How Large, Lazy Galaxies Grow: A Terrible Feast!

Our large and magnificent starlit Galaxy, the Milky Way, is doomed to end its existence as a spiral Galaxy in approximately 5 billion years when it experiences a devastating collision with another large galaxy--the somewhat larger neighboring spiral, the Andromeda Galaxy (M31). In September 2014, a team of Australian astronomers announced that large, massive galaxies, bobbing around in the Universe, have almost stopped giving birth to beautiful, fiery baby stars, and are instead lazily snacking on nearby galaxies in order to grow. Our Milky Way has not merged with and devoured another large galaxy for a very long time--although the relics of such terrible feasts can still be observed. However, as the old saying goes, "what goes around comes around," and technically Andromeda will cannibalize our Milky Way, billions of years from now, because Andromeda is the more massive galaxy!

The team of astronomers carefully observed over 22,000 galaxies and discovered that while smaller galaxies were still very efficient at producing brilliant neonatal stars, from their rich and fertile supplies of nourishing gas, the most massive galaxies had grown much more lazy over time and were considerably less efficient at producing brand new stellar sparklers. In fact, large, lazy galaxies, such as our own and Andromeda, instead grow by cannibalizing other galaxies.

The new research was released on September 19, 2014 in the journal Monthly Notices of the Royal Astronomical Society, published by Oxford University Press. Dr. Aaron Robotham, who is based at the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR), noted in a September 19, 2014 ICRAR Press Release that smaller "dwarf" galaxies were being devoured by their larger galactic cousins.

"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars. Then every now and then they get completely cannibalized by some much larger galaxy," Dr. Robotham added.

Dr. Robotham, who led the new study, known as the Galaxy And Mass Assembly (GAMA) survey--that made use of the Anglo-Australian Telescope in New South Wales--went on to explain that our own Milky Way was currently at the tipping point and is expected to now gain weight primarily by cannibalizing smaller galaxies--instead of by collecting nourishing 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," he continued to explain.

What Goes Around Comes Around!

Both twirling, starry pin-wheels in space--our Milky Way and Andromeda--are the two largest galactic constituents of the Local Group of galaxies, which also harbors approximately 20 smaller galaxies. The Local Group is a few million light-years across. In turn, our Local Group is situated near the outer fringes of the Virgo Cluster of galaxies, whose big, glowing galactic heart is about 50 million light-years away from us. The numerous groups of galaxies and clusters of galaxies are themselves smaller constituents of immense web-like filaments and broad, thin expanses, such as the playfully termed Great Wall of galaxies. The Great Wall of galaxies is a sheet-like congregation of galaxies that is situated approximately 200 million light-years from Earth. A similar structure, called the Great Attractor, is tugging relentlessly, with its powerful gravitational grasp, on the entire Virgo Cluster of galaxies. The Milky Way, Andromeda, and the rest of the Local Group are carried along for the ride at a breathtaking speed of about several hundred kilometers per second.

In the distant future, when our Galaxy collides with the Andromeda Galaxy, the duo will experience a metamorphosis. Billions of years from now, when this smash-up occurs, the two spirals will merge to become an entirely new Galaxy--one that probably shows an elliptical shape, rather than the better organized, elegant pin-wheel shape currently sported by the two galactic travelers. The strange, new Galaxy that will emerge from the wreckage of our Milky Way and Andromeda has been dubbed the Milkomeda Galaxy by astronomers.

At present, the Milky Way and Andromeda are flying towards each other through intergalactic space at the truly stupendous speed of 250,000 miles per hour. Astronomers have suspected for years that the two large starlit pin-wheels are doomed to crash into each other catastrophically--and that this galactic wreck will be quite messy. Indeed, the Andromeda galaxy is heading straight in our direction, and when it finally crashes into our Galaxy, it will eat it.

A Terrible Feast

"Technically, Andromeda will eat us because it's the more massive one," Dr. Robotham commented in the September 19, 2014 ICRAR Press Release.

What is really going on? Is it a case of fatal gravitational attraction? Dr. Robotham suggests that when galaxies gain weight and grow, they also acquire a very powerful gravitational field that enables them to snare and then dine on their neighboring galaxies with ghastly ease. But why do large galaxies essentially stop producing their own sparkling baby stars? Is it because they have used up their entire necessary supply of nourishing gaseous fuel? Dr. Robotham thinks that star formation begins to get sluggish in very massive galaxies because of extreme feedback events in a brilliant region situated at the center of a galaxy called an active galactic nucleus (AGN).

"The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars," Dr. Robotham explained in the ICRAR Press Release.

Eventually, gravity is expected to cause all of the galaxies dancing around in bound groups and clusters to cannibalize each other and merge into only a few super-giant galaxies--although it will still be many billions of years before this can occur.

"If you waited a really, really long time that would eventually happen, but by really long I mean many times the age of the Universe so far," Dr. Robotham told the press on September 19, 2014.

The coming merger between our Milky Way and Andromeda will completely change the appearance of Earth's night sky. If human beings are lucky enough to still be around some 3.75 billion years from now, they will look up into the starry darkness above and see Andromeda literally filling the entire sky as it mercilessly travels toward our doomed Galaxy. For the next few billion years, or so, what is possibly left of life on our pitiful planet, will look up at the sky in shock at the merger--which will ignite violent blasts of fiery, dazzling star-birth.

In approximately 7 billion years, the glaring heart of the newborn elliptical Milkomeda galaxy, will take over the entire night sky. The chance that human beings will actually be able to observe this stupendous sight, however, is extremely low because (by this time) our Star, the Sun, will have likely become a bloated, enormous red giant star. Our Sun will enter its red giant phase in about 5 or 6 billion years, and it will probably devour the inner, rocky planets--Mercury, Venus, and our Earth.

Both the Milky Way and Andromeda are about the same age--and both are very ancient. Although the two galaxies were once thought to literally be twins, it is now thought that Andromeda is sufficiently larger to be the one that prevails at dinner.

The GAMA survey took seven years and more than 90 scientists to complete--and it didn't produce only one lone revelation. From this work there have been more than 60 publications--with yet another 180 in progress.