The Strange Case Of A Gigantic Galaxy Scale Fountain

A galaxy is a gravitationally bound structure that hosts myriad stars, the sad stellar relics that dead stars leave behind as their legacy, interstellar gas, dust, and dark matter. Dark matter is thought to be composed of exotic non-atomic particles that do not dance with light or any other form of electromagnetic radiation, which is the reason why this strange substance is invisible. Dark matter is also much more abundant than the "ordinary" atomic matter that we are familiar with. The formation and evolution of the billions of galaxies that dwell in the observable Universe remains a tantalizing mystery, enchanting curious scientific detectives who seek to uncover its many beguiling secrets. In November 2018, a team of astronomers announced their discovery that, a billion light-years from Earth, lurks one of the Universe's most massive structures--a giant elliptical galaxy that is surrounded by a sparkling, sprawling cluster of swarming galaxies known as Abell 2597. Residing in sinister secret within the dark heart of the central behemoth galaxy, a supermassive black hole powers the cosmic equivalent of an enormous fountain, pulling in huge amounts of cold molecular gas, and then launching it screaming back out again time after time after time in a continuous cycle.
Supermassive black holes lie in voracious secret within the hearts of almost all currently known massive galaxies in the Universe. These enormous gravitational beasts are the largest type of black hole, on the order of millions to billions of times the mass of our Sun. Our own Milky Way harbors just such a dark heart that corresponds with the location of Sagittarius A* (pronounced Saj-A-star).
"The supermassive black hole at the center of this giant galaxy acts like a mechanical 'pump' in a water fountain. This is one of the first systems in which we find clear evidence for both cold molecular gas inflow toward the black hole and outflow or uplift from the jets that the black hole launches," explained study lead author Dr. Grant Tremblay in a November 6, 2018 National Radio Astronomy Observatory (NRAO) Press Release. Dr. Tremblay is an astrophysicist at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.
A Galactic Dance In Spacetime
The galaxies that trip the light fantastic in Spacetime are usually members of groups, clusters, and superclusters--with groups being considerably smaller than clusters, and superclusters being much larger and heavily populated than clusters. Our own barred-spiral Milky Way Galaxy is a member of the Local Group that hosts more than 54 galaxies, most of them relatively small galactic dwarfs. Our Milky Way and another immense spiral galaxy named Andromeda are the largest constituents of the Local Group, and the twirling pair resemble enormous starlit pin-wheels in Space.
According to recent studies, our Milky Way and Andromeda reside in what is called the green valley of the Galaxy Color-Magnitude Diagram. The green valley of this diagram refers to galaxies currently in the process of evolving from the blue cloud (galaxies actively giving birth to new baby stars) to the red sequence (galaxies that no longer give birth to new stars). Starbirth activity in green valley galaxies is in the midst of slowing down. This is because the galaxies are running out of star-birthing gas floating around in the interstellar medium. For simulated galaxies that sport similar attributes to our Milky Way and Andromeda, starbirth will normally be extinguished within approximately five billion years from the present--even with the predicted, brief increase in the rate of stellar formation resulting from the collision that will occur between the Milky Way and Andromeda about 5 billion years from now. Our Milky Way and Andromeda actually make up a binary system of giant spiral galaxies inhabiting the Local Group. Measurements of other galaxies similar to our own indicate that it is among the reddest and brightest spiral galaxies still able to give birth to sparkling, fiery new baby stars--and it is also only slightly bluer than the bluest red sequence galaxies.
The Local Group of galaxies is surrounded by the Local Void, and the entire galactic group is a constituent of the Virgo Supercluster. Encircling the Virgo Supercluster are a number of voids, which are barren regions that are almost, though not entirely, bereft of galaxies. The Microscopium Void is situated to the "north", the Sculptor Void to the "left", the Bootes Void to the "right" and the Canis-Major Void to the South. In addition, these black, cavernous voids are shape-changers. Over time, these sparsely populated regions create filamentous structures of galaxies. For example, the Virgo Supercluster is being gravitationally pulled towards the Great Attractor, which in turn is part of the immense Laniakea Supercluster.
A duo of smaller galaxies as well as a number of dwarf galaxies inhabiting the Local Group are in orbit around our Milky Way. The largest of these is the Large Magellanic Cloud (LMC) that sports a diameter of 14,000 light-years. The LMC has a smaller close companon, the Small Magellanic Cloud (SMC).
Ever since the first generation of stars were born in the Universe, the Milky Way has been growing in size as a result of both galaxy mergers (early in the Milky Way's formation) and the accretion of gas directly from the Galactic halo. The Milky Way today is is accreting material from several small galaxies, including both the LMC and SMC. Indeed, certain attributes displayed by our Milky Way, such as angular momentum, stellar mass, and metallicity in its outer limits, indicate that it has experienced no mergers with large galaxies in the last 10 billion years. The term metallicity, as used by astronomers, refers to all atomic elements heavier than helium. The Big Bang produced only hydrogen, helium, and traces of lithium--but the stars are responsible for all atomic elements heavier than helium. The lack of recent major mergers is considered to be rather odd among similar spiral galaxies. In fact, the neighboring large spiral galaxy Andromeda appears to have experienced a more typical history. Andromeda has apparently been shaped by more recent mergers with relatively large galaxies than our Milky Way.
Recent calculations of the number of galaxies inhabiting the observable Universe range from 200 billion to a trillion--or more. The galaxies of the Universe host more stars than all of the grains of sand on Earth! Most of the galaxies are about 3000 to 300,000 light-years in diameter, and are separated by distances of millions of parsecs (megaparsecs). By comparison, our Galaxy sports a diameter of at least 100,000 light-years and is separated from the Andromeda Galaxy by 2.5 million light-years.
The space between galaxies inhabiting the visible Universe is brimming with a tenuous gas (the intergalactic medium). The intergalactic medium has an average density of less than one atom per cubic meter.
The galaxies of the Universe formed very long ago, and began their ancient dance in the primordial Cosmos less than a billion years after the Big Bang. The most widely accepted theory of galactic formation among astronomers is called the bottom-up theory. This theory basically proposes that that the large galaxies of the Universe--like our Milky Way and Andromeda--only slowly attained their vast sizes as the result of ancient mergers between much smaller, amorphous protogalactic blobs. The galaxies themselves trace out an enormous and mysterious web-like structure called the Cosmic Web. The Cosmic Web is the largest-scale structure in the Universe, and it is woven of filaments composed of the weird, invisible dark matter. The starlit galaxies that do their fantastic celestial ballet in Spacetime congregate together in groups and clusters, and their starry constituents outline with their stellar fires that which we are not able to see.
In the primeval Cosmos, these opague shapeless clouds of mostly hydrogen gas collided and merged together along the massive filaments woven of the mysterious, invisible dark matter. The ancient "seeds" from which today's large galaxies eventually emerged were strung out along the transparent filaments like dewdrops collecting on the web of a hidden spider.
Galaxy-Scale Fountain Seen In Full Glory
According to the scientists who observed the galaxy-scale fountain in all of its full glory, the entire system operates like a self-regulating feedback loop. This means that the infalling material provides power for the "fountain" as it drains toward the central supermassive black hole. This is similar to water entering the pump of a familiar fountain. The gas swirling inward causes the black hole to furiously light up with fiery activity, and as a result it shoots high velocity jets of super-heated material shrieking out of the galaxy. As it flees through space, the material pushes out clumps and streamers of gas into the galaxy's expansive halo, where it eventually rains back on the black hole--and the entire process begins anew.
In a previous study conducted by the same team of astronomers, published in the journal Nature in 2016, the scientists were able to verify the relationship between the supermassive beast and the enormous fountain. They accomplished this feat by observing the region across a range of electromagnetic wavelengths, or portions of the spectrum. By determining the motion and location of molecules of carbon monoxide with the Atacama Large Millimeter/submillimeter Array (ALMA)--which shine brilliantly at millimeter wavelength light--the astronomers were able to measure the motion of the gas as it tumbled inward toward the waiting black hole. ALMA is an astronomical interferometer located in the Atacama Desert of northern Chile, which observes electromagnetic radiation at millimeter and submillimeter wavelengths.
Earlier data that had been obtained from the Multi-Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory's (ESO's) Very Large Telescope (VLT) revealed warm ionized gas being expelled from the galaxy--which was essentially the fountain's plume. The more recent ALMA observations revealed cold clumps of molecular gas in exactly the same locations as the warm gas observed in earlier studies.
"The unique aspect here is a very detailed coupled analysis of the source using data from ALMA and the MUSE instrument. The two facilities make for an incredibly powerful combination. ALMA revealed the distribution and motions of the cold molecular gas clouds, and MUSE did the same for the warm ionized gas," Dr. Tremblay explained in the November 6, 2018 NRAO Press Release.
The ALMA and MUSE data were then combined with a new, ultra-deep observation of the cluster using NASA's Chandra X-ray Observatory, revealing the hot phase of the enormous fountain in great detail.
The new observations also greatly strengthen the hypothesis that the warm ionized and cold molecular nebulas are one-and-the-same, with the warm ionized gas simply being the "shell" surrounding the cold molecular cores that churn within this immense, galaxy-scale fountain.
This multiwavelength approach provides an admirably complete picture of this system. As Dr. Tremblay commented in the November 6, 2018 NRAO Press Release, "It's like observing the rain cloud, rain, and puddle all at the same time."
While this research constitutes only one observation of a single lone galaxy, the astronomers speculate that they may be observing a process that is common in galaxies and fundamental to their mysterious evolution.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. 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 some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
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