One galaxy’s core spins at right angles to everything around it

NGC 4698 looks like an ordinary spiral galaxy from a distance, but its center is spinning in a direction that has nothing to do with the rest of it. The galaxy sits about 55 million light-years away in the constellation Virgo, and its central bulge extends outward at a right angle to the flat disk of stars, gas, and dust that surrounds it. Stars and gas closest to that bulge rotate perpendicular to everything farther out.

A new image from the NASA/ESA Hubble Space Telescope captured the galaxy in enough detail to show the mismatch directly, with the ghostly glow of the bulge visible peeking above and below the dusty disk that should otherwise contain it. NGC 4698 is a member of the Virgo Cluster, home to more than a thousand galaxies bound together by gravity, and it resembles the Milky Way closely enough at first glance that the anomaly at its center is easy to miss.

A bulge built at the wrong angle

Spiral arms typically wind all the way down to a galaxy’s glowing center, but NGC 4698’s arms pull back from it instead, forming a ring-like structure around the galaxy’s perimeter rather than reaching the core. The galaxy’s elongated central bulge is made of stars smaller, older, and cooler than the bright blue stars scattered across the outer disk, and those bulge stars orbit tightly around a supermassive black hole holding millions of times the Sun’s mass. NGC 4698 is one of only a handful of known spiral galaxies whose bulge extends from the disk at a right angle, and it is rarer still for that mismatch to extend into how the gas and stars near the center actually move.

The perpendicular rotation is the detail that turns an unusual bulge into a genuine puzzle. In a normal spiral, everything from the innermost bulge to the outermost arm rotates around the same axis, more or less in the same plane. In NGC 4698, the material nearest the black hole is rotating on a completely different axis than the disk holding the spiral arms, meaning the galaxy currently has two rotational systems that do not share an orientation.

A minor merger left a hydrogen trail

Astronomers studying the galaxy have concluded the cause likely originated outside NGC 4698 rather than from anything happening natively within its disk. If the galaxy pulled in gas from an external source, that newly captured material could have settled into its own disk near the center, tilted at an angle relative to the older, larger disk already in place. Some observations point to a specific culprit: a short tail of hydrogen gas streaming away from one side of the galaxy, evidence consistent with a minor galactic merger sometime in NGC 4698’s past.

That inflowing gas would also help explain why the supermassive black hole at the galaxy’s center appears to be actively growing. Signs that the black hole is drawing in gas with its own gravity suggest the same merger event, or something like it, is still feeding material toward the core years after the initial disruption.

Minor mergers of this kind are common inside a dense environment like the Virgo Cluster, where galaxies pass close enough to one another that gravity can strip gas from a smaller neighbor without the two galaxies fully colliding or merging into one body. What makes NGC 4698 useful to astronomers is that the borrowed gas never blended smoothly into the existing disk. Instead it kept its own orbital plane, leaving two rotating structures stacked at right angles rather than one disk that gradually absorbed the disruption and settled back into a single alignment.

The Virgo Cluster survey behind the image

The image comes from an observing program aimed squarely at the Virgo Cluster, led by Johns Hopkins University astronomer David Thilker under Hubble proposal 18103, part of the broader MAUVE effort to study how cluster environments reshape individual galaxies. The wider MAUVE survey combines that Hubble imaging with ground-based spectroscopy from the Very Large Telescope’s MUSE instrument and radio observations from the Atacama Large Millimeter/submillimeter Array, covering 40 late-type Virgo Cluster galaxies chosen to span different stages of falling into the cluster. The program zooms in on fine structure inside relatively nearby galaxies, including individual star clusters and glowing nebulae, while a parallel goal is tracking how a galaxy’s path through the crowded cluster affects its ability to keep forming new stars.

NGC 4698 gives the Hubble Space Telescope survey an unusually clean example of a galaxy whose evolution went sideways, literally. Most galaxies that pull in outside gas do so quietly enough that the disruption smooths itself out within the disk’s existing rotation. NGC 4698 instead kept its borrowed material spinning on its own axis, a state that could persist for a long stretch of the galaxy’s history before the two systems eventually align or one consumes the other.

What the MAUVE team has not yet pinned down is timing: how long ago the merger happened, and how much longer the mismatched rotation can hold before gravity and friction finally force the inner and outer disks into agreement. Thilker’s group plans to combine the new Hubble imaging with the survey’s spectroscopic data to attempt an answer.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview