Though the spiral galaxy in this new image from the NASA/ESA Hubble Space Telescope seems serene, it hides a secret. This galaxy is NGC 4698, and it lies about 55 million light-years away in the constellation Virgo. It’s one of over a thousand galaxies in the Virgo Cluster, the nearest large cluster of galaxies bound together by gravity.
As a spiral galaxy similar to our own Milky Way galaxy, NGC 4698 has spiral arms that curl around within a thin disk of stars, gas, and dust. These arms are marked by opaque clumps of brown dust and dotted with small collections of bright blue stars. Unlike many other spiral galaxies, like this one recently photographed by Hubble, NGC 4698’s delicate spiral arms are only prominent in the outer reaches of the disk; spiral arms often wind down to the center of a galaxy, but NGC 4698’s spiral arms appear to shy away from its glowing center. The arms instead hover in a ring-like structure around the perimeter of the galaxy.
NGC 4698’s center is dominated by an elongated galactic bulge. The diffuse off-white glow of the galaxy’s bulge comes from stars smaller, older, and cooler than the large blue stars that dot the galaxy’s outer disk. These tightly packed stars orbit a supermassive black hole containing millions of times the Sun’s mass. Scientists have found signs that this black hole is actively growing, drawing gas inward with its gravitational pull.
With a flat, starry disk and diffusely glowing center, NGC 4698 looks a lot like a typical spiral galaxy — but this galaxy is far from normal. NGC 4698 is one of only a few known spiral galaxies with a bulge that extends out from the disk at a right angle. The elongation is faintly visible in this Hubble image, which shows the ghostly glow of the starry bulge peeking above and below the dusty disk. What’s more, the stars and gas nearest the galaxy’s center rotate perpendicular to the rest of the disk!
What could cause a galaxy to be so out of sync? Astronomers have found that the culprit likely lies outside the galaxy. If NGC 4698 funneled in gas from an outside source, the newly collected gas could have formed a disk of gas and stars in the galaxy’s center, at an angle relative to the rest of the disk. Some observations suggest that this galaxy once experienced a galactic merger, as evidenced by a short ‘tail’ of hydrogen streaming from one side of the galaxy.
The data used to create this image come from an observing program focusing on galaxies in the Virgo Cluster. This program zooms in on the details of these relatively nearby galaxies, such as individual star clusters and nebulae. At the same time, researchers will use these data to take stock of the larger picture, learning how a galaxy’s journey through a cluster affects the galaxy’s evolution and ability to form new stars.
Claire Andreoli NASA's Goddard Space Flight Center, Greenbelt, MD claire.andreoli@nasa.gov
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