It’s a galaxy eat galaxy Universe, where small galaxies collide and merge to create the large magnificent galaxies that we see today. It is well-known that our barred-spiral Milky Way Galaxy attained its majestic size this way, devouring smaller galaxies floating around in its own general neighborhood, thus growing ever larger and larger. The relics of such terrible feasts can still be observed in the form of star streams that are the sad remnants of those dwarf galaxies that our Galaxy devoured long ago. Indeed, a duo of irregular dwarf galaxies, the Large and Small Magellanic Clouds, were in the midst of merging into a single larger galaxy when they tumbled into our own. In August 2018, a team of astronomers announced their new findings that this duo of galactic dwarfs contain enough gas to replenish 50% of our Milky Way’s supply of star-birthing fuel–thus providing the seeds for the brilliant birth of future baby stars.
The new study is published in the Monthly Notices of the Royal Astronomical Society (U.K.), and it sheds new light into the way that large galaxies like ours are able to gravitationally snatch this gas so easily. The scientists simulated the collision of a duo of distant dwarf galaxies in order to understand how their gas gets dispersed during the merger process. In their simulations, they watched the bigger galaxy, NGC 4490, steal gas from its smaller sibling by way of a gravitational effect resulting from their lopsided difference in size. As the duo circled ever closer and closer and closer to one another in this remarkable celestial ballet, the smaller galaxy’s tail of gas was swept ever farther and farther and farther away. This finding supports a study published earlier in 2018 that managed to fingerprint the gas streaming from the Magellanic Clouds into the Milky Way as belonging to the Small Magellanic Cloud.
A Tale Of Two Galactic Dwarfs
The Magellanic Clouds are a pair of nearby, small, and irregular satellite galaxies in orbit around our own–they are also the brightest of our Milky Way’s small galactic satellites. The shapeless duo puff clouds of gas both ahead of and behind them in a long ribbon that is appropriately dubbed the Magellanic Stream. The Magellanic Stream is a long streamer that reaches almost half way around our Milky Way, and performs a rippling dance beyond our Galaxy’s edge. Most of the ribbon was ripped from the Small Magellanic Cloud (SMC) approximately 2 billion years ago, but a small cloud of gas formed more recently from the gas belonging to the Large Magellanic Cloud (LMC).
The LMC and SMC got their names when the explorer Ferdinand Magellan (1480-1521) mistook them for clouds–and the pair of so-called “clouds” were named in his honor.
The LMC is only about 158,200 light-years from Earth, and the SMC is not much further than that at approximately 199,000 light-years. For comparison, our entire Galaxy is about 100,000 light-years across, and it is about three million light-years away from the Andromeda Galaxy (M31), which is another large spiral, as well as the nearest large galactic neighbor of our Milky Way.
More than twenty small satellite galaxies orbit our own, but only the Magellanic Clouds sparkle brightly enough with brilliant starlight to be observed from our planet with the unaided human eye. The Magellanic Clouds–in contrast to our Galaxy’s other orbiting satellites–are filled with gas. Gas is the precious stuff that galaxies use to create bright new fiery baby stars.
The people of several ancient cultures were aware of the existence of the Magellanic Clouds. Probably the most ancient continuous extant references to the duo of “clouds” were made by observers from the Khoisan culture of Southern Africa. The ancestors of these people apparently lived separately from all other living human cultures for thousands of years.
Another lengthy history of cultural association may have re-emerged with the migration of humans south from the Middle East reaching Australia about 50 to 60 thousand years ago. These ancient migrating people were the ancestors of the modern Aborigines, whose various cultures have produced a variety of fascinating myths and folk-tales about this pair of starlit nearby galaxies.
The ancient Polynesians also knew of the existence of the Magellanic Clouds, and they served as important navigation markers. Taken together they were also known to the Maori of New Zealand as Nga Patori-Kaihau or as Te Reporepo. The ancient Maori people believed that the two “clouds” were predictors of winds.
The Magellanic Clouds have been known since the first millennium in Western Asia. The first mention of the LMC is by the Muslim polymath Ibn Qutaybah, in his book on Al-Anwan (stations of the Moon in pre-Islamic Arabian Culture).
The people of ancient Sri Lanka referred to the Clouds as the Maha Mera Paruwathaya (the great mountains). This is because they thought that they looked like the peaks of a faraway mountain range.
In Europe, the Clouds were first reported by the 16th century Italian authors Peter Martyr d’Anghiera and Andrea Corsali, and both were derived from observations on Portuguese voyages. Subsequently, they were reported by Antonio Pigafetta, who was a member of the expedition of the explorer Ferdinand Magellan on its circumnavigation of the globe (1519-1522).
The LMC and its sibling, the SMC, are both conspicuous celestial objects in the southern hemisphere of our planet. The duo of “clouds” look like separated chunks of our Milky Way to the unaided human eye, and the true distance between them is approximately 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, the pair were the closest known galaxies to our own. However, in 2003, the Canis Major Dwarf Galaxy was discovered to be even closer to our Galaxy, and is currently considered to be our nearest galactic neighbor. The total mass of the duo of Clouds is uncertain.
For some time, many astronomers proposed that the Magellanic Clouds had orbited our Galaxy at approximately their current distances for eons. However, new evidence now indicates that it is rare for the duo to travel as close to the Milky Way as they are now. Both observation and theory suggest that the duo have both been significantly distorted by tidal interactions with our much larger Galaxy as they wander closer and closer to it. The LMC displays a very clear elegant and orderly spiral structure in radio-telescope images of neutral hydrogen. Ribbons composed of neutral hydrogen tie them both to our Milky Way and to each other. Both members of the duo look like disrupted barred spiral galaxies. Their gravity has also influenced our Milky Way as well, distorting the outer limits of the Galactic disk.
In addition to their differing structure and smaller mass, the pair of Clouds differ from our Milky Way in two important ways. First, they are more metal-poor than our Galaxy (in astronomy a “metal” is any atomic element heavier than helium). Second, they are heavily laden with gas; a greater percentage of their mass is hydrogen and helium compared to our own Milky Way. Both members of the duo display nebulae and youthful populations of stars. However, like our own Galaxy, their stars range in age from stellar babies to elderly stars. This suggests a long star formation history.
The Primordial Birth Of Galaxies
The Universe was born approximately 13.8 billion years ago in the exponential inflation of the Big Bang. Many scientific cosmologists propose that it started off as an exquistely tiny speck, that was smaller than a proton, only to attain macroscopic size in the tiniest fraction of a second. It has been expanding at a much more stately pace ever since–and it has been cooling off as well. The primordial Universe was much smaller and more crowded than it is today. Primeval protogalaxies were closer together when our Universe was young. For this reason, the ancient galaxies had a considerably greater chance of bumping into one another and merging to form ever larger and larger galaxies.
The first protogalaxies probably were born when the Universe was less than a billion years old. The most widely accepted model of galactic formation proposes that the majestic, large galaxies were uncommon in the ancient Universe, and only eventually reached their enormous sizes after they had snared smaller galaxies and then merged with them.
The star-blazing galaxies of the Cosmos switched on at the end of what is called the Cosmic Dark Ages, and brightened up what had previously been a dark and featureless swath of unimaginable blackness. The first light-emitting objects brought the Cosmic Dark Ages to an end when they sent their newborn light streaming out into Spacetime..
Most scientific cosmologists propose that the first galaxies to be born in the ancient Universe were opaque, dark, and shapeless clouds composed mostly of hydrogen gas. These primordial clouds had silently, slowly gathered within the secretive hidden hearts of dark matter halos. These newborn protogalactic clouds composed primarily of pristine hydrogen gas gravitationally snared the first generation of brilliant, gigantic baby stars. The brightly shining neonatal stars and extremely hot gas then lit up the ancient Cosmos.
The dark matter is a mysterious form of matter–it is not composed of the “ordinary” atomic matter that we are familiar with. Indeed, dark matter is transparent and invisible because it does not dance with light or any other form of electromagnetic radiation. Many scientists think that it is really there because it does exert gravitational effects on objects that can be seen.
For a long time after NGC 4490 collided with its smaller sibling, SN 4485, and merged with it to create a single galaxy, their gas continued to expand. The astronomers who performed the new study found that in another five billion years, the colliding galaxies’ tails of gas will extend over an impressive distance of about 1 million light-years–this amounts to almost double its current length.
“After five billion years, 10 percent of the gas envelope still resides more than 260,000 light-years from the merged remnant, suggesting it takes a very long time before all the gas falls back to the merged remnant,” Dr. Sarah Pearson noted in an August 9, 2018 Columbia University Press Release. Dr. Pearson is now a fellow at the Flatiron Institute’s Center for Computational Astrophysics (Simons Foundation) in New York City.
When the scientists compared their results to the real telescope observations of NGC 4490/4485, the results they obtained matched their simulations. This provided a strong indication that their model was accurate.
The new findings also are consistent with what astronomers know about how gas is recycled in the Cosmos. As clouds of gas grow increasingly extended, the gas becomes looser. This makes it easier for a larger galaxy to meet up with the cloud and eat it for dinner. The simulation indicates that this spreading out process has enabled the Milky Way to effectively strip gas from the SMC. Furthermore, this means that this sort of gas-transfer may be a frequent occurrence throughout the Universe.
“Our study suggests that similar dwarf pairs exist out there. Because their gas is so extended, if they fall into something like the Milky Way, their gas is easily shed,” Dr. Pearson explained in the August 9, 2018 Columbia University Press Release.
In addition, the new study suggests that the declining density of the gas on the outer limits of colliding and merging dwarf galaxies makes it difficult for new stars to be born–a conclusion matched by direct observations. The astronomers plan to continue studying other duos of dwarf galaxies in the process of colliding in order to refine their new model.
The other authors of the study are George Privon (University of Florida), Gurtina Besia (University of Arizona), David Martinez-Delgado (Astronomical Calculation Institute), Kathryn Johnston (Columbia University), R. Jay Gabany (Black Bird II Observatory), David Patton (Trent University), and Nitya Kallivayalil (University of Virginia).
The new study is published in the July 3, 2018 edition of the Monthly Notices of the Royal Astronomical Society under the title: Modeling the Baryon Cycle in Low Mass Galaxy Encounters: the Case of NGC 4490 & NGC 4485.