Billions and billions of starlit galaxies perform their wild ancient dance within the observable Universe. The observable, or visible, Universe is the relatively small domain of our much more vast Cosmos that we are able to observe, because whatever may or may not exist beyond the horizon of the visible Universe, has not had sufficient time to wend its way to us since the Big Bang almost 14 billion years ago. This is because of the universal speed limit set by light. No known signal can travel faster than light in a vacuum. The Universe presents us with the most profound of mysteries, and it keeps it secrets well. In June 2019, astrophysicists using data obtained from the Chandra X-ray Space Telescope, managed to probe into one of these many mysteries. The Chandra scientists identified a strange galaxy that has been isolated for billions of years, and it contains more dark matter packed into its heavy heart than expected.
The bizarre galaxy, named Markarian 1216 (Mrk 1216) hosts stars that are within 10% of the age of the Universe. This indicates that Mrk 1216 is almost as old as the observable Universe itself. The astronomers have discovered that it has experienced a different evolution than garden-variety galaxies, both in respect to its stars and the invisible and transparent dark matter that, through the powerful force of gravity, holds the galaxy together.
Dark matter is a mysterious substance that accounts for about 85% of the matter in the Cosmos, and about 25% of its total energy density–although it has never been observed directly. Most of the dark matter is thought to be an exotic form of undiscovered non-atomic matter that does not interact with light or any other form of electromagnetic radiation, which is the reason why it is invisible. However, its ghostly presence is revealed in a variety of astrophysical observations, including its gravitational effects on objects that can be observed. Dark matter is believed to be the stuff that keeps galaxies from falling apart. Indeed, the primary indication of its existence is based on calculations that demonstrate that without it galaxies would fragment, instead of rotating. Without the existence of this mysterious transparent material, it is also likely that galaxies would not have formed in the first place–or even move the way that they do. Other lines of evidence include observations of gravitational lensing from the cosmic microwave background (CMB) radiation–which is the relic radiation left over from the Big Bang itself. Furthermore, the reality of this ghostly material is suggested from astronomical observations of the observable Universe’s current structure, from the birth and evolution of galaxies, from the location of mass during galactic smash-ups, and from the way that galaxies move within galaxy clusters.
According to the Standard Model of Cosmology, the total mass-energy of the Universe contains a mere 5% ordinary atomic matter and energy, 27% dark matter, and 68% dark energy. Dark energy is even more mysterious than the dark matter, and it is a substance that is causing the Universe to accelerate in its expansion. It has been proposed that the dark energy is a property of space itself. Dark matter accounts for 85% of total mass, while dark energy and dark matter account for 95% of the total mass-energy of the Cosmos. Hence, “most of the Universe is missing.” Ordinary atomic (baryonic) matter is clearly the runt of the universal litter of three, and it is really extraordinary material “Ordinary” atomic matter includes all of the elements listed in the familiar Periodic Table, and it composes the world that we are familiar with.
Since dark matter has not yet been observed directly, if it really does exist, it must barely dance with ordinary baryonic matter and radiation–except through the force of gravity.
Astronomers think that small galaxies (protogalaxies) were the first galactic structures to form in the ancient Cosmos. According to the bottom up theory of galactic formation, smaller protogalaxies emerged first, and then bumped into one another and merged–eventually growing into the immense and majestic galaxies that we observe today, such as our own barred spiral Milky Way. It has been proposed that the birthplace of a galaxy is within a transparent cradle composed of dark matter, termed a halo. In the primeval Universe, astronomers believe, the dark matter and baryonic matter danced together–clumping and, ultimately, weaving a complex web of intertwining, slender filaments. The great Cosmic Web of the Universe, as we see it today, is an enormous structure, composed of heavy filaments of dark matter, traced out by the visible starlight emitted by billions and billions of galaxies that are strung out along this large scale structure like glistening dewdrops on the web of an enormous spider.
The galaxies of today’s Cosmos are divided into specific types. These types include starlit spiral pinwheels in space, like our own Milky Way, that host billions of stars of all ages. There are also elliptical (spherical) galaxies that are considerably larger than spirals, and they host stars that are primarily old and red that zip around chaotically. Ellipticals frequently form after a collision and subsequent merger of two or more spirals. Lenticular galaxies account for yet a third type. Lenticulars are disc galaxies that are often described as “armless” spirals, and they are considered to be intermediate between spirals and ellipticals.
But the galaxies that formed in the primordial Universe were different from those observed today. According to the Chandra astrophysicists, Mrk 1216 is a member of a family of galaxies that are elliptical in shape, and are more densely packed with stars in their centers than most other galaxy types. Astronomers think that these galaxies are descendants of compact, reddish galaxies, nicknamed red nuggets, that formed about a billion years after the Big Bang, but then became victims of a galactic form of “arrested development” when they stalled in their growth approximately 10 billion years ago.
If this theory proves to be correct, then the dark matter content in Mrk 1216 and its galactic kin should also be tightly packed. In order to test this idea for the first time, two astronomers carefully studied the X-ray brightness and temperature of the searing-hot gas at varying distances from Mrk 1216’s center. The pair did this in order to “weigh” the amount of dark matter residing in the middle of the intriguing galaxy.
“When we compared the Chandra data to our computer models, we found a much stronger concentration of dark matter was required then we find in other galaxies of similar total mass. This tells us the history of Mrk 1216 is very different from a typical galaxy. Essentially all of its stars and dark matter was assembled long ago with little added in the past 10 billion years,” explained Dr. David Buote in a June 3, 2019 Chandra Press Release. Dr. Buote is of the University of California at Irvine (UCI).
Precious Red Nugget Galaxies
Red Nuggets are small galaxies that are packed with a large number of red stars that were originally detected by the Hubble Space Telescope (HST) back in 2005. They are very ancient relics of the first massive galaxies. The environment of red nuggets is generally consistent with that of elliptical galaxies. Most red nuggets have collided and merged with other galaxies. However, some have managed to escape that ordeal.
Red Nuggets acquired their nickname not only as a result of their size and color, but also as a result of how precious their discovery was for curious astronomers. This is because the discovery of these small galaxies challenged accepted theories of galactic formation.
Blue Nugget galaxies are thought to be the progenitors of Red Nuggets. Blue Nuggets are stream-nourished, star-forming, ancient systems that are quenched inside-out wiithin the inner parsec and dissipatively compacted into red nuggets at their peak formation rate. Galaxies that sport more mass generally quench sooner than galaxies that are not as richly endowed, and contain low amounts of mass because galaxies with small quantities of mass attempt to quench several times. Blue Nuggets apparently quench at a totally constant stellar surface density. The compaction occurs as a result of a ferocious episode of inflow involving (primarily small) mergers and counter-rotating streams or recycled gas. It is also frequently associated with extreme disc instability. The quenching occurs because of the extremely high rate of star formation and feedback from supernovae–and also, possibly, as a result of the high gas density in the center of the red nugget.
Data obtained from Chandra has revealed that the central black hole of Mrk 1216 has suppressed star birth. This is because of the black hole’s heat, and also because it devours surrounding gas. Thus the interesting question arises: how could Mrk 1216 possibly be packed so densely with stars? Results indicate that the answer to this question may be that Mrk 1216 and other red nuggets contain unused stellar “fuel” enabling them to give birth to their unusually large stellar population. However, there is another viable theory that proposes that red nuggets are really young elliptical galaxies, therefore evolving the same way that they do.
A team led by Dr. Ivana Damjanov discovered more than 600 red nuggets in the Sloan Digital Sky Survey (SDSS) database. These small galaxies were overlooked for such a long time because their size made them appear to be stars in the images. However, examination of their spectra revealed their true rare and precious nature.
Before Dr. Damjanov and her colleagues sifted through the SDSS database, no astronomer could detect the elusive galaxies after their original discovery back in 2005.
Mrk 1216’s Heavy Heart
According to the new study, conducted by the UCI astronomers, a halo–which can be described as a “fuzzy sphere”–composed of the mysterious dark matter, formed around the center of Mrk 1216 approximately 3 or 4 billion years after the Big Bang. This halo is thought to have extended over a larger area than the constituent stars within that galaxy. The formation of this type of red nugget galaxy was typical for many elliptical galaxies that dwell in our Cosmos today. However, in a way that differed from others of its kind, Mrk 1216 seems to have suffered from an atypical case of galactic arrested development. This is because most of the enormous elliptical galaxies continued to slowly grow larger and larger over cosmic time, when smaller galaxies collided and merged with them. However, little Mrk 1216 apparently traveled to the beat of a different drummer, and did not experience this more typical growth.
“The old ages and dense concentration of the stars in compact elliptical galaxies like Mrk 1216 seen relatively nearby provided the first key evidence that they are the descendants of red nuggets seen at great distances. We think the compact size of the dark matter halo seen here clinches the case,” noted study co-author Dr. Aaron Barth in the June 3, 2019 UCI Press Release. Dr. Barth is also of UCI.
Previously, astronomers calculated that the resident supermassive black hole that lurks hungrily in Mrk 1216 is more massive than expected for a galaxy of its relatively small mass. This more recent study, however, demonstrated that the black hole probably weighs-in at less than approximately 4 billion solar-masses. Even though this estimate indicates that Mrk’s 1216’s central black hole is quite hefty, it may really not be unusually massive for a galaxy as large as Mrk 1216.
The UCI scientists also went on the hunt for signs of outbursts emanating from Mrk 1216’s resident heavy dark heart. They observed tantalizing hints of cavities within the searing-hot gas akin to those seen in other massive galaxies and clusters of galaxies like Perseus. However, additional data are necessary in order to confirm their presence.
The Mrk 1216 data also provide important information about the nature of the mysterious and elusive dark matter. Because this invisible material has never been observed directly, some scientists are not convinced that it exists. In their study, Dr. Buole and Dr. Barth interpreted the Chandra data using both standard “Newtonian” models of gravity and a viable alternative theory called modified Newtonian dynamics (MOND). According to MOND, there is no need for dark matter in typical galaxies. However, the results of the new theory demonstrate that both theories of gravity need approximately the same immense quantity of dark matter to lurk in the heavy heart of Mrk 1216. Therefore, the two authors conclude that their study removes the need for the MOND interpretation.
Dr. Buole commented in the June 3, 2019 Chandra Press Release that “In the future we hope to go a step further and study the nature of dark matter. The dense accumulation of dark matter in the middle of Mrk 1216 may provide an interesting test for non-standard theories that predict less centrally concentrated dark matter, such as dark matter particles that interact with each other by an additional means other than gravity.”
A paper describing the new study was published in the June 1, 2019 issue of The Astrophysical Journal.