The Ancient Collision That Changed Our Galaxy
By Judith E Braffman-Miller
Large galaxies start off small and then grow larger and larger by gathering gas clouds that ultimately experience a sea-change into fiery young stars. Our own large star-splattered spiral Galaxy, the Milky Way, has grown to its majestic size by snacking on smaller nearby galaxies and stealing their stars and gas, as well as by merging with other large neighboring galaxies. In the ancient Universe, small protogalaxies bumped into one another and merged, thus creating the large galaxies that we observe today. In July 2018, an international team of astronomers reported their discovery of a very ancient and dramatic head-on collision between our Milky Way and a smaller galactic object, dubbed the Gaia Sausage galaxy. This smash-up, that occurred very long ago, was a defining event in our Galaxy’s primordial past and it served to reshape the very structure of our lovely Milky Way, that now twirls like a starlit pin-wheel in space.
This very violent collision between our Galaxy and the unfortunate Gaia Sausage served to reshape both our Milky Way’s bulge and its outer halo. Today, the Sausage is the lingering leftovers of what was once an ancient dwarf galaxy, consumed by our own 8 to 11 billion years ago. At least eight globular clusters were also added to our Milky Way along with about 50 billion solar masses of stars, gas and dark matter. The Gaia Sausage acquired its name because of its characteristic shape in velocity space. When observing the distributon of stellar velocities in the Milky Way, the stars inhabiting the Sausage create a characteristic sausage-like shape. This unusual shape is the result of strong radial motions of the stars. As our own Star, the Sun, is situated at the center of this vast cloud of stars, the distribution does not include the stars that have been slowed-down. These slow-moving stars are currently in the act of making a U turn back toward our Milky Way’s core. The stolen stars that have been incorporated into our Milky Way sport orbits that are highly radial. The outermost points of their orbits are approximately 20 kiloparsecs from the Galactic center at what is termed the halo break.
The stolen Sausage globulars are NGC 1851, NGC 1904 NGC 2298, NGC 2808, NGC 2808, NGC 6864, NGC 6779, and NGC 7089. NGC 2808 is possibly the old galactic core of the Gaia Sausage.
The stars that our own Galaxy stole from the unfortunate galactic dwarf display eccentricities of approximately 0.9. Their metallicity is also typically higher than other halo stars. In the terminology used by astronomers a metal is any atomic element heavier than helium. Only hydrogen, helium, and trace quantities of lithium were produced in the Big Bang birth of the Universe almost 14 billion years ago (Big Bang nucleosynthesis). The stars created all the rest of the elements that are listed in the familiar Periodic Table, by way of the process of nuclear fusion (stellar nucleosynthesis). However, the heaviest atomic elements–such as gold and uranium–were born in the raging fiery fury of supernovae explosions that heralded the demise of massive stars (Supernova nucleosynthesis).
The Gaia Sausage reconstructed our Milky Way long ago. The doomed dwarf galaxy did this by puffing up our Galaxy’s thin disk to change it into a thick disk. In addition, the gas that the Sausage carried into the Milky Way set off a fresh new episode of baby star birth and replenished the thin disk. The dwarf galaxy did not survive the violent head-on collision, and it very quickly fell apart, leaving it wreckage all around us. Indeed, this tattle tale wreckage of the erstwhile Gaia Sausage galaxy provides most of the metal-rich halo of our Milky Way.
Elegant spiral galaxies, like our Milky Way, are orderly structures composed of a bulge, halo, and disk. The bulge is at the center of a spiral galaxy and hosts primarily old stars, while the disk is where the spiral arms are located. Disks host stars of all ages. The halo contains both individual elderly stars and clusters of elderly stars (globular clusters). It also contains a large amount of mysterious non-atomic dark matter. Our Galaxy’s halo may extend 130,000 light-years across.
“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits” that are long and narrow like needles, explained Dr. Vasily Belokurov in a July 4, 2018 Simons Foundation Press Release. Dr. Belokurov is of the University of Cambridge in the UK and the Center for Computational Astrophysics at the Flatiron Institute in New York City. The paths that the stars took carry them “very close to the center of our Galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed,” he continued to note.
What Goes Around Comes Around
A glittering host of stars light up the more than 100 billion galaxies that trip the light fantastic within the observable Universe. The observable, or visible, Universe is that relatively small domain within the entire unimaginably enormous Universe that we can observe. This is because the light that flows out from those remote objects has not had enough time to reach us since the Big Bang birth of the Cosmos about 13.8 billion years ago. This is because of the expansion of Space. Whatever objects may (or may not) dwell beyond the cosmological horizon of our visibility remain both hidden and mysterious. Indeed, the very secret of our existence may be hiding beyond our cosmological horizon.
Most galaxies dwell in groups or clusters, with clusters being considerably more vast than groups. The galaxies themselves switched on very long ago, and began to shed their magnificent new light on the primordial Universe less than a billion years after the Big Bang. The most widely accepted theory of galaxy formation, the bottom up theory, proposes that large galaxies were uncommon denizens of the ancient Cosmos, and they only eventually reached their enormous sizes when protogalactic blobs collided and then merged when our Universe was young. The most ancient galaxies were only approximately one-tenth the size of our Milky Way, but they were just as dazzling. This is because they were furiously giving birth to myriad fiery baby stars. These extremely luminous and relatively small ancient protogalaxies served as the “seeds” from which the mature, large galaxies of the Universe ultimately grew.
In the primordial Universe, opaque clouds of pristine gas met up with one another and converged along the enormous filaments of the invisible and mysterious Cosmic Web. This large-scale structure of the Universe is believed to be woven of exotic particles of dark matter that are invisible and transparent. This is because dark matter will not “dance” with light or any other form of electromagnetic radiation. Although scientists do not know the identity of the particles that compose the dark matter, they do realize that it is probably not made up of the so-called “ordinary” matter that is the stuff of stars, planets, moons, and people–the material that creates the Cosmos that we are familiar with. Indeed, “ordinary” atomic matter (baryonic matter) accounts for only 4% of the mass-energy of the Universe.
In the ancient Universe, opaque clouds of gas–which were composed mostly of pristine hydrogen–collected along the heavy filaments of the invisible dark matter of the Cosmic Web. The densest areas of dark matter lured the clouds of hydrogen gas with their powerful gravitational attraction. Although dark matter does not interact with electromagnetic radiation, it does interact with gravity, thus giving its ghostly presence away. This is because the gravity of dark matter distorts and bends light (gravitational lensing). Gravitational lensing is a phenomenon proposed by Albert Einstein in his Theory of General Relativity (1915) when he came to the realization that the gravity of an object in space could warp passing light and therefore have lens-like effects.
The dark matter grabbed at the clouds of gas, and the gas eventually evolved into the cradles of the first generation of stars to light up the Cosmos. The gravity of the Cosmic Web tugged on its atomic prey until the captured clouds of hydrogen formed blobs like pearls within invisible halos of dark matter. The blobs of gas than floated down into the dark depths of these primordial halos that were strung out along the cosmological “spider web”.
As time passed, the whirling sea of pristine gases and the exotic, phantom-like non-atomic dark matter traveled throughout the ancient Universe, mixing themselves up together to create the distinct and familiar structures that we see today. Dense regions of the dark matter served as the “seeds” that grabbed at the primordial gases that ultimately formed galaxies. The gravitational pull of those “seeds” slowly pulled the gas into ever tighter and tighter knots. If the “seed” was large, a large protogalactic knot formed; if the “seed” was small, a small protogalactic knot formed. These ancient protogalactic knots began to “dance” together gravitationally and then cluster together. The protogalaxies, both large and small, swarmed together like bees around a picnic table on a hot July day. The protogalaxies embraced one another in their ancient “dance”, forming ever-larger and larger galactic structures that became the galaxies we see today. Like balls of clay in the hands of a pottery maker, the protogalaxies smacked into each other forming shapeless masses. The primordial Universe was small and crowded. The amorphous protogalaxies were relatively close to one another. For this reason, they frequently bumped into one another and merged.
Our Milky Way Galaxy has not merged with another large galaxy for a very long time. However, the sad leftovers of those ancient, terrible feasts can still be seen. The alarming truth about our Galaxy is that it still continues to devour its smaller kin. Large galaxies, like large fish, eat the smaller members of their kind.
Our Milky Way and the Andromeda Galaxy, which is another big spiral, are the largest galactic members of the Local Group of galaxies. But as enormous as our Galaxy is, there are other galaxies that are considerably larger than it is inhabiting the observable Universe. In addition to our Milky Way and Andromeda there are over 50 smaller galaxies that are members of the Local Group, and most of them are dwarf galaxies that are about the same size as the ill-fated Gaia Sausage. The entire Local Group is situated close to the outer limits of the Virgo Cluster of galaxies, whose big, bright galactic heart resides approximately 50 light-years from Earth.
It is often said that “what goes around comes around”. In the distant future, our Milky Way is doomed to collide and merge with Andromeda. Technically, Andromeda will devour our Milky Way, billions of years from now, because it is the more massive of the two spirals. When the two galaxies collide, the duo will undergo a dramatic metamorphosis. Billions of years from now, when this monumental smash-up occurs, the two spirals will merge to become an entirely new Galaxy. The new Galaxy, that is composed of both members of the merging duo, will probably sport an elliptical shape instead of the elegant and better organized spiral shape of both the Milky Way and Andromeda. The future giant Galaxy will rise from the wreckage of both erstwhile spirals. Astronomers have already given the future Galaxy a name–the great Milkomeda Galaxy.
Today, the Milky Way and Andromeda are flying rapidly towards one another through intergalactic space, at the truly breathtaking speed of 250,000 miles per hour. Astronomers have suspected for a very long time that the starlit spiral duo are doomed to perish in a head-on collision–and that this catastrophic event will be rather messy. Indeed, the Andromeda galaxy is traveling straight in our direction, and when it finally hits our Milky Way, it will eat it.
The future merger between the two large spirals will totally alter the appearance of our planet’s night sky. If human beings have somehow managed to still be around 3.75 billion years from now, they will stare up at the starlit darkness and observe Andromeda literally filling the entire sky, as it comes screaming towards us. For the next few billion years, or so, what may be left of life on Earth, will see a sky blasted by the merger–which will trigger a burst of dazzling, flaming star birth.
In about 7 billion years, the brilliant heart of the newborn elliptical Milkomeda Galaxy will take over the entire sky above Earth. However, the chance that human beings will still be around to witness this stupendous sight is rather slim. That is because (by this time) our Sun will have likely undergone a metamorphosis into a swollen, immense, red giant star. Our Sun will enter its red giant phase in approximately 5 or 6 billion years, and when it does so it will likely incinerate the quartet of inner, solid planets–Mercury, Venus, Earth, and Mars.
Our Milky Way and Andromeda are approximately the same age. Both spirals are very ancient and, although very similar, Andromeda is sufficiently larger to be the galaxy that prevails at the celestial feast.
The Milky Way’s Last Major Merger
Our Milky Way continues to collide with other galaxies, such as the tiny, and unfortunate, Sagittarius Dwarf Galaxy. However, the Gaia Sausage Galaxy was considerably more massive. The total mass of the Sausage in stars, gas, and dark matter was more than 10 billion times solar-mass. When the doomed Sausage blasted into the much more youthful Milky Way, its particular piercing trajectory created a complete mess. The Milky Way’s disk was likely puffed up or even fractured during the mayhem resulting from the crash, and would have needed to regrow. Furthermore, debris from the Sausage was splattered all over the inner regions of the Milky Way, thus forming the bulge now seen at our Galaxy’s center and the surrounding stellar halo.
The paths that the stars took as a result of the fatal merger earned them the name Gaia Sausage. “We plotted the velocities of the stars, and the sausage shape just jumped out at us. As the smaller galaxy broke up, its stars were thrown onto very radial orbits. These Sausage stars are what’s left of the last major merger of the Milky Way,” explained Dr. Wyn Evans in the July 4, 2018 Simons Foundation Press Release. Dr. Evans is of the University of Cambridge in the UK.
Numerical supercomputer simulations of the terrible galactic smash-up can reproduce the very features that are now observed by astronomers, explained Dr. Denis Erkal in the same Simons Foundation Press Release. Dr. Erkal is of the University of Surrey in the UK. In the simulations run by Dr. Erkal and his team, stars from the Gaia Sausage enter into stretched-out orbits. The orbits are then even further elongated by the growing disk of the Milky Way, which has been puffed up as a result of the collision.
Evidence of this Galactic remodeling is observed in the paths of the stars that our Milky Way inherited from the unfortunate dwarf galaxy, according to Dr. Alis Deason in the July 4, 2018 Press Release. Dr. Deason is of Durham University in the UK. These stellar U-turns then caused the Milky Way’s halo of stars to dramatically decrease where the stars change directions. This discovery was especially pleasing for Dr. Deason, who predicted the orbital pileup in 2013. This new research explains how the stars entered into such narrow orbits in the first place.
The new observations also identified at least eight large, spherical globular clusters, which are spherical clumps of stars that were carried into the Milky Way by the doomed Gaia Sausage. Small galaxies usually do not possess a population of globular clusters of their own. This means that the original Sausage galaxy must have been large enough to host its own retinue of globulars.
Dr. Sergey Koposov of Carnegie Mellon University in Pittsburgh, Pennsylvania, who has studied the kinematics of the Sausage stars and globular clusters in detail, commented to the press on July 4, 2018 that “While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, 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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