NASA's Hubble Space Telescope has imaged NGC 4698, a spiral galaxy in Virgo whose central bulge and inner rotation axis are set at right angles to its disk, hinting at a past shaped by external gas or a minor merger.
Galaxies don't always follow the rules. In a recent Hubble Space Telescope image, NGC 4698-a spiral galaxy in the Virgo Cluster-shows a rare mismatch: its central bulge stretches at a right angle to the disk, and the stars and gas near the core rotate on a different axis than the rest of the galaxy. This odd arrangement makes NGC 4698 stand out among its neighbors and raises questions about how such a structure forms. NASA notes that NGC 4698 is one of the few known spirals with both a bulge and inner rotation axis misaligned at right angles to the disk, making it a useful case for studying galactic dynamics.
What sets NGC 4698 apart
NGC 4698 sits about 55 million light-years away in Virgo, part of the nearest large cluster of galaxies. Its outer disk has the familiar spiral arms, marked by brown dust and blue star clusters, but these arms don't reach the center. Instead, they form a ring around the edge, leaving the bright core exposed. The central bulge, made up of older stars, is stretched out and oriented at a right angle to the disk-a detail visible in the Hubble image as a faint glow above and below the dusty plane. Catalogs and reviews consistently list NGC 4698 as a spiral in Virgo with a misaligned core, making it a prime example for research on galaxies with unusual internal structure.
The core's rotation is even more unusual. Instead of following the disk's spin, the stars and gas near the center orbit along an axis nearly perpendicular to the main plane. This kind of rotation is extremely rare among spirals and points to a complicated past. Both NASA and the European Space Agency have highlighted this perpendicular structure as important for understanding the range of shapes galaxies can take.
How did it get this way?
Astronomers think NGC 4698's odd structure comes from outside influences. One idea is that the galaxy pulled in gas from its surroundings, which then settled into a central disk misaligned with the original one. Another possibility is a minor merger-a smaller galaxy collided with NGC 4698, bringing in material on a different orbital plane and disrupting the symmetry. A short hydrogen tail seen streaming from one side of the galaxy supports the idea of a past merger. This fits with models published in journals like Monthly Notices of the Royal Astronomical Society, which recognize external accretion and minor mergers as ways to create these kinds of misalignments in galactic centers.
At the heart of NGC 4698 is a supermassive black hole with millions of times the Sun's mass. Evidence suggests this black hole is still growing, pulling in gas from its surroundings. The dense stars in the bulge orbit this central mass, creating the diffuse off-white glow seen in Hubble's image. The combination of a misaligned bulge and perpendicular rotation makes NGC 4698 a valuable test case for models of galactic evolution and the effects of outside material. Research teams at NASA and institutions like the Max Planck Society continue to study galaxies like this to better understand how they form.
Hubble's view of the Virgo Cluster
The image of NGC 4698 comes from a Hubble program focused on galaxies in the Virgo Cluster. By targeting nearby systems, the project can resolve individual star clusters and nebulae, giving a detailed look at galactic structure. These data help researchers see how galaxies interact with their environment and how those interactions affect star formation and shape. The campaign builds on earlier Hubble work, such as previous imaging of superbubbles in the Large Magellanic Cloud, which showed how massive stars can reshape their surroundings. NASA and ESA's involvement ensures the data are reliable and available to researchers worldwide.
NGC 4698's structure is more than a visual oddity-it gives astronomers a direct look at the processes that can disrupt or reshape spiral galaxies. By comparing images and motion data, researchers can test whether outside gas or minor mergers are behind these misalignments, and how often such events happen in the local universe. Studies in journals like Nature and Science have pointed out the value of rare cases like this for testing theories of galaxy evolution.
What we still don't know
Even with Hubble's sharp images, the exact sequence of events that led to NGC 4698's perpendicular core isn't clear. The evidence for outside gas and a possible minor merger is indirect, based on the hydrogen tail and the unusual rotation. Without direct observation of the event, astronomers rely on models and comparisons with other galaxies. The rarity of perpendicular bulges among spirals suggests these processes are uncommon or need special conditions. Ongoing research at places like MIT and Harvard aims to find more examples and refine how we spot galaxies with misaligned cores.
NGC 4698's core and disk don't line up, challenging simple models of how galaxies grow. As more high-resolution data become available, researchers hope to better understand how galaxies like this get their distinctive shapes.
To study galaxies like NGC 4698, astronomers use both images and spectroscopy. Spectroscopy lets them measure the speed of stars and gas along different lines of sight, revealing rotation patterns and misalignments that aren't obvious in pictures. By combining these measurements with detailed images, scientists can reconstruct a galaxy's three-dimensional structure and look for signs of past interactions or mergers. This approach is key to telling apart changes caused by internal processes from those driven by outside influences.
