David LinckThe billions and billions of dazzling galaxies, that do their mysterious dance in our Universe today, came into being very long ago and began to illuminate what was once a vast swath of incredible, featureless darkness less than a billion years after the Big Bang. The prevailing current explanation for the birth of galaxies so long ago–playfully termed the “bottom-up” theory–suggests that large galaxies were rare denizens of the ancient Cosmos, and that galaxies eventually attained their majestic, large sizes as a result of mergers between much smaller amorphous protogalactic blobs. For decades astronomers have thought that such galaxy mergers result in the birth of elliptical galaxies. But, in September 2014, a team of astronomers announced–for the first time–that they have discovered direct evidence that merging galaxies can instead form disk galaxies, and that this particular outcome is actually a very frequent occurrence. This new and very surprising discovery could well explain why there are so many spiral galaxies like our own Milky Way lighting up the Universe!
The new observations were made with the Atacama Large Millimeter/submillimeter Array (ALMA) and a host of other radio telescopes, which provide strong evidence that merging galaxies can indeed produce disk galaxies that are the cosmic kissing cousins of our own starlit barred-spiral Milky Way.
By studying a collection of 37 galaxies that were born as a result of earlier mergers, the international research team led by Dr. Junko Ueda, a postdoctoral fellow with the Japan Society for the Promotion of Science, discovered that a sizable majority–24 of the galaxies surveyed–appear to have gas disks!
Disk galaxies, which also include galaxies with lovely, star-blasted spiral arms– like our own Milky Way–and those with less well-defined attributes known as lenticular galaxies, are all characterized by a circular, flattened region composed of gas and dust. Indeed, lenticular galaxies are defined by their possession of pancake-shaped regions of dust and gas that distinguish them from their elliptical galactic kin.
According to galaxy classification, spiral galaxies, like our own Milky Way, consist of flat, rotating disks composed of stars, dust, and gas, and a central concentration of stars termed a bulge. These are surrounded by a much dimmer halo of stars, many of which dwell in globular clusters. Spirals are named for their spiral arms that extend from the center into the disk. Elliptical galaxies, on the other hand, sport an approximately ellipsoidal shape and a smooth and almost featureless brightness profile. Unlike flat spirals that possess both structure and organization, ellipticals are somewhat more three-dimensional, with little in the way of structure, and their stellar inhabitants are in, more or less, random orbits around their centers. Lenticular galaxies are intermediate between spirals and ellipticals–they share kinematic properties with both spirals and ellipticals. Indeed, lenticulars are sometimes referred to as “armless spiral galaxies,” because they sport a bulge, but no spiral arms.
Based on computer simulations dating from the 1970s, astronomers predicted that the merger of a duo of similar galaxies would produce an elliptical galaxy. These mergers would have ignited a blast of brilliant star-birth, and the ensuing gravitational chaos would have destroyed the original structures to give rise to an elliptically shaped galaxy that sported no clearly defined disk. However, more recent models do indicate that such mergers can also give birth to disk galaxies, even though astronomers had not as yet discovered the “smoking gun” evidence in merger remains.
A Host Of Starlit Galaxies
More than 100 billion galaxies dwell in our observable, or visible, Universe. The visible Universe is that relatively small region of the entire unimaginably vast Cosmos that we are able to observe. Most of the Universe exists far beyond what we can observe, and this is because the light flowing out from those unimaginably remote regions–far, far beyond the reach of our visibility–has not had sufficient time to travel to us since our Universe was born in the wild expansion of the Big Bang almost 14 billion years ago.
According to the so-called bottom-up theory of galactic formation, large galaxies grew to their immense and majestic sizes as a result of mergers between much smaller protogalaxies bobbing around in the primordial Cosmos. The most ancient galaxies furiously gave birth to fiery newborn stars and, even though they were only approximately one-tenth the size of our Milky Way, they were just as dazzling and brilliant because of these ferocious rounds of stellar fireworks.
Before the first generation of stars caught fire, and lit up the vast expanse of incredible, featureless darkness that was our primordial Universe, opaque clouds of mostly hydrogen gas collected together along heavy filaments of transparent dark matter. Although scientists do not know what particles make up the dark matter, they understand that it is not composed of so-called “ordinary” atomic matter, termed baryonic matter. The badly misnamed “ordinary” atomic matter is the stuff of stars, planets, moons, people, and all of the elements listed in the familiar Periodic Table of the Elements. “Ordinary” atomic matter accounts for a relatively puny 4.6% of the Universe, while dark matter accounts for 24% of it. Most of the Universe–71.4% of it–is composed of the bizarre dark energy. Dark energy is a weird substance, thought by many scientists to be a property of space itself, that is causing the Universe to accelerate in its expansion.
In the primordial Cosmos, dense regions of the dark matter grasped at wandering clouds of pristine gas with the powerful pull of their gravity. Dark matter does not interact with “ordinary” atomic matter or electromagnetic radiation except through the force of gravity. However, since it does interact with baryonic matter gravitationally, and it warps and bends the path light takes (gravitational lensing), it reveals its ghostly presence–despite its eerie invisibility. Gravitational lensing is a phenomenon suggested by Albert Einstein when he came to the realization that gravity could warp light and therefore exert lens-like effects.
Transparent, ghostly dark matter relentlessly snared clouds of pristine gas. These clouds of primordial gases became the very cradles of the first generation of fiery baby stars to light up the ancient Universe. The heavy filaments of dark matter, spinning a mysterious Cosmic Web throughout Space and Time, pulled and pulled on its baryonic prey until the doomed gas clouds created blobs that sunk like beads of onyx within the transparent halos of the dark matter. The clouds of pristine gas floated down, down, down into the centers of these ghostly, invisible halos of the dark stuff–strung out like beads on this unimaginably magnificent and profoundly mysterious cosmic spider’s web.
Slowly, the swirling sea of ancient gases and the eerie, ghostly dark matter, flowed throughout the ancient Universe, mixing together to eventually form the familiar and distinct structures that we see today. Areas of greater density within the filaments of dark matter, weaving the great Cosmic Web, flooded the baby Universe and served as the precious seeds from which the galaxies were born and evolved. The gravitational pull of those primordial seeds lazily lured the ancient gases into ever tighter and tighter clouds. Most astronomers think that these clouds of gas began to cluster together, and that these protogalaxies, both large and small, danced around together forming ever larger and larger galactic structures. The protogalaxies interacted with each other through the force of gravity, hugging one another, forming ever larger and larger structures that ultimately grew into the immense, majestic galaxies of the Universe we see today. Like tiny blobs of clay in the small hands of a toddler, the protogalaxies bumped into one another to form ever-larger shapeless masses. The ancient Cosmos was smaller than what we are used to today–and very crowded. Therefore, the protogalaxies frequently collided in this relatively small and crowded environment–and stuck together to create larger and larger structures.
Violent Origins Of Disk Galaxies
In order to determine which types of galaxies were formed by merging disk galaxies, the team of astronomers used ALMA and a number of other radio telescopes to observe the emission emanating from carbon monoxide (CO) gas. CO gas can be used as a tracer for molecular gas in galaxies. These new studies indicate that as the merger of two galaxies begins to come to an end, signs of rotation and a flattened disk-like structure begin to emerge.
The team of astronomers spotted this rotation by observing how the wavelengths of radio emission shifted to higher and lower frequencies as a result of the movement of the gas. The astronomers saw that the radio emission shifted toward the higher “blue” end of the electromagnetic spectrum in one region of the disk, meaning that it was moving toward us–while it moved toward the lower “red” end of the spectrum in the other region, meaning that it was moving away. This tattletale Doppler shift is indicative of rotation in a disk.
“For the first time, there is observational evidence for merging galaxies, resulting in disk galaxies, not elliptical galaxies. This is a large and unrecognized step towards understanding the mystery of the birth of disk galaxies,” Dr. Ueda explained in a September 16, 2014 National Radio Astronomy Observatory (NRAO) Press Release.
The galaxies in this study range anywhere between 40 million to 600 million light-years from our planet.
“We have to start focusing on the formation of stars in these gas disks. Furthermore, we need to look farther out in the more distant Universe. We know that the majority of galaxies in the more distant Universe also have disks. We, however, do not yet know whether galaxy mergers are also responsible for these, or whether they are formed by cold gas gradually falling into the galaxy. Maybe we have found a general mechanism that applies throughout the history of the Universe,” Dr. Ueda added in the NRAO Press Release.
The results of this study are published in the August 2014 issue of The Astrophysical Journal Supplement. It is titled Cold Molecular Gas in Merger Remnants I. Formation of Molecular Gas Disks,” written by Ueda et al.
