Mysterious Dark Matter Is Missing In Ancient Galaxies
By Judith E Braffman-Miller
The Universe keeps its secrets well. Long before there was anything around with eyes to see, the galaxies formed, and the myriad of sparkling, brilliant stars were born–lighting up what had previously been a barren swath of featureless darkness. The most widely accepted theory of how the galaxies were born proposes that, in the primordial Universe, opaque clouds of pristine gas collected along immense, massive filaments composed of the transparent, mysterious, and ghostly dark matter–which is an unidentified material that is invisible because it does not interact with light or any other form of electromagnetic radiation. It is thought that the dark matter–the most abundant form of matter in the Cosmos–formed the bizarre cradles of newborn galaxies. However, in March 2017, astronomers announced that their new observations of rotating galaxies at the peak era of galactic birth, 10 billion years ago, surprisingly reveal that these massive, star-birthing ancient galaxies are completely dominated by the “ordinary” atomic (baryonic) matter that constructs our familiar world–with dark matter playing a considerably less important role, in comparable regions of their outer disks, than it does in modern galaxies inhabiting the local Universe.
The international team of astronomers, led by the Max Planck Institute for Extraterrestrial Physics in Germany, mapped the rotation curves of six galaxies in the ancient Universe to distances of approximately 65,000 light-years from their secretive hearts and found that their rotation velocities are not constant but drop with radius. These new findings have been supported by observations of over 200 more galaxies, where varying estimates of their dynamical conditions also show a high baryonic mass fraction. Furthermore, the new calculations suggest that these very early galaxies had a much thicker disk, with turbulent motion accounting for a percentage of the dynamical support.
For decades, numerous different studies of galaxies inhabiting the local Universe have unveiled the existence, as well as the importance, of the dark matter. While “ordinary”, or baryonic matter, can be observed as dazzling stars or glowing clouds of gas and dust, the dark matter exclusively dances with “ordinary” matter through the force of gravity. Most importantly, the dark matter is generally thought to be responsible for flat rotation curves in spiral galaxies–that are similar to our own Milky Way. This means that the rotation velocities of spiral galaxies are either constant or increasing with radius.
In The Dark
Scientists are much more certain about what the dark matter is not than what it is. By fitting a theoretical model of the composition of the Cosmos to the combined set of cosmological observations, astronomers have determined that the approximate composition of the Cosmos is 68% dark energy, 27% dark matter, and only 5% baryonic–or “ordinary” atomic matter. Even though atomic matter is obviously the runt of the Cosmic litter of three, it is really extraordinary because it is the material that brought life into the Universe. Atomic matter accounts for literally every atomic element listed in the familiar Periodic Table. The Big Bang birth of the Universe, almost 14 billion years ago, manufactured only the lightest of atomic elements–hydrogen, helium, and scant quantities of lithium and beryllium. All of the atomic elements heavier than helium were created in the searing-hot nuclear-fusing furnaces of the stars, or in the supernova explosions that herald the demise of the most massive stars in the Universe. Atomic elements heavier than helium are termed metals by astronomers–and, therefore, the term metal carries a different meaning for astronomers than it does for chemists.
As early as 1915, physicists began to suspect that an invisible form of matter–meaning matter that is not detectable using electromagnetic radiation–might lurk in the Universe secretly. The term dark matter was coined by the Dutch astronomer Jacobus Kapteyn (1851-1922) who, at the beginning of the 20th century, observed the movements of the stars within our Milky Way Galaxy. However, Kapteyn came to the conclusion that no such matter could really exist in the Universe.
In 1932, the Dutch astronomer Jan Oort (1900-1992), in an effort to explain the orbital velocities of stars in our Galaxy, was the first to actually propose dark matter’s true existence in nature. In 1933, Fritz Zwicky (1898-1974), a Swiss-American astrophysicist at the California Institute of Technology (Caltech) in Pasadena, California, also proposed the real existence of an abundant form of transparent matter. Zwicky did this in an effort to explain evidence of “missing mass” hiding phantom-like in the Cosmos, revealing its presence only by the way it influenced the orbital velocities of galaxies within distant galaxy clusters. Strong evidence derived from galactic rotation curves was obtained by the Caltech astrophysicist Horace W. Babcock (1912-2003) in 1939, but he did not attribute his highly suggestive observations to the existence of the dark matter.
The American astronomer, Vera Rubin (1928-2016), performed pioneering work on galaxy rotation rates. She was responsible for uncovering the discrepancy between the predicted angular motion of galaxies and their observed motion. This nagging discrepancy became known as the galaxy rotation problem, and it originally met with considerable skepticism. However, Rubin’s results were ultimately confirmed over subsequent decades.
Rubin, who was of the Carnegie Institute’s Department of Terrestrial Magnetism in Washington D.C., began work that was related to her controversial thesis regarding galaxy clusters with her colleague, the instrument-maker, Kent Ford (b. 1931). Together Rubin and Ford made hundreds of observations, resulting in what is known as the Rubin-Ford Effect. Rubin’s work powerfully indicated the real existence of this strange form of invisible matter–the dark matter.
Following closely on the heels of Rubin’s findings, a number of important observations were made by other scientists suggesting that the ghostly dark matter was hiding secretly in the Cosmos. These later discoveries were based on observations that included the gravitational lensing of background objects by foreground galaxy clusters such as the Bullet Cluster; the temperature and distribution of searing-hot gas within galaxies and galaxy clusters and–more recently–the pattern of anisotropies observed in the Cosmic Microwave Background (CMB) radiation. The CMB radiation is the relic radiation of the Big Bang birth of the Universe about 13.8 billion years ago. The anisotropies observed in the CMB originated from temperature variations in the newborn Cosmos. Gravitational lensing is a phenomenon proposed by Albert Einstein in his Theory of General Relativity (1915), when he suggested that gravity could warp, bend, and magnify Spacetime and, as a result, distort the path that traveling light takes through the Universe–thus having lens-like effects.
The dark matter hypothesis plays a prominent role in current modeling of the formation of cosmic structure, as well as galaxy birth and evolution. The presence of dark matter has also been used by scientists to explain the anisotropies observed in the CMB radiation. All of the lines of evidence, collected by scientists so far, indicate that galaxies, clusters of galaxies, and the Universe as a whole contain more matter than what can be observed using electromagnetic signals.
Currently, the favored theory proposing the identity of dark matter suggests that it is composed of weakly interacting massive particles (WIMPS) that interact only through the force of gravity and (to a smaller degree) the weak nuclear force that accounts for some forms of radioactive decay (beta decay).
The most widely accepted theory proposes that the starry galaxies of the Universe trace out immense, heavy, massive web-like structures that are made up of the dark matter. The luminous starlit galaxies that dance together in groups and clusters light up and trace out this invisible Cosmic Web–outlining with their tattle-tale light that which otherwise could not be seen.
The galaxies were born less than a billion years after the Big Bang. The favored theory, called the “bottom-up” theory, proposes that large galaxies were rare denizens of the early Universe, and that large galaxies only eventually reached their magnificent, mature sizes when smaller protogalactic blobs merged together to create bigger things. The earliest galaxies were likely only about one-tenth the size of our large spiral Milky Way Galaxy–but they were just as brilliant because they were fiercely producing a multitude of dazzling newborn baby stars. It is thought that these very bright, relatively small ancient galactic structures served as the “seeds” from which modern galaxies grew.
In the baby Universe, opaque clouds of gas collected together along the massive filaments of the great Cosmic Web. During that very early, murky epoch, long before the first stars ignited, these opaque clouds of mostly hydrogen gas bumped into one another along the dark matter filaments. The dark matter gravitationally snatched these clouds of pristine, primordial gas, which then evolved into the nurseries of the first generation of stars to light up the Cosmos.
It is generally thought that the first galaxies were blobs of gas, pooling at the mysterious hearts of halos composed of the invisible, ghostly dark matter–and that these halos hoisted in the first batches of newborn baby stars. The dazzling, sparkling baby stars and searing-hot glaring clouds of gas lit up what had previously been a dark, dismal expanse.
However, at this point, there are still some alternative possibilities. For example, “ordinary” baryonic matter could still account for what we call the dark matter if it is contained in brown dwarfs. Brown dwarfs are “failed stars” that were probably born the same way as their successful stellar kin–from the collapse of dense regions embedded within giant, cold, dark molecular clouds. Alas, little brown dwarfs never managed to accrete the necessary mass to trigger nuclear-fusion reactions that could light their starry fires–and, therefore, they never sent their brilliant light shining throughout the Universe
Mysterious Dark Matter Is Missing In Ancient Galaxies
An international team of astronomers, led by Dr. Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany, collected deep imaging spectroscopy of several hundred massive, star-birthing galaxies in the distant Universe. This newly obtained information enabled the astronomers to determine the rotation curves, which provide important constraints on the mass distribution out to the very edge of the visible galactic disks for both dark matter and “ordinary” atomic matter at the peak epoch of galaxy formation. For six of the galaxies with the highest quality of obtained data, the astronomers were able to calculate individual rotation curves, while they used a clever stacking approach for about 100 galaxies in order to constrain, on average, representative rotation curves.
“Surprisingly, the rotation velocities are not constant, but drop with radius. The reason for this is twofold: First, most of these early massive galaxies are strongly dominated by normal matter with dark matter playing a much smaller role than in the local Universe. Second, these early disks were much more turbulent than the spiral galaxies we see in our cosmic neighborhood, so they did not need as much circular motion to be dynamically supported,” Dr. Genzel explained in a March 16, 2017 Max Planck Institute for Extraterrestrial Physics (Mpe.Mpg) Press Release. Dr. Genzel is first author of a paper about the new findings of this study appearing in the March 16, 2017 issue of the journal Nature.
Both effects apparently increase with distance–this means that they played a larger role at earlier cosmic times. In astronomy, the more distant an object is in Space, the older it is in Time, as a result of the expansion of the Universe. This indicates that in the young Universe, approximately 3 to 4 billion years after the Big Bang, the gas contained in galaxies had already very efficiently condensed at the center of large halos of dark matter. It took billions of years for the dark matter in the halo to condense as well, and it was not until later times that it exerted a dominant effect on the rotation velocities of modern disk galaxies. This explanation also fits well with the fact that the most distant–and most ancient–galaxies were more richly endowed with gas, and were also more compact, than modern disk galaxies that are relatively close. Such a plentiful amount of gas helps to dissipate angular momentum and to force the gas to travel inwards.
“We have to be very careful when comparing these early massive and gas-rich galaxies to the ones in our local Universe. Present-day spirals, such as our Milky Way, require additional dark matter in various amounts. On the other hand, local passive galaxies–which are dominated by a spheroidal component and are the likely descendants of the galaxies in our study–show similarly low dark matter fractions on galactic scales,” explained Dr. Natascha Forster Schreiber in the March 16, 2017 Mpe.Mpg Press Release. Dr. Schreiber, a study co-author, is of the Mpe.Mpg.
In addition, two studies of 240 star-birthing disks support these findings. Detailed dynamical modeling reveals that, while “ordinary” atomic matter accounts for about 56% of the entire mass fraction of all galaxies, it completely dominates the inner matter distribution for galaxies at the greatest distances in the early Universe.
Dr. Stijn Wuyts of the University of Bath in the UK, and a co-author on all four papers, noted in the March 16, 2017 Mpe.Mpg Press Release that “You can do the arithmetic, the dynamics reveal the total mass present. Subtract from that what we see in the form of stars and gas, and there really is not much room left for dark matter within these early disk galaxies. The dropping rotation curves are not only in line with these results, they provide a more direct indication of the baryon dominated nature, especially to researchers that have a healthy skepticism about the accuracy with which one can measure the amount of stars and gas in these distant objects.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. 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.