A galaxy 55 million light-years away in space spins in two directions at once, and astronomers suspect it ended up that way after swallowing a smaller neighbor.
The Hubble Space Telescope, NASA‘s optical light telescope in orbit around Earth, has now photographed the galaxy, called NGC 4698, in sharp detail. It sits in the constellation Virgo, one of more than 1,000 galaxies that gravity holds together in the Virgo Cluster, the closest big group of galaxies.
Although galaxies can grow on their own, they can also evolve by consuming gas and smaller galaxies it bumps up against. The two scenarios leave different clues, and NGC 4698 leans toward a galaxy that ate something. Because only a few known spiral galaxies spin against themselves like this, astronomers want to study NGC 4698 to figure out how it happened.
Picture a doughnut spinning around your finger. That is the body of this distant galaxy. Its bright arms wrap around the outside in a ring and leave the middle open. The ring turns flat — the way nearly every spiral galaxy revolves — including our own Milky Way.
Now picture another mini doughnut sitting in its hole, standing on its edge and spinning perpendicular to the big one. That is what’s going on in the middle of NGC 4698, except the little doughnut is really more of a dough ball, stretched into an oval and missing its own hole. The stars packed inside it ride up over the top of the galaxy and down under the bottom. Hubble’s image shows those bulges as a faint glow above and below the dusty plane.
The new image comes from a Hubble project aimed at the Virgo Cluster. It captures small features inside these nearby galaxies, down to single clusters of stars and glowing clouds of gas. The bigger goal is to learn what happens to a galaxy as it travels through a crowded cluster, and whether the trip helps it build new stars or shuts that down.
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Nothing in a galaxy is glued down, which is how one galaxy can manage two spins. Each star follows its own loop around the center. One crowd can loop flat while another loops crosswise, and neither forces the other to turn around.
Scientists believe some type of collision caused these two spins. Gas from somewhere else could have fallen into the galaxy’s gap, gathered at the center, and started spinning its own way. Some observations back that up, NASA says, suggesting the galaxy swallowed another smaller galaxy long ago. A short tail of hydrogen trailing off one side might have formed in the crash.
Credit: NASA / ESA
A recent study on the Milky Way’s center also traces a family of stars back to an extremely ancient galactic merger. The research, which appeared in Nature Astronomy last month, looked at the ages and chemicals in several of the galaxy’s globular clusters — tightly packed balls of thousands to millions of stars.
That information, combined with the direction of the stars’ orbits, helped a team of scientists parse three distinct groups: native Milky Way stars, stars from the Gaia-Sausage-Enceladus merger, and a third unknown population. Using advanced tools, scientists estimated the collision that brought them into the galaxy happened 11.8 billion years ago.
Everything else about NGC 4698 looks rather ordinary. Dark brown clumps of dust run through the outer arms, dotted with bunches of bright blue stars — the huge, hot kind that burn fast and mark spots where stars recently formed. The middle glows soft white because those stars are older, smaller, and cooler.
A black hole as heavy as millions of suns sits at the center, dragging gas into itself. Scientists see signs it’s actually still growing.
