Less than a billion years after the Universe’s Big Bang beginning almost 14 billion years ago, the first primordial galaxies were born. The most popular theory of galactic birth and evolution among astronomers is playfully called the “bottom up” theory, which basically states that large galaxies were uncommon in the very ancient Universe, and that larger, majestic galaxies only later attained their impressive sizes as a result of collisions and mergers between smaller, protogalactic blobs–that were akin to the dwarf galaxies that inhabit our Universe today in great abundance. Many scientists think that the most ancient protogalaxies, both large and small, swarmed chaotically together like bees around a pool of sticky syrup. However, in July 2014, an international team of astronomers announced their observations of dwarf galaxies not behaving as expected. Instead, they seem to do their enchanting dance in orderly disk-shaped orbits around larger galaxies–and these bewitching, beguiling, and bewildering galactic nonconformists present a tantalizing mystery!
The paper describing the new observations of a number of small, dwarf galaxies that seem to dance to the beat of a different drum, was published in the July 31, 2014 issue of the journal Nature. “Early in 2013 we announced our startling discovery that half of the dwarf galaxies surrounding the Andromeda Galaxy are orbiting in an immense plane,” Dr. Geraint Lewis noted in a July 18, 2014 University of Sydney (Australia) Press Release. Dr. Lewis is a member of the international team of astronomers, and a professor in the University of Sydney’s Physics Department.
In the Nature paper titled Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe, the authors note: “Recent work has shown that the Milky Way and the Andromeda galaxies both possess the unexpected property that their dwarf satellite galaxies are aligned in thin and kinematically coherent planar structures. It is interesting to evaluate the incidence of such planar structures in the larger galactic population, because the Local Group may not be a representative environment.” The authors of this paper are Dr. Neil G. Ibata, Dr. Rodrigo A. Ibata, Dr. Benoit Famaey, and Dr. Geraint F. Lewis.
The Local Group of galaxies is situated near the outer limits of the heavily populated Virgo cluster of galaxies whose core is about 50 million light-years from Earth. Our own Galaxy, the Milky Way, and the Andromeda Galaxy, are the two largest galactic constituents of the Local Group–and they are both lovely spirals that look like starlit, twirling pin-wheels in intergalactic space. There are also about 40 or so smaller member galaxies dwelling in the Local Group. Most galaxies inhabit groups or clusters–with groups being considerably smaller than clusters. Clusters and superclusters of galaxies are the largest structures known to exist in the Universe, and they are commonly populated by hundreds to thousands of separate galaxies all bound together by their mutual gravitational attraction, thus creating the densest portion of the large-scale structure of the Universe.
What is going on with these dwarf galaxies, and why is their mysterious dance so mesmerizing?
Myriads Of Dancing, Primordial Protogalaxies
The most ancient galaxies, the small protogalaxies, were only approximately one-tenth the size of our Milky Way and Andromeda, but they were equally dazzling because they were furiously creating a myriad of very hot, very bright baby stars. These extremely brilliant, relatively petite ancient protogalactic structures served as the “seeds” that eventually flourished and grew to become the large, mature galaxies seen in the Universe today–such as our Milky Way and Andromeda.
Most astronomers think that in the ancient Universe, clouds of opaque gas merged together along massive, enormous filaments composed of mysterious and invisible dark matter that constructs the great Cosmic Web. Although scientists have not as yet determined what particles compose the dark matter, they do not think that it is made of “ordinary” atomic matter that is the stuff of stars, galaxies, planets, moons, people, and all of the familiar atomic elements of the Periodic Table. In fact, the “ordinary” atomic matter, or baryonic matter, accounts for a puny 4% of the mass-energy of the Universe.
Envision now that very ancient time, long before the first batch of stars were born to cast their fires into the bleak desolation of darkness, as pristine clouds of mostly hydrogen gas swirled along the heavy filaments of transparent dark matter. The dense regions of the dark matter hoisted in these ancient clouds of gas with the irresistible lure of their gravitational snares. Dark matter cannot interact with atomic matter or electromagnetic radiation except through gravitation. However, because it apparently does interact with “ordinary” atomic matter by way of gravitation, and it warps and bends light (gravitational lensing), most scientists think that it is there. Gravitational lensing is a phenomenon suggested by Albert Einstein in his Theory of General Relativity when he came to the realization that gravity could bend light and therefore have lens-like effects.
Now, with your mind’s eye, envision how this bizarre, transparent, ghostly form of matter embraces the pristine clouds of gas with its intense gravitational hug. These collecting, merging clouds of pristine gas eventually evolved into stellar nurseries for the very first generation of baby stars–that then set the ancient Cosmos on wonderful fire with their sparkling light. The gravity of the great Cosmic Web hoisted in its prey until these snared clouds of pristine gas collected like beads of onyx within the transparent halos composed of the strange, ghostly dark matter. The gas then tumbled and somersaulted down, down, down into the centers of these transparent halos, strung out like beads along this mysterious, magnificent, invisible Cosmic spider’s web.
Most scientists think that the first galaxies were opaque, shapeless clouds of pristine mostly hydrogen gas, collecting at the hearts of dark matter halos–and that eventually the brilliant, fiery first generation of baby stars and seething-hot glowing gas lit up what was previously a dismal, dark swath, setting the Universe reeling with the light of their brilliant fires!
Gradually, the swirling, undulating clouds of pristine, primordial gases and the ghostly, transparent dark matter flowed throughout the very ancient Universe, mixing together to form the familiar structures that we see today. Regions of higher density within the heavy filaments of dark matter that created the Cosmic Web filled the early Cosmos, and served as the seeds from which the galaxies were born, evolved, and grew. The gravitational embrace of those seeds ultimately pulled in the primordial gases, squeezing them into ever tighter and tighter blobs. Structures of varying sizes formed depending on the size of the dark matter blob–if the blob was large, a large protogalactic structure formed and, conversely, if the blob was small, a small protogalactic structure was born. These protogalactic structures then began to do a gravitational jitter-bug and cluster together.
It is commonly thought among theorists that these protogalaxies, both large and small, swarmed chaotically like bees buzzing around a delightful pool of sweet stuff. Then, like balls of clay in the playful hands of a small child, these flying protogalaxies smacked into each other to form ever larger and larger shapeless masses. The very ancient Universe was very small and crowded compared to what we are used to today. The amorphous protogalaxies were relatively close together and, as a result, frequently bumped into one another and merged to form larger and larger galactic structures.
However, the new study suggests that the choreography of the bumblebees’ dance was somewhat more orderly. The study published in 2014 follows from research conducted in 2013 that indicates that half of Andromeda’s circling satellite dwarf galaxies are in a single plane about a million light-years in diameter, but only 300,000 light-years thick. A survey of thousands of galaxies using the Sloan Digital Sky Survey (SDSS) suggests something interesting–first revealed when the scientists observed the massive Andromeda Galaxy relatively near Earth. The dwarf galaxies orbited the larger ones in disk-shaped orbits that did not display the random distribution of swarming bees.
“Everywhere we looked, we saw this strangely coherent coordinate motion of dwarf galaxies,” noted Dr. Lewis in the July 18, 2014 University of Sydney Press Release. He added that “From this we can extrapolate that these circular planes of dancing dwarfs are universal, seen in about 50 percent of galaxies.”
Flight Of The Protogalactic Bumblebees
The international team of astronomers, led by scientists at the Observatoire Astronomique de Strasbourg (CNRS/Universite de Strasbourg) in France, observed 380 galaxies and demonstrated that their dwarf satellite galaxies always seem to move in rotating disks. However, this is not predicted by current models of the formation of structure in the Cosmos, so it seems that further observational and theoretical studies are needed.
It has been known for years that there are large numbers of dwarf galaxies flying around large galaxies–such as our own and Andromeda–in a way that is analogous to the dance of the ancient horde of protogalaxies. More recently, the orbits of these modern dwarf galaxies around our own and Andromeda have raised some new and exciting questions of interpretation. The reason for this is that the observed orbits are arranged in orderly, flat, rather large rotating structures–and the most widely favored current models of galaxy formation, based on the Standard Model of Cosmology, predicts that they should fly around in all directions in a chaotic swarm. At first glance, it seemed that the Milky Way and Andromeda were statistical anomalies among the billions upon billions of starlit galaxies that compose the visible Universe–as an international study recently confirmed.
However, the study conducted in Strasbourg and Sydney, that is based on the SDSS that surveyed a third of the entire sky, made it possible to observe the properties of remote galaxies. The study found that of the almost 400 galaxies observed–dwelling approximately 30 to 700 million light-years from Earth and sporting at least a duo of visible satellite dwarf galaxies–showed that their satellite dwarf galaxies also orbited their hosts in orderly, rotating disks. The scientists went on to estimate that about 50% of the satellite dwarf galaxies inhabiting the Local Universe must also dance around in rotating disks in order to agree with their observations.
“We were supposed to find that a large proportion of pairs of satellite galaxies have oppositely directed velocities if they are situated on opposite sides of their giant galaxy hosts,” noted lead author Dr. Neil Ibata of the Lycee International in Strasbourg, France to the press in July 2014.
Dr. Lewis also explained to the press that “This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the Universe works including the nature of dark matter.”
The problem is that if this obsered phenomenon were tied in with the accretion of satellite galaxies flying along filaments of dark matter in the Universe, it would demand an explanation for why these rotating structures are considerably thinner than the filaments that gave rise to them–and also why the duo of the most brilliant satellite galaxies–which are the two that can be observed–systematically always come from the same filament.
These new findings, if strengthened by additional observational and theoretical work, could potentially result in revisions of current models. Most scientists today think that everything seems to suggest that the Standard Model still provides the best representation of observations at the largest scales of the Universe–but that, perhaps, something is being missed that occurs on smaller scales.
The researchers believe the answer to this riddle may be well hidden in some currently undiscovered physical process that governs how gas flows in the Universe–although, at this point, there is still no obvious mechanism known that can herd dwarf galaxies into narrow streams.
There are suggestions that something more radical may be going on, which eventually could demand revisions of the laws of gravity and motion.
“Throwing out seemingly established laws of physics is unpalatable, but if our observations of nature are pointing us in this direction, we have to keep an open mind. That’s what science is all about,” Dr. Lewis told the press in July 2014. He also noted that it would take many more studies to solve this bewitching, beguiling, and bewildering scientific mystery.