Diving into an out-of-sync galaxy

Galaxies are like snowflakes: No two are exactly alike, even the spirals. The Hubble Space Telescope took a close look at the distant spiral NGC 4693 and found intriguing hints about its past. For one thing, this member of the Virgo Cluster seems to have a central region that’s rotating out of sync with its spiral arms and bulge. Those arms don’t seem to connect to the central region, and the bulge itself seems to extend above the plane of the galaxy. In other words, there seems to be a decoupling between the arms and the bulge.

Why is that? Did NGC 4693 evolve with these characteristics? Or did something happen to throw its interior stars and gas clouds out of sync? Let’s look at the bigger picture before diving into its past.

Exploring the Virgo Cluster

This galaxy lies about 55 million light-years from us and is a member of a larger grouping of galaxies called the Virgo Cluster. The collection may contain up to 2,000 galaxies of different types and shapes and is part of an even larger grouping called the Virgo Supercluster (itself consisting of more than 100 galaxy groups and clusters). The Virgo Cluster gives us a look at several types of galaxies in a relatively nearby region of space, around 54 million light-years from us.

The larger Virgo Supercluster includes several groups of galaxies, including our Milky Way’s Local Group. There’s evidence that the Virgo Cluster is still assembling. That affects the evolution of the galaxies it contains, which appear to be grouped by type—spirals arrayed in one region, ellipticals centered around the giant elliptical M87, and so on.

The groupings appear to be in an ongoing merger, and most members seem to be losing gas through a process called “ram-pressure stripping.” That loss of star-forming material affects star formation rates.

The best estimates suggest that the Virgo Cluster itself has been accumulating galaxies for at least 7 billion years, with the formation of individual galaxies stretching back around 11 billion or 12 billion years. Interactions between galaxies in the cluster have influenced their evolutionary histories.

What threw NGC 4693 out of sync?

The unusual rotation of gas and stars and the extended shape of NGC 4693’s bulge have prompted a couple of suggestions from astronomers about their origins. One theory suggests that the galaxy, like most of its siblings, experienced a merger or at least a close encounter with another galaxy sometime in the past. If that happened, it could have destabilized stellar orbits, and shock fronts could have moved clouds of gas around.

Another idea is that the galaxy somehow managed to draw in gas from the intergalactic medium in the larger cluster. There’s evidence of quite a lot of superheated plasma between galaxies, along with rogue stars and planetary nebulae somehow ejected from cluster galaxies during close encounters and collisions.

If NGC 4693 did draw in gas from other regions, that material could have gathered in the central region and assumed the strange, perpendicular orbital motions observed there. Add to the mix the fact that there’s a supermassive black hole at the galaxy’s core. It’s actively drawing in gas from surrounding regions, which also affects the motions of stars and gas clouds.

In addition to the unusual central motions, the galaxy’s arms show evidence of star formation, likely due to the same merger activity. When galaxies interact, shock fronts often blow through clouds of gas and dust, setting off bursts of star formation.

Studies of clusters such as the Virgo Cluster and Virgo Supercluster give astronomers insights into the evolution of member galaxies. Understanding the interactions between galaxies also provides background on the star formation that shapes them. The universe is filled with galaxy clusters and superclusters, so studying the relatively nearby Virgo Cluster gives us a look at processes that likely have occurred throughout time and space since the Big Bang and the births of the first stars and galaxies.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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