Our starlit spiral Milky Way Galaxy is very ancient. In fact, it is almost as old as the visible Universe itself. The most recent estimates indicate that our Galaxy is 13.51 billion years old – an elegant and majestic primeval pin-wheel whirling in the vastness of Spacetime. The galaxies that perform a twirling dance through the cosmos were born in very ancient times, and their stars began to light up the primordial Universe less than a billion years after its Big Bang birth almost 14 billion years ago. In August 2018, a team of international astronomers released their new discoveries demonstrating that some of the faintest satellites galaxies orbiting our own Milky Way are among the very first to be born in the Universe. With the birth of the first galaxies, the Universe lit up like a candle flame, bringing an end to what is called the Cosmic Dark Ages when it was an expanse of incredible and unimaginable blackness.
The new research was conducted by a team of scientists, including physicists Dr. Carlos Frenk and Dr. Alis Deason of the Institute for Computational Cosmology (ICC) at Durham University (UK) and Dr. Sownak Bose from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts (US). Their findings suggest that the satellite galaxies dubbed Seque-1, Bootes I, Tucana II and Ursa Major I are over 13 billion years old.
Professor Carlos Frenk, Director of Durham University's ICC noted in an August 17, 2018 Durham University Press Release that "Finding some of the very first galaxies that formed in our Universe orbiting in the Milky Way's own backyard is the astronomical equivalent of finding the remains of the first humans that inhabited the Earth. It is hugely exciting. Our finding supports the current model for the evolution of our Universe, the Lambda-cold-dark-matter model in which the elementary particles that make up the dark matter drive cosmic evolution. "
The dark matter is a mysterious form of matter that is unlike the familiar atomic matter listed in the Periodic Table of the Elements. Many physicists suggest that the dark matter is composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation – which is why the dark matter is invisible. Despite its intriguing transparency, scientists are almost certain that the dark matter really exists because its gravitational effects can be observed on those objects that astronomers are able to see – such as stars, starlit galaxies, and clouds of glowing gas. The dark matter is much more abundantly than the "ordinary" atomic matter that composes the components of the Cosmos that human beings on Earth find familiar.
The End Of The Cosmological Dark Ages
The first atoms were hydrogen atoms – hydrogen is the simplest, as well as the most abundant and lightest, atomic element in the Universe. Hydrogen first formed in the exponential expanding fireball of the Big Bang itself (Big Bang Nucleosynthesis) . Hydrogen atoms that existed when the Universe was only 380,000 years old gathered together to create clouds – and then began to slowly cool off and congeal into small blobs, or halos , that were composed of the exotic dark matter.
This cooling phase is generally referred to as Cosmic Dark Ages, and it lasted for about 100 million years. As time went by, the gas that had finally cooled off within the halos became unstable – and, as a result of this instability, the first generation of fiery baby stars were born. These newborn objects were the first galaxies to inhabit the primordial Universe.
The birth of the first galaxies lit the Universe's fires, and brought the Cosmic Dark Ages to a brilliant end. The most widely accepted model explaining galaxy formation, the bottom-up scenario , suggests that large galaxies like the Milky Way were rare in the ancient Cosmos, and that they ever acquired their hefty more mature masses as the result of collisions and mergers of relatively small protogalactic structures in the ancient Universe. Most of the primeval galaxies were only about one-tenth the size of our own large Galaxy, but they were just as luminous because they were quickly creating myriad fiery baby stars. These very bright, albeit small, ancient galactic structures served as the "embryos" that extremely became the mature galaxies that trip the light fantastic in the Universe today.
In the swath of strange darkness that was the primeval Universe, opaque clouds of pristine gas met up with one another and then gathered along the awful, extremely massive filaments that weave what cosmologists refer to as the Cosmic Web . This strange web-like structure has also been likened to a natural sponge or, alternatively, a honeycomb. These massive filaments – that form the large scale structure of the Universe – are composed of the exotic dark matter . These dark matter filaments are the scaffolding that holds on to galaxies, groups of galaxies, and clusters of galaxies, that are all strung out along this strange cosmic web like glittering dewdrops on the web of some mysterious, hidden spider. The dark matter is much more abundantly than the so-called "ordinary" atomic matter. Even though "ordinary" atomic matter accounts for a puny 4% of the mass-energy of the Cosmos, it is the stuff that made life possible. Most of the heavier elements, that compose "ordinary" atomic matter, were formed in the searing-hot hearts of the Universe's billions upon billions of stars that were created tremendously heavier and heavier than atmospheric elements of lighter ones. The Big Bang only created the lightest atomic elements – hydrogen, helium, and tracks of lithium – but the stars created the rest. Furthermore, when massive stars ended their nuclear-fusing "lives" in catastrophic supernovae blasts, they created the heaviest atomic elements of all – such as gold. The iron in your blood, the calcium in your bones, the water that you drink, the oxygen you breathe were all created courtesy of the stars. We are star-stuff.
The primeval galaxies were dark, opaque blobs of pristine hydrogen gas, tumbling gently down into the hearts of dark matter halos , and these protogalaxies snatched up the first generation of stars. The dazzling stars and hot glowing gas lit up what had been a murky expanse.
Our Galaxy's Circus Satellites
Our Milky Way has several smaller galaxies that are gravitationally bound to it as part of our Galaxy's subgroup. Our host Galaxy, and its accompanying entourage of little satellite galaxies, are part of the local galaxy cluster, appropriately dubbed the Local Group.
There are 59 smaller galaxies known to do their cosmic dance within 1.4 million light-years of the Milky Way. However, not all of them are in orbit around our Galaxy. Indeed, some of these smaller galaxies may actually be orbiting other satellite galaxies. The Large and Small Magellanic Clouds are the only satellite galaxies that can be seen without the aid of binoculars or telescopes, and both have been observed with the unaided human eye since prehistoric times. Measurements obtained by astronomers in 2006, using the Hubble Space Telescope (HST) , indicate that the duo of Magellanic Clouds may really be traveling too fast to be in orbit around the Milky Way. Of the little galaxies confirmed to be in orbit, the largest is the Sagittarius Dwarf Elliptical Galaxy , which sports a diamenter of about 8,500 light-years – or about one fifth that of our Milky Way.
Our Galaxy's satellites orbit from about 1,000 light-years of the outer edge of its disc to the outer edge of its dark matter halo at about 980 x 10 to the third power light-years from the Galactic center. The little circulating galactic satellites are generally depleted of hydrogen gas when compared to those that are in orbit farther away. This is due to the strict searing-hot gas of the halo of the Milky Way strips cold gas from the unfortunates satellites. Satellites that stay beyond that region still manage to keep a firm grip on copious depths of gas.
The tragic Sagittarius Dwarf Spheroidal Galaxy is currently being devoured by the Milky Way, and it is doomed to travel through it within the next 100 million years. The Sagittarius Stream is a ribbon of stars that is thought to have been an orbiting dwarf galaxy disk that has been completely disrupted by our own Galaxy's powerful and merciless gravitational embrace.
Tripping The Light Fantastic
The team of international scientists who released their new findings in August 2018 identified two distinct populations of satellite galaxies circling our own.
The first population consistors of very faint galaxies that were actually born during the primordial Cosmic Dark Ages . The second population is somewhat brighter and is made up of galaxies that were born hundreds of millions of years later. This latter population of slightly brighter satellites came into being when the hydrogen that had been ionized by the intense ultraviolet radiation (hurled out into space by the first stars) was able to cool into more massive halos of the invisible dark matter.
The researchers were further encouraged when they found that a model of galaxy formation, that they had previously described, agreed perfectly with the new data. This allowed them to go on to determine the formation times of the satellite galaxies.
"A nice aspect of this work is that it highlights the complementarity between the predictions of a theoretical model and real data. A decade ago, the faestest galaxies in the vicinity of the Milky Way would have gone under the radar. present and future galaxy censuses, a whole new trove of the tiniest galaxies has come into the light, allowing us to test theoretical models in new regimes, "explained Dr. Sownak Bose in the August 17, 2018 University of Durham Press Release. Dr. Bose was a doctor student at the ICC when this research began and is now a research fellow at the CfA.
The powerful ultraviolet radiation, hurled out into Spacetime by the first generation of galaxies to dance in the Cosmos, destroyed the lingering hydrogen atoms by ionizing them – meaning that it knocked out their electrons. This made it difficult for this gas to cool down adequately to give birth to fiery baby stars. Although it seems counterintuitive, things have to become very cold in order for a hot new star to form.
The ancient process of galactic formation came to a screeching halt and no new galaxies were able to form for the next billion years. However, at long last, the dark matter halos grew so massive that even ionized gas was able to cool off. Galaxy formation resumed, ever creating brilliantly starlit and magnificent galaxies like our own Milky Way.
Dr. Alis Deason, who is a Royal Society (UK) University Research Fellow at the ICC commented to the press on August 27, 2018 that "This is a wonderful example of how observations of the tiniest dwarf galaxies residing in our own Milky Way can be used to learn about the early Universe. "
The new finds are published in The Astrophysical Journal.