This image from the NASA/ESA Hubble Space Telescope shows a galaxy that is difficult to categorize. The galaxy in question is NGC 2775, located 67 million light years away in the constellation Cancer (the Crab). NGC 2775 sports a smooth, featureless center devoid of gas, resembling an elliptical galaxy. It also has a dusty ring with uneven star clusters, like a spiral galaxy. What is it: spiral or elliptical – or neither?
Since we can only see NGC 2775 from one angle, it’s hard to say for sure. Some researchers classify NGC 2775 as a spiral galaxy due to its feathery ring of stars and dust, while others classify it as a lenticular galaxy. Lenticular galaxies have characteristics common to spiral and elliptical galaxies.
Astronomers don’t know exactly how lenticular galaxies form, and they could form in a variety of ways. Lenticular galaxies could be spiral galaxies that have merged with other galaxies, or that have mostly run out of star-forming gas and lost their prominent spiral arms. They could also have started more like elliptical galaxies and then collected gas into a disk around them.
Some evidence suggests that NGC 2775 merged with other galaxies in the past. Invisible in this Hubble image, NGC 2775 has a tail of hydrogen gas that extends nearly 100,000 light years around the galaxy. This faint tail could be the remnant of one or more galaxies that strayed too close to NGC 2775 before being stretched and absorbed. If NGC 2775 had merged with other galaxies in the past, it could explain the galaxy’s strange appearance today.
Most astronomers classify NGC 2775 as a flaky spiral galaxy. Flocculated spirals have ill-defined, discontinuous arms that are often described as “feathery” or as “clumps” of stars that loosely form spiral arms.
Hubble previously released an image of NGC 2775 in 2020. This new version adds observations of a specific wavelength of red light emitted by clouds of hydrogen gas surrounding massive young stars, visible as bright, pinkish clusters in the image. This extra wavelength of light helps astronomers better define where new stars are forming in the galaxy.
