Sunday, February 28, 2021

Observe this galaxy carefully. Notice anything strange?

If you try to compare this image with those of other galaxies you will notice that in this case the colors are much duller than the gaudy blue of typical spiral galaxies. This feature is intrinsic to the galaxy, cataloged as NGC 4921 and located 320 million light years away from Earth.

NGC 4921 is an excellent example of an anemic galaxy. These galaxies are characterized by a very low content of neutral hydrogen, an essential material for the creation of new stars. This results in a low star formation rate. Visually, anemic galaxies are characterized by a very diffuse structure, a low surface brilliance and virtually indistinguishable spiral arms.

Anemic galaxies are easy to find inside clusters (NGC 4921 itself is found inside the Coma Cluster), while they are very rare to find as isolated galaxies. This difference seems to suggest that anemic galaxies form when their gas is stripped away following interactions with other galaxies in the cluster or with the intergalactic medium.

Credit: NASA, ESA, K. Cook (LLNL).

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