Cosmic Cannibalism Reveals How Galaxies Grow

The Umbrella galaxy takes its name from a mysterious feature (seen at left), which has now been found to be debris from a tiny galaxy that was shredded by gravity. Courtesy of R. Jay Gabany

The largest telescopes in the world have spied a Milky Way lookalike, some 62 million light-years away, that is gobbling up a smaller neighboring galaxy.

Astronomers hope that observations of the cannibalistic feast will help explain how galaxies grow over billions of years, according to an upcoming report.

The powerful W. M. Keck Observatory and Subaru Observatory on the summit of Mauna Kea, Hawaii, acquired the up-close view of NGC 4651, which is also known as the Umbrella galaxy, located some 62 million light-years away.

Tens of thousands of stars have formed a faint stream connected to the main, brighter galaxy, which is estimated to be about 50,000 light-years across. The faint stream is now thought to be the remnants of a smaller galaxy that has been stretched and pulled apart by intense tidal forces from the much larger galaxy.

Astronomers believe the hapless little galaxy will eventually be completely devoured and absorbed.


Cosmic mergers like this are nothing new, but such snack-size galaxies are usually too faint for even the largest scopes to see. Until now.

“This is important because our whole concept about what galaxies are and how they grow has not been fully verified,” said co-author Aaron Romanowsky, an astronomer at both the San José State University and University of California Observatories.

“We think they are constantly consuming smaller galaxies as part of a cosmic food chain, all pulled together by a mysterious form of invisible ‘dark matter.’ When a galaxy is torn apart, we sometimes get a glimpse of the hidden vista because the stripping process lights it up. That’s what occurred here.”

The Umbrella galaxy study will be published in the Monthly Notices of the Royal Astronomical Society this week.

See for Yourself

The Umbrella lies in the northern constellation Coma Berenices, and it is part of the Virgo cluster of galaxies.

Skychart showing the general location of the Umbrella galaxy (NGC 4651) in the constellation Coma Berenices and near the planet Mars. Credit: SkySafari
This sky chart shows the general location of the Umbrella galaxy (NGC 4651) in the constellation Coma Berenices, near the planet Mars. Courtesy of SkySafari

Shining at 10.8 magnitude, it is visible as a faint, elongated smudge in the eyepiece when viewed through a small or medium-size amateur telescope at high magnification.

You can start your hunt with Spica and Mars in the constellation Virgo. The bright orange, starlike Mars lies about 20 degrees southeast of the distant galaxy.

Look for the smudge in the sky beneath Coma Berenices. The flowing constellation’s name is a classical reference to “Berenice’s Hair,” the legend of Queen Berenice II of Egypt, who offered her long hair to the gods.

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Changing Planet

Meet the Author
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.