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An Ancient Baby Galaxy Is Caught In The Web

by Judith E Braffman-Miller

posted in Reference and Education

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How do galaxies, like our own large, majestic, star-splattered Milky Way form, and how do they evolve through time? When we gaze in wonder up at the night sky above our planet, we see that it is dazzling with the distant fires of a host of brilliant stars. However, most of the Universe is dark, made up of exotic, transparent material, the identity of which constitutes one of the most profound and bewitching of all mysteries. In August 2015, a team of astronomers led by the California Institute of Technology (Caltech) in Pasadena, California, announced the discovery of a gigantic, whirling disk composed of gas that is a very remote 10 billion light-years away. This enchanting, bewildering, bewitching ancient structure is thought to be a galaxy-in-the-making--and it is actively being fed a nutritious formula of cool pristine, primordial gas that can be traced all the way back to the very beginning--the Big Bang birth of the Universe almost 14 billion years ago, and its discovery sheds new light on this great and profound mystery.
Using Palomar Observatory's Cosmic Web Imager (CWI), that was designed and built by Caltech, the astronomers were able to image the distant protogalaxy and found that it is bound to a filament of the intergalactic medium--the great Cosmic Web that is constructed of diffuse gas that weaves its way between galaxies and extends throughout the entire Universe.
The enormous Cosmic Web is a large-scale, web-like structure that is embellished with the starry luminous fires of the galaxies, and it is thought to have played a major role in the evolution of galaxies that occurred long ago and far away in the ancient Universe--only a few billion years after the Big Bang.
The way that galaxies and matter are distributed in the Universe is not random. The distribution of galaxies, up to the present time, resembles an enormous network--the transparent Cosmic Web of ghostly invisibility--a strange transparent structure flecked with countless stars. This weird, ghostly web has denser regions composed of dazzling groups and clusters of galaxies. There are also regions that are almost--but not entirely empty--which are the cosmic voids. The filaments link the regions of greatest density, somewhat like bridges that connect the densest regions of the Cosmic Web. This filamentary structure has been compared to threads woven into the web.
Galaxies located in the regions of lesser density have a greater probability of actively giving birth to brilliant, new baby stars (protostars). In contrast, galaxies situated in denser regions give birth to their stellar inhabitants much more slowly. Our own Milky Way Galaxy is located in a region of lesser density.
The billions of starlit galaxies and enormous clusters of galaxies are embedded in mysterious, invisible halos of transparent, ghostly dark matter. Dark matter is a bizarre and bewildering form of exotic matter that is generally believed to exist because it exerts gravitational effects on objects that can be observed--such as galaxies that blaze with starlight and glowing clouds of gas. However, the true identity of the dark matter is unknown, even though it is the most abundant form of matter in the Universe. Dark matter is thought to be composed of exotic non-atomic particles that do not interact with light, or any other form of electromagnetic radiation. The starry galaxies are suspended throughout this invisible, enormous structure in a way that evokes the haunting image of glittering dewdrops on the web of a waiting spider.
Even more abundant, and more mysterious, is the dark energy--a strange substance that is causing our Universe to speed up in its expansion. Some scientists even propose that, billions and billions of years from now, the bizarre dark energy will tear our entire Universe apart--even ripping atoms into non-existence.
The most recent measurements suggest that the dark energy accounts for most of the mass-energy of the Cosmos--68.3% of it. The dark matter accounts for 26.8% of the Universe, while familiar atomic matter--the stuff of planets, moons, people, and literally all of the elements listed in the Periodic Table of the Elements--accounts for a mere 4.9% of the Cosmos. The runt of the Cosmic litter, so-called "ordinary" atomic matter, is really very extraordinary. Without it, life would not be possible.
Most astronomers think that the Cosmos was born about 13.8 billion years ago in the Big Bang. It began as an unimaginably tiny Patch, that was smaller than an elementary particle, and then--in the briefest instant--expanded exponentially to reach macroscopic size. Something--it is not known precisely what--caused that very small Patch to experience this wild period of inflation. This little Patch, that was too small for a human being to see with the naked eye, was so exquisitely tiny that it was almost, but not quite, nothing--and it was so extremely hot and dense that everything that we are, and everything that we know, originated from it.
The neonatal Universe was filled with energetic radiation, a violent, stormy sea of searing-hot particles of light, that we call photons. The entire baby Cosmos was brilliant with light, and it resembled the glaring, blinding surface of a star. What we now witness almost 14 billion years after our Universe's mysterious birth, is the dimming and greatly expanded and still-expanding aftermath of that primordial birth. As our Universe evolved and grew to its present unimaginably enormous size, the ancient fires of its birth cooled--and now we bear witness as we watch from our tiny, rocky, obscure little planet as our Universe grows ever larger and larger, darker and darker, colder and colder, fading like the lingering, eerie, haunting grin of the Cheshire Cat in a Wonderland dream.
The most widely accepted theory suggests that the Universe, at the instant of its birth, underwent a brief period of accelerated expansion termed inflation. Even though inflation still remains in the realm of theory, the most recent measurements and observations show that it is the most likely explanation known that could have caused the Cosmos to evolve in the way that it has over the course of billions and billions of years. In the smallest fraction of a second, it is thought that inflation blew up like an extraordinary bubble each and every region of our tiny Patch of space by a factor of at least 10 to the 27th power--that is, 10 followed by 26 zeroes. Before inflation enlarged this Patch, the region of the Universe that we can observe today--the visible Universe--was a smooth little bit that was smaller than a proton. At this very ancient era, our Cosmos was composed of a strange plasma of elementary particles. Speedy, high-energy photons slowly lost their energy as time went by and started to travel more slowly. In other words, they cooled off as the Universe continued to expand. The energy flowed into the expansion. In the almost 14 billion years since our Universe was born, it has expanded by yet another 10 to the 27th power.
It is generally thought that the dark matter dominated Cosmic Web formed in the very ancient Universe, and that it began as exquisitely small initial fluctuations in the primordial Universe. Such a strange skeletal Cosmos must have played a starring role in galactic birth and evolution, but historically this has proven to incredibly difficult to study and understand.
An Ancient Baby Galaxy Is Caught In The Web
The new research conducted by Caltech astronomers provides the most powerful observational support yet for what is termed the cold-flow model of galaxy formation. According to this model, in the ancient Universe, relatively cool gas flowed down from the Cosmic Web directly into the ancient galaxies, and this triggered rapid star birth.
The paper describing this finding, as well how CWI made the observation possible, appears in the August 13, 2015 print issue of the journal Nature, under the title A giant protogalactic disk linked to the cosmic web.
"This is the first smoking-gun evidence for how galaxies form. Even as simulations and theoretical work have increasingly stressed the importance of cold flows, observational evidence of their role in galaxy formation has been lacking," noted Dr. Christopher Martin in an August 5, 2015 Caltech Press Release. Dr. Martin is a professor of physics at Caltech.
The protogalactic disk that the team of astronomers identified is approximately 400,000 light-years across. This makes it about four times larger in diameter than our Milky Way. The distant disk is located in a system that is dominated by two brilliant quasars. Quasars are extremely bright, and especially energetic, active galactic nuclei (AGN) that are most often found residing in the ancient Universe. They are, in fact, the swirling, glaring accretion disks surrounding supermassive black holes inhabiting the dark hearts of galaxies. The closest quasar inhabiting this particular system is designated UM287, and it is fortuitously situated in just such a way that its emission is beamed like a flashlight. This helps to illuminate the otherwise invisible Cosmic Web filament that is funneling gas into the voracious, spiraling, newborn protogalactic disk.
In 2014, Dr. Sebastiano Cantalupo of ETH in Zurich, Switzerland (then at the University of California at Santa Cruz) and his team published a paper that appeared in the February 6, 2014 issue of the journal Nature, announcing that they had detected what they believed was a large filament close to UM287. The structure that they saw was considerably more brilliant than it should have been--if it had been only a filament. The astronomers began to suspect that there was something else hiding there in secret.
In September 2014, Dr. Martin and his colleagues, including Dr. Cantalupo, followed up on their intriguing observations of the system with the CWI. The CWI is an integral field spectrograph, and it enabled the scientists to gather images in the vicinity surrounding UM287 at hundreds of different wavelengths at the same time. The images showed details of the system's mass distribution, velocity, and composition.
Dr. Martin and his colleagues targeted a range of wavelengths around an emission line in the ultraviolet termed the Lyman-alpha line. That line serves as a tattle-tale fingerprint of atomic hydrogen gas, and is frequently used by astronomers to trace primordial matter in the ancient Universe.
The scientists gathered a series of spectral images that they combined to create a multiwavelength map of a region of the sky surrounding the brilliant quasar duo. This newly acquired data showed the areas where gas is emitting in the Lyman-alpha line, and it indicated the velocities at which the primordial gas was traveling with respect to the center of the system.
"The images plainly show that there is a rotating disk--you can see that one side is moving closer to us and the other is moving away. And you can also see that there's a filament that extends beyond the disk," Dr. Martin explained in the August 5, 2015 Caltech Press Release. The astronomers' measurements suggested that the disk is rotating at a speed of about 400 kilometers per second. This is somewhat faster than our own Milky Way's rate of rotation.
"The filament has more or less constant velocity. It is basically funneling gas into the disk at a fixed rate. Once the gas merges with the disk inside the dark matter halo, it is pulled around by the rotating gas and dark matter in the halo," Dr. Matt Matuszewski noted in the August 5, 2015 Caltech Press Release. Dr. Matuszewski is an instrument scientist in Dr. Martin's group and a coauthor of the paper. Galaxies are thought to be born within extended dark matter halos.
The observations and measurements provide the very first direct confirmation of the cold-flow model of galaxy formation. The subject of heated debate among astronomers since 2003, this model stands in stark contrast to the older, standard view of galaxy formation. The standard model indicates that when dark matter halos collapse, they hoist in a large amount of "ordinary" atomic matter in the form of gas along with them, heating up to searing-hot temperatures. The gas then slowly cools, and this provides a slow and steady flow of cold gas that can give birth to fiery, brilliant baby stars in growing young galaxies inhabiting the ancient Cosmos. Although it may seem counterintuitive, gas has to be very cold in order to give birth to a hot neonatal star.
The older model was generally accepted until 1996, when Dr. Charles Steidel, who is Caltech's Lee A. DuBridge Professor of Astronomy, discovered a remote population of galaxies giving birth to stars at a furious rate a mere 2 billion years after the Big Bang. The standard model is unable to explain the enormous fuel supply for these rapidly-forming galaxies in the ancient Universe.
However, the cold-flow model does provide a potential explanation for this nagging mystery. Scientific theorists suggested that relatively cool gas, delivered by filaments of the great Cosmic Web, funnels directly into the forming primordial protogalaxies. Once it is there, the cool gas can rapidly condense to give birth to a brilliant, fiery host of dazzling newborn stars. Supercomputer simulations reveal that as the gas tumbles in, it possesses enormous amounts of angular momentum, or spin, and creates extended rotating disks.
Dr. Martin explained in the August 5, 2015 Caltech Press Release: "That's a direct prediction of the cold-flow model, and this is exactly what we see--an extended disk with lots of angular momentum that we can measure."
Dr. Phil Hopkins called the study "very compelling" in the same Press Release. Dr. Hopkins is an assistant professor of theoretical astrophysics at Caltech, who was not involved in the research. He added that "As a proof that a protogalaxy connected to the Cosmic Web exists and that we can detect it, this is really exciting. Of course, now you want to know a million things about what the gas falling into galaxies is actually doing, so I'm sure there is going to be more follow up."
Dr. Martin told the press on August 5, 2015 that the team has already detected two additional disks that appear to be receiving gas directly from filaments of the Cosmic Web in the same way.
About the Author: Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, newspapers, and magazines. 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.
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