Galaxies On The Edge



Galaxies On The Edge
By Judith E Braffman-Miller

Glittering like a trillion fireflies on a summer’s night, the galaxies of the Universe host a swarming multitude of dazzling stars, and these stellar performers do their strange cosmic dance within an invisible structure called the Great Cosmic Web. This enormous web-like structure is woven of extremely massive, transparent filaments that are lit by flecks of brilliant starlight. The heavy filaments of the Great Cosmic Web surround almost-empty, very black, cavernous, and barren Voids, that contain relatively few galaxies. Indeed, on the largest scales, the entire Universe looks the same wherever we observe it, displaying a foamy, bubbly appearance. Voids commonly have sizes of hundreds of millions of light-years, and account for about 90% of known Space. In January 2018, a team of astronomers released their new observations of galaxies growing up on the edge of a Void. These lonely galaxies, on the edge, serve as effective natural laboratories for studying the impact of nature on galactic evolution without the influence of nurture. This is because galaxies that dwell within cosmic Voids “live” in isolation from others of their kind.

Unlike their lonely cousins hidden in the darkness of the Voids, most galaxies are not normally found in isolation–and they are not randomly scattered throughout the Cosmos. Most of the galaxies, that skip the light fantastic around the Universe, are accompanied by an encircling swarm of satellite galaxies, and they are themselves embedded in larger collections of galaxies termed groups or clusters–with clusters being considerably larger than groups. These enormous collections of galactic fireflies light up the larger-scale structures, that form galactic filaments and sheets, that are populated by millions and millions of galaxies. Between these enormous walls of galaxies are the sparsely populated Voids, that may contain only a few lost and lonely galactic denizens, whose feeble light can do little to dispel the cavernous darkness. These few galactic inhabitants of a murky Void, primarily exist in dim filaments–and they are the poor, pitiful relatives of their much brighter siblings found at the boundaries between Voids.

Big Bubbles In Space And Time

Cosmic Voids are roughly spherical domains within the Great Cosmic Web that host a lower-than-average density of matter. Even though these deep, dark, mysterious regions are less populated than dense galaxy clusters, Cosmic Voids are not as empty as they appear at first glance. Indeed, the delicate filaments, within the eerie Voids, on which glittering galactic diamonds are strung out across their centers, are actually the nurturing cradles of galaxy birth. This is because of their low density and, because of this, Voids are natural laboratories within which the properties of galaxies and their evolution are primarily determined independently of the interfering influence of nuisance galactic neighbors.

What is it like for a poor, pitiful, solitary galaxy in the proximity of a Cosmic Void? In order to answer this intriguing question, Dr. Elena Ricciardetti of the Ecole Polytechnique Fedderale de Lausanne (Switzerland) and her collaborators analyzed the properties of galaxies dwelling within and swarming around Cosmic Voids in the “nearby” Universe–meaning that they are “nearby” by cosmological standards.

Dr. Ricciardetti and her colleagues studied the influence Cosmic Voids have on galaxy morphology by analyzing a sample of galaxies taken from the Sloan Digital Sky Survey (SDSS). In total, the Void galaxies the team of astronomers studied numbered approximately 6,000–while a control sample of about 200,000 galaxies from environments of average density were considered. The astronomers used the Galaxy Zoo morphological classification in order to identify the spiral and elliptical galaxies of their sample. Spirals, like our own large Galaxy, the Milky Way, are enormous pin-wheels whirling their way in Space and Time. Spirals have a system of arms that rotate around a central bulge, and these galaxies are populated by stars of all ages. Ellipticals are football-shaped galaxies, whose stellar constituents are usually elderly and red, and zip about their galactic host in a much less well-organized way than their more orderly kin, that dance around in spirals.

Finally, Dr. Ricciardetti and her colleagues went on to calculate the percentage of spiral and elliptical galaxies present in their Void and control group sample galaxies. The team of astronomers also corrected for the unfortunate fact that dim spirals are more likely to be misclassified as ellipticals than their brighter cousins. The team found that galaxies that swarm near Voids are much more likely to be spirals than galaxies that dwell far from Voids. This suggests that nearby Cosmic Voids have a definite effect on galactic evolution.

The Rich Get Richer And The Poor Get Poorer

It is generally thought that Voids were formed via the hierarchical clustering of galaxies that existed around primordial density fluctuations in the ancient fireball of the Big Bang–meaning quantum mechanical fluctuations in the density of the Universe during the very first moments of its existence. This occurred on the heels of its inflationary Big Bang birth almost 14 billion years ago. Many astronomers propose that matter accumulated around regions of higher density in the early Universe, while matter was lost from the lower density regions. This process went on to lower the densities in these almost empty regions even further. These low density regions are what we observe today as the Voids.

If hierarchical clustering is indeed responsible for the formation of Voids, sheets, and filaments, it is interesting to consider what these enormous structures can reveal to astronomers about the Universe today. In addition, they can also reveal something about the processes on the very smallest scales (quantum) in the ancient Universe.

The dark and mysterious Voids of the Great Cosmic Web were first discovered back in 1978 in an important study by Dr. Stephen Gregory and Dr. Laird A. Thompson, who used the Kitt Peak National Observatory in Arizona to make their discovery.

Many scientists propose that the Cosmic Voids formed as a result of baryon acoustic oscillations (BAOs) in the primordial Cosmos. This suggests that the collapse of mass soon followed implosions of compressed atomic (baryonic) matter. In cosmology, the BAO are episodic and regular fluctuations in the density of visible atomic matter in the Universe. In a way that is similar to how supernova explosions provide astronomers with a standard candle that they can use to make astronomical observations, BAO matter clustering gives astronomers a standard ruler that they can use in order to measure the length scale in cosmology. The length of this standard ruler–which is about 400 million light-years in today’s Cosmos–can be measured by observing the large-scale structure of matter using astronomical surveys.

Beginning from what started as extremely tiny anisotropies, resulting from quantum fluctuations in the primordial fireball of the Big Bang, the anisotropies grew larger, and larger, and larger with the expansion of the Universe. In physics, a quantum is the minimum quantity of any physical entity that is part of an interaction.

The regions of higher density collapsed much more quickly under the extremely heavy pull of their own merciless gravity–ultimately creating the foam-like, large-scale structure of the Great Cosmic Web that is composed of massive, invisible dark matter filaments and almost-empty Voids that wrap around one another in an intricate tangle. The mysterious dark matter is the most abundant form of matter in the Universe, and it is thought to be composed of exotic, non-atomic particles that cannot interact with light or any other form of electromagetic radiation, which is why it is transparent and invisible. There is much more dark matter than the relatively small amount of atomic matter that composes the world we are familiar with. Current measurements indicate that the Universe is made up of approximately 27% dark matter and 68% dark energy. The true composition of the dark matter is still unknown, but the dark energy’s true identity is even more mysterious than that of the dark matter. The origin and attributes of the dark energy remain elusive, but it is often thought to be a property of Space itself. A mere 5% of the Cosmos is composed of the so-called “ordinary” atomic matter, and yet this “ordinary” form of matter accounts for literally all of the elements listed in the familiar Periodic Table. “Ordinary” atomic matter is extraordinary–it accounts for literally all of the Universe that human beings on Earth directly experience with their senses. This familiar runt of the Cosmic litter of three is the stuff of stars, and the stars are responsible for bring life into the Cosmos. We are such stuff as stars are made of.

Modern scientific cosmology started with Albert Einstein who applied his two theories of RelativitySpecial Relativity (1905) and General Relativity (1915)–to the Universe. At the beginning of the 20th-century, astronomers thought that our Milky Way Galaxy was the entire Universe, and that the Universe was both eternal and static. However, this is no longer the case. There are billions upon billions of galaxies bobbing around in the Cosmos, and our Universe is dynamic–not static. The Universe is thought to have been born about 13.8 billion years ago, and because it had a beginning, it may also have an ending.

The large-scale structure of the Universe, as revealed by the mysterious Cosmic Web, may have emerged with no true physical differences between domains of higher and lower density. This is possible because, if the current large-scale structure of the Universe is really the outcome of random fluctuations on the quantum level, occurring in the newborn Universe, this is exactly what the most straightforward models indicate. According to this point of view, some domains of the primordial Cosmos received a much greater density of matter than others simply as the result of chance. The distribution of wealth in the baby Universe was random–and unfair. Some regions simply were lucky, while others were not. There was nothing particularly special about those domains that received more of the cosmological “wealth” than the poorer, less fortunate domains–the “wealthy” domains simply were lucky. The rich get richer and the poor get poorer.

The otherwise invisible filaments of the Great Cosmic Web are traced out by the dazzling light of starry fires, that sparkle within vast sheets of twisted, tangled, and intertwining structure. The almost-empty, enormous, cavernous black Voids–which interrupt the strange, transparent filaments–are traced out by the brilliant flames of a myriad of stars. Because the Voids contain very few galaxies, this makes them appear to be almost, but not quite, empty–in dramatic contrast to the brilliantly lit, star-blazing heavy filaments of the Cosmic Web. The filaments braid themselves around these very black, and nearly empty, Voids.

Galaxies On The Edge

Dr. Ricciardetti and her team find that not only does a galaxy’s distance from the Void affect its properties, but also the size of the adjacent Void plays an important, measurable role as well. The team of astronomers find that, within the Void, there is a larger percentage of spirals than there are within the control sample. This effect persists even after removing the mass bias that results from the fact that low-density Void environments are preferentially populated by low-mass galaxies. For a given mass or absolute magnitude, Voids host a higher proportion of spirals than the control sample.

This effect is not limited to the volume within the Voids. Indeed, Dr. Ricciardetti and collaborators find that the properties of Void-adjacent galaxies are changed. This alteration occurs out to about twice the radius of the Void itself, with a higher percentage of spiral galaxies discovered to be closer to Voids. The size of a given Void plays a role as well; larger-than-average Voids tend to host a larger-percentage of spirals than the smaller-than-average Voids.

However, Dr. Ricciardetti and her colleagues caution that their first result depends on how the Voids are defined. This is because the effect vanishes if the Voids are defined using their dynamical properties instead of their size. Future research will help to disentangle the role that Cosmic Voids play in the evolution of galaxies.

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.

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