Unraveling NGC 2775: The Galaxy That Defies Classification

This image from the NASA/ESA Hubble Space Telescope shows a galaxy that is difficult to categorize. The galaxy, NGC 2775, lies 67 million light-years away in the constellation Cancer (the Crab). Its center is smooth and featureless, lacking gas, resembling an elliptical galaxy. Yet it also has a dusty ring with patchy star clusters, a trait seen in spiral galaxies. So, is it a spiral, an elliptical, or something else entirely?

Because we can only see NGC 2775 from one angle, its exact classification is uncertain. Some astronomers call it a spiral galaxy due to its feathery ring of stars and dust, while others classify it as a lenticular galaxy-a type that shows features of both spirals and ellipticals.

The origins of lenticular galaxies are not fully understood and may vary. They could be spirals that merged with other galaxies, spirals that have mostly exhausted their star-forming gas and lost prominent arms, or ellipticals that later accumulated gas into a surrounding disk.

Evidence suggests NGC 2775 may have undergone mergers in the past. While invisible in this Hubble image, the galaxy has a hydrogen gas tail stretching nearly 100,000 light-years, likely the remnant of one or more galaxies that came too close and were torn apart. Such past mergers could explain NGC 2775’s unusual appearance.

Most astronomers classify NGC 2775 as a flocculent spiral galaxy. Flocculent spirals have loosely defined, discontinuous arms, often described as “feathery” or “tufts” of stars forming spiral patterns.

Hubble previously imaged NGC 2775 in 2020. The new version adds observations in a specific wavelength of red light emitted by hydrogen clouds around massive young stars, visible as bright pink clumps. This additional wavelength helps astronomers pinpoint where new stars are forming within the galaxy.