It’s A Galaxy-Eat-Galaxy Cosmos
By Judith E Braffman-Miller
Big galaxies are our Universe’s bullies. Snacking ruthlessly on their smaller galactic kin, these massive galaxies have stopped producing stars of their own, and are instead feasting on tiny galaxies inhabiting their cosmic neighborhood. Like the unappetizing bullies that terrorize school playgrounds, these big galactic buffoons victimize their helpless smaller, starlit classmates. All galaxies start off small and grow to their enormous sizes by gathering clouds of gas and giving birth to new sparkling, beautiful baby stars–but every so often they get completely devoured by a much larger galaxy. In July 2017, astronomers announced their discovery of a small primitive, primordial galaxy, named the Little Cub, that could provide precious clues about the galaxy-eat-galaxy ancient Universe, as it begins to be cannibalized by a gigantic, neighboring galaxy.
The Little Cub galaxy got its name because it is located in the Ursa Major (Great Bear) constellation. The doomed small galaxy is being robbed of its gas by its larger galactic companion, and this gas is necessary for the Little Cub to continue producing baby stars. This recent observation means that astronomers now have the rare chance to observe a dwarf galaxy as its gas is being stolen by a nearby giant galaxy. This opportunity means that the scientists can now learn more about how this process occurs.
The Little Cub has managed to remain almost pristine since its ancient formation. Because of this, astronomers also hope its elements will tell them more about the chemical signature of the Universe only minutes after its Big Bang birth almost 14 billion years ago.
This new research, conducted by University of California at Santa Cruz (UCSC) and Durham University, UK, scientists, was presented on July 4, 2017 at the UK’s Royal Astronomical Society’s (RAS’s) National Astronomy Meeting (NAM).
The Little Cub and its ruthless larger galactic neighbor in space, a spiral galaxy named NGC 3359, are approximately 200 to 300 thousand light-years apart, and about 50 million light-years from our own planet. Gas belonging to the Little Cub is being stripped away as a result of its unfortunate interaction with NGC 3359, which hosts as many as 10,000 times more stars than its tiny galactic victim. NGC 3359 is similar to our own Milky Way Galaxy, and so by observing this terrible cosmic feast, astronomers hope to gain a better understanding about exactly how and when gas is stolen from smaller galaxies by larger neighboring galaxies.
“We may be witnessing the quenching of a near-pristine galaxy as it makes its first passage about a Milky Way-like galaxy. It is rare for such a tiny galaxy to still contain gas and be forming stars when it is in close proximity to a much larger galaxy so this is a great opportunity to see just how this process works. Essentially the larger galaxy is removing the fuel that the Little Cub needs to form stars, which will eventually shut down star formation and lead to the smaller galaxy’s demise,” explained study lead author Tiffany Hsyu in a July 3, 2017 UCSC Press Release. Ms. Hsyu is a graduate student in the Department of Astronomy and Astrophysics at UCSC.
What Goes Around Comes Around
The first galaxies are thought to have formed when the Universe was less than one billion years old. Currently, the favored theory of galactic formation, the “bottom-up” theory, indicates that large galaxies, such as NGC 3359 and our own Milky Way, were rare denizens of the ancient Universe, and that large galaxies only eventually reached their current mature, enormous sizes when they devoured their doomed, smaller galactic snacks.
The first star-blazing galaxies ignited at the end of the Cosmic Dark Ages, lighting up with their stellar flames what had previously been a strange swath of unimaginable, featureless blackness. These first light-emitting starlit structures brought the Dark Ages to a dramatic conclusion, and this event occurred approximately 380 thousand to 150 million years after the Big Bang. Scientists generally believe that the first galaxies were opaque and dark amorphous blobs composed primarily of hydrogen gas, gradually collecting within the dense and hidden hearts of dark matter halos, and that these shapeless structures hoisted in the first generation of fiery baby stars, pulling at them with a powerful gravitational embrace. The sparkling newborn baby stars and searing-hot glaring clouds of gas then lit up the Universe, changing it utterly from what it had been, to what it now is. Dark matter is an invisible, mysterious material that is very different from the so-called “ordinary” atomic matter that composes the world that we are familiar with. This is because the dark matter is not made up of the atomic elements listed in the Periodic Table–the elements that create stars, planets, moons, and people, and everything else that we have evolved to experience with our senses. However, the so-called “ordinary” atomic matter is not at all ordinary–indeed, it is quite extraordinary. Even though atomic (baryonic) matter accounts for only a puny 4% of the mass-energy of the Cosmos, it is what brought life into our cosmic home.
The first galaxies were only approximately one-tenth the size of galaxies such as our own and NGC 3359, but they were just as dazzling. This is because these small primeval galactic structures were rapidly giving birth to a myriad of brilliant, searing-hot baby stars. These small, but nonetheless very bright and extremely hot early galactic structures (protogalaxies) functioned as the “seeds” from which large galaxies grew–the majestic galaxies that we are familiar with today.
Our Milky Way Galaxy and the Andromeda Galaxy (M31) are the two largest denizens of the Local Group of galaxies, which also contains 64 or so smaller galactic constituents. Both our Milky Way and Andromeda are magnificent, large spirals–starlit pinwheels twirling in the space between galaxies. Currently, Andromeda is a safe 2 million light-years away from our Galaxy. Alas, this will not always be the case. The relentless pull of gravity is mercilessly tugging Andromeda closer and closer to us at the stupendous speed of about 100 kilometers per second.
In 2014, a team of astronomers released their new findings based on more than 22,000 galaxies that they had been observing. Their study showed that even though smaller galaxies were very efficient at forming new baby stars from their supply of gas, the most massive galaxies were considerably less efficient at the task of producing new stars. In fact, the more massive galaxies created hardly any new baby stars themselves, and instead grew by devouring other galaxies. That study was released in the September 9, 2014 issue of the journal Monthly Notices of the Royal Astronomical Society (UK).
Dr. Aaron Robotham, an astronomer based at the University of Western Australia base of the International Centre for Radio Astronomy Research (ICRAR) in Australia, noted in a September 9, 2014 ICRAR Press Release that tiny “dwarf” galaxies were being routinely eaten by their larger, bullying galactic counterparts.
Dr. Robotham, who led the research team, added that our own Milky Way is now at the point where it is expected to grow primarily by devouring smaller galaxies as snacks–instead of by the process of collecting gas.
“The Milky Way hasn’t merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we’ve cannibalized. We’re also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years,” he continued to explain.
Dr. Robotham went on to note that our Galaxy would eventually get what it has coming to it–after all, what goes around comes around. In about 5 billion years, our Galaxy is doomed to merge with nearby Andromeda. However, according to Dr. Robotham, “Technically, Andromeda will eat us because it’s the more massive one.”
Dr. Robotham added that as the galaxies became larger and larger, their gravity grew stronger and stronger. For this reason, they could more easily and powerfully pull in their smaller neighbors. He added that the reason why star birth decreased in very massive galaxies was possibly due to extreme feedback events in a very bright area at the center of a galaxy. This very bright region is termed an active galactic nucleus (AGN).
“The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cools the gas and prevents it from cooling down to form stars,” Dr. Robotham continued to explain.
In the distant future, gravity is expected to cause all of the galaxies that dwell in groups and clusters to merge into a few super-giant galaxies. However, this will not occur for many billions of years.
“If you waited a really, really, really long time that would eventually happen but by really long I mean many times the age of the Universe so far,” he continued to comment.
It’s A Galaxy-Eat-Galaxy Cosmos
Tiffany Hsyu and her team hope to gain a new understanding of the composition of the ancient Universe by studying the hydrogen and helium atoms that are being illuminated by the small number of extremely bright stars inhabiting the Little Cub. This small, doomed little galaxy also goes by the telephone book sounding name of J1044+6306.
Because J1044+6306 is so primitive, it may still preserve the pristine hydrogen and helium atoms that formed only minutes after the Big Bang. Only hydrogen, helium, and traces of lithium and beryllium were formed in the Big Bang (Big Bang nucleosynthesis). All of the rest of the atomic elements in the Universe were created in the nuclear-fusing, searing-hot furnaces of the stars–their stellar fires progressively fusing increasingly heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). However, the heaviest atomic elements of all, such as gold and uranium, formed in the explosive, brilliant, and furious death throes of very massive stars when they went supernova (supernova nucleosynthesis).
Dr. Ryan Cooke, a co-author of the study, explained in the July 3, 2017 UCSC Press Release that “We know by studying the chemistry of the Little Cub that it is one of the most primitive objects currently known in our cosmic neighborhood. Such galaxies, which have remained dormant for most of their lives, are believed to contain the chemical elements forged a few minutes after the Big Bang. By measuring the relative number of hydrogen and helium atoms in the Little Cub we might be able to learn more abut what made up the Universe in the moments after it began 13.7 billion years ago.” Dr. Cooke is a Royal Society (UK) Research Fellow in Durham University’s Centre for Extragalactic Astronomy.
The astronomers hope that further observations will detect still more pristine, primordial little galaxies where the chemical signatures of the ancient Universe might–like a hidden treasure–be found.
The Little Cub was first identified as a potentially pristine dwarf galaxy in data derived from the Sloan Digital Sky Survey (SDSS). Follow up observations were conducted by astronomers using the 3-meter Shane Telescope at Lick Observatory and the 10-meter Keck II telescope located at the W.M. Keck Observatory.
“The Little Cub’s discovery is a terrific example of using the smaller 3-meter-class Lick Observatory to scan through hundreds of candidates before focusing on the best sources with UC’s 10-meter Keck telescope,” commented co-author Dr. J. Xavier Prochaska in the July 3, 2017 UCSC Press Release. Dr. Prochaska is professor of astronomy and astrophysics at UCSC.
A research paper describing the discovery of the doomed Little Cub has been submitted for publication in the Astrophysical Journal.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. 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 the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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