Astronomers Have Finally Found Our Galaxy’s Missing Sister
By Judith E Braffman-Miller
The first galaxies formed very long ago, when our almost 14 billion year old Universe was less than a billion years of age. Our own Milky Way Galaxy likewise is very ancient–a large starlit pinwheel twirling in space that is thought to be about 13.6 billion years old–give or take 8 million years. Indeed, the oldest known star in our Galaxy is 13.7 billion years old. Altogether, the Milky Way is thought to host approximately 300 billion stars. But, even though our Galaxy has many galactic neighbors, one of its enormous starlit siblings has gone missing, disappearing mysteriously billions of years ago. In July 2018, astronomers at the University of Michigan, Ann Arbor, announced that they have finally found our Milky Way’s long lost sibling. Alas, the team of scientists have deduced that our current closest large galactic neighbor shredded and cannibalized this massive sister of our Milky Way two billion years ago.
Even though it was mostly devoured and shredded, this massive sister galaxy left behind, as a lingering tattle-tale relic of its former existence, a trail of evidence revealing that it was once here. This rich trail of evidence is composed of an almost invisible halo of stars that is larger than our Milky Way’s largest spiral neighbor, the Andromeda galaxy itself. The evidence also consists of an elusive stream of stars, as well as a separate mysterious and enigmatic galaxy named M32. Discovering and observing this partly devoured doomed galaxy will help astronomers understand how disk galaxies like our Milky Way evolve and manage to survive large and violent mergers wth other enormous galaxies.
Our Galaxy And Its General Neighborhood
The group of galaxies that includes our Milky Way is appropriately named the Local Group, and it hosts more than 54 galaxies, most of which are relatively small dwarfs. Astronomers have predicted that sometime between 1 billion and 1 trillion years from now, all of the galactic constituents of the Local Group will crash into one another, and these collisions and resulting mergers will create a single enormous galaxy. The gravitational center of the Local Group today is situated between our Milky Way and Andromeda, and the entire group sports the impressive diameter of about 3.1 million parsecs. It also displays a binary (dumbell) distribution. The Local Group itself is a constituent of the larger Virgo Supercluster that may, in turn, be a part of the recently discovered Laniakea Supercluster.
The unfortunate, decimated galaxy, dubbed M32p, was once the third-largest member of the Local Group, after our Milky Way and Andromeda. Using supercomputer models, Dr. Richard D’Souza and Dr. Eric Bell of the University of Michigan’s Department of Astronomy were able to piece together the lingering tattle-tale evidence of this galactic crime, revealing all that is left of the tragically cannibalized sister of our own Galaxy.
Currently, the three largest member galaxies of the Local Group (in decreasing order) are the Andromeda Galaxy, the Milky Way, and the Triangulum Galaxy. The larger duo of these three spiral galaxies each have their own system of orbiting satellite galaxies. Both the Milky Way and Andromeda are majestic spirals that display starlit spiral arms that whirl majestically in space. Andromeda is, at present, a safe 2 million light-years away from our Milky Way. However, this will not always be the case. The relentless and merciless pull of powerful gravity is tugging Andromeda towards our Galaxy at the breathtaking speed of 250,000 miles per hour. In about 5 billion years, our Milky Way and Andromeda will crash into one another, merging to create one single enormous Galaxy.
Indeed, the future collision of our Galaxy with Andromeda will create an entirely new Galaxy, one that will likely display an elliptical shape, instead of the elegant starlit spiral “pinwheel” arms of its two badly disrupted galactic parents. This strange new Galaxy has been given the name Milkomeda, even though there will likely be no human life left on Earth to witness the enormous new Galaxy that will rise from the wreckage of this monumental merger.
Such galactic wrecks may not be quite as violent as once thought. These collisions have been observed in distant galaxies throughout the Cosmos, and even though galaxies have been seen smashing into one another, it is not likely that any two of their constituent stars will meet up and merge. The splattered wreckage that would be left behind in the wake of a two-star collision would create a big stellar mess. The good news is that the space between stars within a host galaxy is usually vast. For this reason violent stellar smash-ups rarely occur.
In contrast, the floating clouds of gas and dust that swirl around together within their host galaxies, will probably suffer as a result of a smash-up and merger. That sort of unfortunate and catastrophic event will be violent and make a horrific mess. This is because such a wreck will trigger star-birth within churning, writhing clouds of gas and dust. These cold dark clouds serve as the strange cradles of bright new baby stars, that are born in a dramatic, brilliant, blaze of newborn glory.
Galactic head-long collisions occur over long stretches of time–they can last as long as millions to billions of years, and they are not quickly over for the suffering parties. However, our Milky Way has been lucky because a violent collision with a similarly large galaxy has not occurred throughout its entire 13.6 billion year history–at least, not yet.
When Andromeda crashes into our Milky Way, our entire night sky will experience a sea-change. About 3.75 billion years from now, the sky above our planet will literally be filled with Andromeda, as it mercilessly makes its fatal approach towards our Galaxy. For the next few billion years, as a result of Andromeda’s approach, there will be brilliant blasts of fiery stellar birth lighting up Earth’s night sky.
In about 7 billion years, the sky above our planet will become even more strange and alien. The glaring core of the newborn Milkomeda Galaxy–now our own host Galaxy–will take over the entire sky. However, the prospect of human beings still being around to view this sight is remote. This is because our Sun will probably evolve into an enormous, swollen, dying red giant star approximately 5 billion years from now, and will have already incinerated its inner planets–Mercury, Venus, and Earth–long before the head-long smash-up between the two galaxies has occurred.
Both our Milky Way and Andromeda are approximately the same age. Although the two sister galaxies are considered to be almost identical twins, it is a little difficult to predict which one of the doomed duo will suffer the most when the end comes. However, since Andromeda is a bit larger than our own Galaxy, technically it will be Andromeda that will feast on our Milky Way.
A Sister Galaxy Gone Missing
Astronomers have known for a long time that nearly invisible large halos of stars surround galaxies, and that these halos contain the sad relics of smaller cannibalized galaxies. Indeed, a large galaxy like Andromeda is thought to have devoured literally hundreds of its smaller companions in this galaxy-eat-galaxy Universe. For this reason, many astronomers believed it would be a difficult task to learn about the history of any particular one of these unfortunate little galaxies.
However, the team of astronomers using new supercomputer simulations were able to come to a new understanding. The scientists found that even though a large number of companion galaxies were devoured by Andromeda, most of the stellar inhabitants of that galaxy’s outer dim halo were the unfortunate children of a shredded single large galaxy.
“It was a ‘eureka’ moment. We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies,” commented study lead author Dr. D’Souza in a July 23, 2018 University of Michigan Press Release. Dr. D’Souza is a postdoctoral researcher at the University of Michigan.
“Astronomers have been studying the Local Group–the Milky Way, Andromeda and their companions–for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said co-author Dr. Bell in the same Press Release. Dr. Bell is a University of Michigan professor of astronomy.
This unfortunate sister galaxy of our Milky Way, M32p, which was shredded mercilessly by the voracious Andromeda galaxy, was at least 20 times larger than any galaxy which merged with the Milky Way over the course of its more than 13 billion year existence. M32p would have been quite massive, and likely would have been the third largest galaxy in the Local Group, after Andromeda and our Milky Way, had it not been shredded and consumed by Andromeda.
This new study might also solve an intriguing mystery: the formation of Andromeda’s puzzling M32 satellite galaxy. The astronomers now suggest that the compact and dense M32 is really the surviving central heart of our Milky Way’s long-lost sister. The team of astronomers compare M32 to the pit of a plum.
“M32 is a weirdo. While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the Universe. There isn’t another galaxy like it,” Dr. Bell noted in the July 23, 2018 University of Michigan Press Release.
The new research may change the currently most widely accepted scientific understanding of the way galaxies evolve. The astronomers realized that Andromeda’s disk had managed to survive a smash-up with a massive galaxy. This impact would challenge the traditional viewpoint that such large interactions would invariably destroy the orderly disks of spirals, thus creating only elliptical galaxies.
The timing of the impact may also shed new light on the thickening of Andromeda’s disk, as well as on a mysterious blast of brilliant star-birth that occurred about two billion years ago. This finding was independently reached by a team of French astronomers early in 2018.
“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago. When I was at graduate school, I was told that understanding how the Andromeda Galaxy and its satellite galaxy M32 formed would go a long way towards unraveling the mysteries of galaxy formation,” Dr. Bell explained in the July 23, 2018 University of Michigan Press Release.
The good news is that this study can also be used for other galaxies. This would enable astronomers to measure their most massive past galaxy mergers. Armed with this new knowledge, scientists can go on to untangle the intricate and complicated tapestry of cause and effect that triggers galaxy growth, as well as learn about what mergers do to the galaxies that must suffer through them.
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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