Star-blazing galaxies outline mysterious and enormous web-like filaments composed of weird invisible dark matter whose identity we do not know. However, we strongly suspect that the dark matter is composed of so-far undiscovered exotic particles that do not interact with light, and are therefore invisible. All of the majestic galaxies, glistening with billions and billions of incandescent stars, seem to gather like swarms of bees around the perimeters of immense empty (or almost-empty) Voids. These great cavities can be as much as 150 light-years across, and they are quite black because they contain very little luminous matter. One light year is the distance that light can travel in a vacuum, which is 5,850,000,000,000 miles!
The prevailing theory of galaxy formation suggests that small galactic embryos, or protogalaxies, formed first in the ancient Cosmic gloom that was swathed in starless darkness. These protogalaxies eventually collided and merged together in early times to create the majestic, enormous galaxies that we see today. This so-called bottom-up theory, indicates that large galaxies were a rarity in the very early Universe, and that they eventually grew to their magnificent size by way of the bonding together of smaller, protogalactic globs. The galaxies were born in cradles of the invisible dark matter, which are termed halos. In the early Universe, it is believed that the dark matter and “normal” atomic matter clumped and danced together to create an intricate web of thin and intertwining filaments. “Normal” matter, of which stars, planets, moons, and people are made, is the stuff of atoms, and it composes a mere 4% of the matter-energy content of the Universe. The dark matter is not atomic matter, and it is far more abundant in the Universe. The most abundant gas in these ancient filaments was hydrogen–the most plentiful “normal” atomic element in the Universe, as well as the lightest. “Normal” atomic matter composes all of the elements that we find in the Periodic Table. Hundreds of millions of years later, the hydrogen condensed to create galaxies and clusters and superclusters of galaxies along the twisting filaments of the Cosmic Web.
For many, many years, astronomers assumed that the greatest quantity of mass contained in a galaxy dwelled where the galaxy was most luminous–in its heart where there are the greatest number of incandescent stars. However, in the early 1980s, observations of remote spiral galaxies surprisingly indicated that this is not the case. If it were, stars further out from the galactic heart would be circling more slowly in their orbits around the core than stars closer to the heart of the galaxy. This expected motion of stars orbiting within a galaxy is analogous to the way planets in our Solar System circle the Sun. The outer planets–Jupiter, Saturn, Uranus, and Neptune–circle more slowly in their orbits around the Sun than the inner planets–Mercury, Venus, our Earth, and Mars. The closer a planet is to our Star, the swifter its orbit. This was what scientists expected to see when they applied Sir Isaac Newton’s law of gravity to the way they thought stars would behave in a typical spiral galaxy.
However, in the early 1980s, when astronomers observed the rotation of stars in distant spiral galaxies, they were in for a rude shock. What they found was that, in virtually every instance, the speed at which the stars circled around the galactic heart was the same all the way across the disk! The stars further from the galactic core orbited at precisely the same speed as the stars closer to the core. That would be like Neptune orbiting the Sun just as fast as Mercury. This discovery indicated that either Newton was wrong, or there must be a huge amount of matter of the invisible sort spread in a roughly spherical ring around every spiral galaxy in the Universe. The invisible matter would gravitationally pull the fiery stars around with it as it rotated. The invisible matter–the dark matter–does not interact with “normal”, luminous matter except through gravitation. Scientists finally came to the realization that most of a spiral galaxy’s mass could not possibly be located in the same place as the abundant, brilliant stars dwelling in the galactic heart. Galaxies, like our own Milky Way, studding the entire Cosmos, are all solidly lodged in immense halos of the mysterious, invisible dark stuff.
Dark galaxies are relatively diminutive, gas-laden globs that inhabited the early Universe. Although they are thought to be the protogalactic precursors of today’s brilliant, star-blazing galaxies, they themselves were dark because they were devoid of stars–and being dim little structures, they were very difficult to spot.
For a very long time, astronomers have been attempting to devise new techniques that could spot these dark little galaxies in the distant Universe. Tiny absorption drops in the spectra of background sources of light had previously indicated that they were really there! Such clues were tantalizing.
Now, for the first time, the elusive dim galaxies have been observed. Astronomers in Chile have reported seeing what appears to be the first evidence of the existence of these tiny galaxies inhabiting the early Universe. Astronomers at the European Southern Observatory (ESO) announced their important discovery on July 11, 2012. The ESO is an intergovernmental organization supported by 15 nations.
“For the first time, dark galaxies–an early phase of galaxy formation, predicted by theory but unobserved until now–may have been spotted,” the ESO announced in a statement.
“Using the ESO’s Very Large Telescope, an international team thinks they have detected these elusive objects by observing them glowing as they are illuminated by a quasar,” it continued to note. Quasars light up the hearts of ancient galaxies in the early Universe, and they are thought to be powered by voracious supermassive black holes greedily devouring gas and star-stuff that ventured too close to their gravitational embrace. The dazzling brightness of quasars makes them brilliant beacons that can illuminate the surrounding Space. This enables astronomers to peer at the ancient era when the very first stars and galaxies were born from pristine primordial gas.
The ESO team spotted almost 100 gaseous, dim objects dwelling within “only” a few million light-years of a quasar. Dr. Simon Lilly (ETH Zurich, Switzerland), a co-author of the paper, explained to the press in July 2012 that “Our approach to the problem of detecting a dark galaxy was simply to shine a light upon it. We searched for the fluorescent glow of the gas in dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar. The light from the quasar makes the dark galaxies light up in a process similar to how white clothes are illuminated by ultraviolet lamps in a nightclub.”
The team was also able to observe some of the attributes of the dark galaxies. They estimate that the dim objects weigh in at about 1 billion times the mass of our Sun. This is typical of low-mass, gas-bloated galaxies dwelling in the early Universe. The astronomers were also able to determine that the star-formation efficiency is dampened by a factor of more than 100 relative to a typical star-forming galaxy discovered at a similar stage in the history of the Universe.
“Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds. With this study, we’ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas,” Lilly’s colleague, Dr. Sebastiano Cantalupo of the University of California at Santa Cruz, explained to the press in July 2012.
The ESO operates three sites in Chile. The VLT array is a cluster of four telescopes that can observe objects that are four million times fainter than objects visible to the naked eye.
This research was published in a paper entitled “Detection of dark galaxies and circum-galactic filaments fluorescently illuminated by a quasar at z=2.4,” by Cantalupo et al in the Monthly Notices of the Royal Astronomical Society.