Monsters Gobble Galaxies to Grow

In the direction of the constellation Canis Major, some 55 million light years from Earth, two  galaxies appear to be at the beginning stages of a merger that has been captured by the Hubble Space Telescope and its Wide Field Planetary Camera 2. Credit: NASA/ESA and The Hubble Heritage Team (STScI)
In the constellation Canis Major, some 55 million light-years from Earth, two galaxies begin a merger. Courtesy of 
NASA/ESA and the Hubble Heritage Team (STScI)

When it comes to galaxies, it’s survival of the largest. Monster galaxies grow by cannibalizing their smaller neighbors, instead of birthing new stars on their own.

In a new study, led by Aaron Robotham of the University of Western Australia, that looked at 22,000 galaxies, astronomers found that the more massive the galaxy, the more it grows in size by gobbling up their smaller “dwarf” counterparts.

“All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars,” said Robotham in a press statement“Then every now and then they get completely cannibalized by some much larger galaxy.”

This galactic cannibalism appears to be self-perpetuating, since as the galaxies grow they gain more gravitational strength and therefore more easily pull in their neighbors, according the study released this month in the journal Monthly Notices of the Royal Astronomical Society.

And it turns out our own home galaxy, the Milky Way, may now be tipping the scale on the obese side—growing only by snacking on wayward islands of stars.

“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,” Robotham explained.

“We’re also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years.”

Andromeda Big Bully

What goes around comes around, however, and the Milky Way too may fall victim to galactic cannibalization. Our neighbor, the Andromeda galaxy, now some 2.6 million light-years away from Earth, will likely approach and devour the Milky Way in about five billion years.

“Technically, Andromeda will eat us because it’s the more massive one,” Robotham said.

This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth's night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger
This illustration shows a stage in the predicted merger between the Milky Way and Andromeda galaxies. In this image, representing Earth’s night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. Courtesy of NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

Eventually, a similar fate may await most monster galaxies. At one point in the distant future, the cluster members will slowly merge together into only a few supercolossal galaxies that will end up dominating the cosmos.

“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,” which is about 13.8 billion years old, said Robotham.

See for Yourself

Even if you can’t wait for the Milky Way merger, other opportunities abound to see galactic smashups. One of the best examples of interacting galaxies is the famous Whirlpool galaxy in the northern constellation Canes Venatici. It is a small stellar pattern just beside the famous constellation Ursa Major, the Great Bear.

This image of the Whirlpool galaxy was captured using the WIYN 3.5-meter telescope on Kitt Peak, Arizona. Credit: K. Rhode, M. Young, and WIYN/NOAO/AURA/NSF

Visible with small telescopes throughout the Northern Hemisphere, the Whirlpool galaxy, also known as Messier 51, is a spiral galaxy that has a visible bridge of gas, dust, and stars that connect it to a smaller, irregular galaxy known as NGC 5195.

Under dark skies you may even be able to glimpse the magnitude 8.4 galaxy with binoculars.

This starchart shows the location of the Whirlpool galaxy (M51) just underneath Alkaid - the last star in the handle of the Big Dipper as seen in the early evenings of September. Credit: SkySafari
This star chart shows the location of the Whirlpool galaxy (M51) just underneath Alkaid, the last star in the handle of the Big Dipper as seen in the early evenings of September. Credit: SkySafari

You can find M51 by looking 3.5 degrees southeast of the star Alkaid. Alkaid marks the end of the handle of the Big Dipper.

The Whirlpool is estimated to contain over 150 billion stars, making it at least as massive as our own Milky Way.

While you watch, ponder the amazing fact that its ghostly pinwheel structure sits a whopping 27 million light-years away from Earth.

Happy hunting!

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Changing Planet

Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.