Less than a billion years after the Universe’s Big Bang birth almost 14 billion years ago, the starlit galaxies of the Cosmos ignited, and began to cast their magnificent light into the swath of incredible, featureless darkness that characterized the primordial Universe before the first stars were born. Most galaxies do not live alone, but dwell together in groups or clusters, with clusters being considerably larger than groups. In fact, clusters and superclusters of galaxies are the largest structures known to inhabit the Cosmos, and they frequently host a multitude of separate galactic constituents that are bound together by the force of gravity. In April 2015, using University of Arizona observatories in Arizona, as well as observatories in space, astronomers announced their discovery of the likely precursors of the galaxy clusters we see today–and they reveal precious glimpses into the tantalizing mystery of how galaxies like our own starry, barred-spiral Milky Way came to be. For their new research, the astronomers combined observations made by the European Space Agency’s (ESA’s) Herschel and Planck space telescopes.
Galaxies like our Milky Way are not usually spotted dwelling in isolation. In the Cosmos today, most galaxies inhabit dense clusters of tens or even hundreds of galaxies. However, these immense galactic clusters have not always existed. How did such massive structures assemble in the ancient Cosmos? That is the question. Discovering when and how they came to be should provide valuable insight into the process of galaxy cluster evolution, including the important role played by the bizarre dark matter in constructing these large galactic cosmic cities.
A Vast Cosmic Web Lit By Starlight
A host of stellar sparklers have set fabulous fire to the more than 100 billion galaxies that dance around in our visible Universe. The visible Universe is that relatively small domain of the entire unimaginably vast Universe that we are able to observe. Most of the Universe exists far beyond what we can see, because the light that is sent forth from luminous objects dwelling in those very distant regions has not had enough time to travel to Earth since the Big Bang.
Our own Galaxy is an inhabitant of the Local Group that hosts more than 40 galaxies. In turn, our Local Group is situated close to the outer limits of the Virgo Cluster of galaxies, whose core is about 50 million light-years from us. The starlit galaxies of our Cosmos trace out immense, massive, and mysterious web-like filaments that are constructed of weird, transparent dark matter–that is of unknown composition. However, scientists strongly suspect that the strange dark matter is made up of some unknown, undiscovered exotic particles that do not interact with light–or any other form of electromagnetic radiation–and, as a result, are invisible. The starry galaxies that dance around together in groups and clusters light up this invisible great Cosmic Web, outlining with their revealing light, that which otherwise would not be seen.
Currently, the most favored theory of galactic formation among astronomers is humorously referred to as the “bottom-up” scenario. This widely accepted theory suggests that majestic, large galaxies were uncommon inhabitants of the ancient Cosmos, and that galaxies only eventually attained their more impressive sizes as a result of mergers between smaller, protogalactic structures. The earliest galaxies are thought to have been only about one-tenth the size of our Milky Way–but they were brilliantly raging with the furious fires of a myriad of very hot, luminous neonatal stars. These very bright, relatively petite ancient galactic structures served as the “seeds” that ultimately grew into the large, mature galaxies observed in today’s Cosmos–such as our own Milky Way.
It is generally thought that, in the primordial Cosmos, opaque clouds of mostly hydrogen gas converged together along the immense and heavy filaments weaving the invisible great Cosmic Web. Even though the identity of the dark matter is unknown, it is likely not composed of the so-called “ordinary” atomic matter that makes up stars, planets, moons, and people–and literally all of the elements listed in the familiar Periodic Table. In fact, the very badly misnamed “ordinary” atomic matter–or baryonic matter–accounts for a mere 5% of the mass-energy of the Cosmos. Dark matter makes up about 27% of the Cosmos, while dark energy accounts for most of it. Scientists know how much dark energy there is because of how it affects the Universe’s accelerating expansion. Other than that, the dark energy is a total mystery–but an important one, because it accounts for about 68% of the Universe.
In that dark and ancient era before the first generation of stars were born, casting their brilliant light into the barren and bizarre expanse of blackness, opaque clouds of primarily hydrogen gas collected along the transparent dark matter filaments weaving the mysterious, transparent Cosmic Web. The dense regions of dark matter pulled in the clouds of gas with their relentless gravity. Dark matter does not interact with “ordinary” matter or electromagnetic radiation except through the force of gravity. However, because it does interact with “ordinary” atomic matter, and it distorts, bends, and warps light (gravitational lensing), it reveals its ghostly presence to curious observers. Gravitational lensing is a generic prediction of Albert Einstein’s Theory of General Relativity (1915), when he realized that gravity had the ability to bend light and could, therefore, have lens-like effects.
It is generally thought that the very first galaxies were dark and opaque blobs of pristine gas, pooling at the hearts of dark matter halos. The clouds of gas floated down, down, down into the centers of these invisible, transparent halos, that danced along the strange filaments of the Cosmic Web. The first galaxies snared, with their gravitational claws that snatch, the first batches of fiery neonatal stars. The brilliantly shining new stars and hot glowing gas lit up what was previously a murky expanse–igniting the entire Cosmos with their fabulous fires.
Gradually, relentlessly, the swirling sea of ancient gases and the ghostly, transparent dark matter traveled throughout the ancient Universe, mixing themselves up together to form the familiar, distinct structures that we can observe today.
Mysterious Fireworks Galaxies
The team of astronomers, using the combined strengths of both the Herschel Space Observatory and the Planck Satellite, found that objects in the distant Cosmos–observed at a long ago and far away time when it was a mere three billion years old–could be the precursors of the large galaxy clusters seen inhabiting it today.
“Because we are looking so far back in time, and because the Universe is assumed to be homogeneous in all directions, we think it’s very similar to looking at the equivalent of what a baby cluster might look like,” commented Dr. Brenda L. Frye in a March 31, 2015 University of Arizona (UA) Press Release. Dr. Frye is an assistant astronomer at the University of Arizona’s Steward Observatory, who was involved in the research. “We now found a real sample of 200 baby clusters,” Dr. Frye added.
The primary mission of the Planck Satellite was to produce the most precise map of the Cosmic Microwave Background (CMB) radiation, which is the relic radiation left over from the Big Bang. In order to accomplish this, Planck surveyed the entire sky in nine different wavelengths ranging from the far-infrared to radio. This was done in order to eliminate foreground emission from our Milky Way, as well as other objects in the Cosmos.
However, those foreground sources can be extremely valuable in other fields of astronomy, and it was in Planck’s short wavelength data that the astronomers were able to detect 234 brilliant sources with characteristics that indicated they were situated in the very distant, ancient Universe. In astronomy, long ago is the same as far away. The more distant a celestial object is in Space, the more ancient it is in Time. This is because the light that emanates from more distant objects must travel a longer way to reach us than the light that flows from relatively nearby sources. No known signal in the Cosmos can travel faster than light, and so it sets something of a universal speed limit.
Another space observatory, Herschel, then observed these 234 bright objects across the far-infrared to submillimeter wavelength range (just a little bit shorter than microwaves), but with much greater sensitivity and angular resolution. Herschel successfully revealed that most of the Planck-detected sources were consistent with dense concentrations of galaxies in the ancient Cosmos–and these were in the process of giving birth to a multitude of fiery, raging baby stars.
Each of the young, early galaxies is observed to be in the act of converting gas and dust into baby stars at the breathtaking rate of a few hundred to 1,500 times the mass of our own Star, the Sun, each year. By comparison, our Galaxy today is giving birth to new stars at an average rate of only one solar mass each year.