017, an international team of astronomers announced they had found that just such a giant, glowing, and mysterious cloud of gas, is apparently part of the great Cosmic Web composed of ghostly and invisible dark matter filaments connecting galaxies to one another–but the mystery here is the unknown identity of the fireworks causing the vast blob of hydrogen gas to light up. This distant cloud has no obvious source of power to illuminate it, and it stands out as the brightest known member of its kind, as well as one of the largest known examples of these rare celestial objects–only a handful of which have ever been observed.
ELANs both surround galaxies, and extend between them, in the strange wonderland of intergalactic space. These vast gas clouds are believed to be components of the network of extremely massive dark matter filaments that connect galaxies in the immense Cosmic Web. Previously discovered ELANs are thought to be lit up by the intense radiation emitted by quasars, but it is still not well understood what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation, which is a characteristic wavelength of light absorbed and emitted by hydrogen atoms. In physics, the lyman-alpha line is a spectral line of hydrogen, or more generally of one-electron ions. Quasars are exceptionally bright Active Galactic Nuclei (AGN)–the glaring, brilliant accretion disks that surround supermassive black holes. Supermassive black holes weigh millions to billions of times more than our Sun, and they haunt the secretive hearts of probably every large galaxy in the Universe. Quasars are usually observed within the dazzling cores of ancient galaxies that inhabited the early Universe.
The newly discovered nebula was discovered at a distance of 10 billion light years, and it is located in the middle of a region with an unusually large number of galaxies. The astronomers found this massive over-density of early galaxies, called a protocluster, as an outcome of a survey project led by Dr. Zheng Cai, who is a Hubble Postdoctoral Fellow at the University of California at Santa Cruz. The discovery of the huge, glowing cloud proved to be a surprise–a case of scientific serendipity. This means that while the astronomers were searching for one thing, they found something else instead.
“Our survey was not trying to find nebulae. We’re looking for the most over-dense environments in the early Universe, the big cities where there are lots of galaxies. We found this enormous nebula in the middle of the protocluster, near the peak density,” Dr. Cai explained in a February 23, 2017 University of California at Santa Cruz (UCSC) Press Release.
Protoclusters are the precursors to galaxy clusters, which are composed of hundreds to thousands of galactic constituents, all bound together by gravitational attraction. Because protoclusters are spread out over a considerably larger area of the sky, they are considerably more difficult for astronomers to detect than modern galaxy clusters.
Dr. Cai is the first author of a research study describing the discovery of this mysteriously glowing cloud of hydrogen gas. The paper has been accepted for publication in The Astrophysical Journal.
The Best Of Dark And Bright
The Standard Cosmological Model of structure formation in the Universe predicts that galaxies are embedded in a mysterious, massive Cosmic Web composed primarily of the dark matter. The gas that collapsed to create galaxies and their multitude of stellar inhabitants traces the distribution of dark matter and reaches far beyond the starlit galaxies situated along the massive filaments of the Cosmic Web. Dark matter is generally thought to compose most of the material content of the Universe, and it is a mysterious substance, believed to be made up of unidentified and exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation–which is why it is invisible. However, astronomers strongly suspect that the dark matter really exists because it exerts gravitational effects on matter that can be seen–such as stars, starlit galaxies, and clouds of glowing gas.
Billions of blazing bright stars blast with their fierce, shining light the more than 100 billion galaxies that inhabit our observable, or visible, Universe. The observable Universe is that relatively small domain of the Cosmos that we are able to see. Most of our Universe is situated far beyond what we can observe. This is because the light flowing out to us from those extremely distant domains has not had enough time to reach us since the Big Bang beginning of the Universe almost 14 billion years ago. The starry galaxies of the Cosmos trace out the mysterious, vast, and massive filaments that are composed of the weird, transparent dark matter–the identity of which is still unknown. The stars of the Cosmos mingle together in groups and clusters, lighting up this otherwise invisible Cosmic spider’s Web, tracing with their brilliant blasts of light that which would otherwise be impossible to observe.
The galaxies, along with their myriad of sparkling stellar inhabitants, ignited a very long time ago–less than a billion years after the birth of the Cosmos. The most widely accepted theory of galaxy formation among astronomers is called the bottom up hypothesis. According to this viewpoint, large and magnificent galaxies like our Milky Way were rare denizens of the young Universe, and these ancient galaxies only gradually attained their majestic sizes when small protogalactic blobs collided and merged together to go into their construction. The primordial galaxies were about one-tenth the size of our Milky Way–but they were equally bright because they were giving birth to newborn baby stars at a breathtaking rate. These very, very bright ancient galaxies were quite small, but they performed the important role of serving as the “seeds” from which large galaxies ultimately grew.
Vast collections of galaxies are arranged within the enormous Cosmic Web, and this great Web is traced out by galaxy clusters and nodes that are bound together by long strings. This large-scale massive structure is very well organized, and it serves the important function of revealing to astronomers very busy intersections, where a multitude of distant galaxies swarm like starlit fireflies around almost–but not entirely–immense and mostly empty Voids.
On the largest scales, the entire Universe appears the same wherever we look–revealing a bubbly and almost foam-like appearance. The gigantic, massive filaments of dark matter, that weave the of the Cosmic Web throughout Space and Time, twist and braid themselves around one another. The dark and almost empty Voids of the Web interrupt this bizarre, invisible structure, and the filaments are outlined by the glaring fires of dazzling stars. The entire web-like structure resembles a natural sponge–or, perhaps, a familiar honeycomb.
The Universe is a profound mystery. It is impossible for us to observe most of it. The multitude of glittering star-fired galaxies and huge clusters and superclusters of galaxies are all captured within halos of the strange, phantom-like dark matter.
The most recent measurements indicate that the Universe is composed of approximately 27% dark matter and 68% dark energy. Dark energy presents an even greater puzzle to be solved than the dark matter, although it is generally thought to be a property of Space itself. The dark energy is causing our Universe to accelerate in its expansion–but its origin and nature have not yet been determined. A mere 5% of the Universe is made up of the so-called “ordinary” atomic matter. Atomic matter is certainly the runt of the Cosmic litter of three, but it is far from “ordinary”. This extraordinary form of matter–though relatively sparse–accounts for literally all of the atomic elements listed in the familiar Periodic Table, and it is the material that gives birth to stars–and the stars are responsible for bringing life into the Universe.
The large-scale structure of the Universe, as revealed by the Cosmic Web, may have emerged with no real physical differences between regions of higher density and regions of lower density. According to this viewpoint, if the current large-scale structure of the Universe is the result of random fluctuations on the quantum level, occurring in the newborn Universe, this is precisely what the most straighforward models indicate. According to this proposal, some regions of the primordial Cosmos received a much greater density of matter than others simply as the result of “hap”. The distribution of wealth in the baby Universe was random–some regions were lucky, some were not.
How A Vast, Mysterious Cloud Got Its Glow
Dr. Cai and his team’s survey project is called Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH), and the recently discovered ELAN is named MAMMOTH-1.
Study coauthor, Dr. J. Xavier Prochaska, professor of astronomy and astrophysics at the University of California, Santa Cruz, noted in the February 23, 2017 UCSC Press Release that previously detected ELANs have been discovered in quasar surveys. In those cases, the powerful radiation emitted from a quasar, that was illuminated by hydrogen gas in the nebula, caused it to emit Lyman-alpha radiation. Dr. Prochaska’s team discovered the very first ELAN, named the Slug Nebula, in 2014 using the 10-meter Keck 1 Telescope at the W.M. Keck Observatory in Hawaii.
The Slug Nebula is a very large, luminous nebula of gas extending approximately 2 million light-years across intergalactic space. This vast nebula of diffuse gas, revealed for the very first time, a portion of the network of filaments thought to connect galaxies in the Cosmic Web. The research study announcing this important finding was published in the January 19, 2014 issue of the journal Nature.
“This is a very exceptional object: it’s huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” explained first author Dr. Sabastiano Cantalupo in a January 19, 2014 UCSC Press Release. Dr. Cantalupo is a postdoctoral fellow at UC Santa Cruz.
Approximately 84% of the Cosmic Web of matter is made up of the transparent and invisible dark matter. The Web reveals itself in the results of supercomputer simulations of the evolution of structure in the Universe. The simulations show the distribution of dark matter on large scales, including the dark matter halos in which galaxies are born, and the Cosmic Web of massive filaments that connect them. The relentless grip of gravity causes “ordinary” atomic matter to follow the distribution of dark matter. As a result, filaments of diffuse, ionized gas are predicted to trace a pattern that is similar to that observed in dark matter supercomputer simulations.
However, until the discovery of the Slug Nebula, the filaments of the Cosmic Web had never before been seen. Intergalactic gas has been spotted due to its absorption of light flowing out from brilliant background sources–but those results do not demonstrate how the gas is actually distributed. In this study, the astronomers spotted the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from the quasar.
“This quasar is illuminating diffuse gas on scales well beyond any we’ve seen before, giving us the first picture of extended gas between galaxies. It provides a terrific insight into the overall structure of our Universe,” commented study coauthor Dr. Prochaska in the January 19, 2014 UCSC Press Release. The hydrogen gas lit up by the powerful, ferocious light coming from the brilliant quasar emits ultraviolet light–the Lyman-alpha radiation.
However, the vast and mysterious cloud dubbed MAMMOTH-1 is the first of its kind not associated with a visible quasar.
“It is extremely bright, and it’s probably larger than the Slug Nebula, but there’s nothing else visible except the faint smudge of a galaxy. So it’s a terrifically energetic phenomenon without an obvious power source,” Dr. Prochaska explained in the February 23, 2017 UCSC Press Release. MAMMOTH-1’s host protocluster is also huge, Dr. Prochaska continued to note.
MAMMOTH-1’s host protocluster is extremely massive, with an unusually high concentration of galactic constituents inhabiting a region that is approximately 50 million light-years across. However, because the protocluster is so far from Earth, astronomers are actually looking back in Time in order to observe it as it was 10 billion years ago–or, about 3 billion years after the birth of the Universe in the Big Bang. Indeed, this era in the Universe’s ancient history marks the peak epoch of galaxy formation. After evolving for 10 billion years, this primordial protocluster would now be a mature cluster of galaxies, perhaps only one million light-years across. This is because the cluster would have collapsed down to a much smaller area, Dr. Prochaska continued to explain in the February 23, 2017 UCSC Press Release.
The Standard Cosmological Model of structure formation predicts that galaxies are embedded in the huge Cosmic Web–and that they are strung out along the heavy filaments of this enormous transparent structure. The MAMMOTH-1 nebula shows a filamentary structure that aligns with the distribution of galaxies in the large-scale structure of the protocluster. This adds support to the idea that ELANs are illuminated segments of the Cosmic Web, Dr. Cai explained in the February 23, 2017 UCSC Press Release.
“From the distribution of galaxies we can infer where the filaments of the Cosmic Web are, and the nebula is perfectly aligned with that structure,” Dr. Cai added.
Dr. Cai and his coauthors considered several potential mechanisms that could be powering the Lyman-alpha emission from the nebula. The most promising proposal involves radiation or outflows from a hidden AGN that is so shrouded by a thick and obscuring veil of dust that only a dim source can be associated with the nebula. AGNs are powered by a supermassive black hole, residing in the secretive heart of a galaxy, that is feasting on unfortunate blobs of gas and doomed stars.
The powerful and intense radiation emitted from an AGN can ionize the gas surrounding it (called photoionization), and this could possibly be one mechanism at work lighting up MAMMOTH-1. When ionized hydrogen in the nebula recombines it would emit Lyman-alpha radiation. However, an alternative scenario suggests that the mechanism powering the Lyman-alpha emissions is shock heating by a powerful outflow of gas from a brilliant AGN.
The astronomers described several lines of evidence supporting the existence of a hidden AGN energizing the nebula, including the dynamics of the gas and emissions from other atomic elements in addition to hydrogen–notably helium and carbon.
Dr. Prochaska noted in the February 23, 2017 UCSC Press Release that “It has all the hallmarks of an AGN, but we don’t see anything in our optical images. I expect there’s a quasar that is so obscured by dust that most of its light is hidden.”
Judith E. Braffman-Miller is a writer and astronomer, whose articles have been published since 1981 in various magazines, newspapers, and journals. 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 the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.