One of the most renowned examples is NGC 4921, as depicted in an image captured by the Hubble Space Telescope. This galaxy lies 320 million light-years away from Earth, situated within the Coma Cluster – one of the closest superclusters of galaxies to our planet.
The distinction between a standard spiral galaxy and an anemic one is not solely based on color, but also on the quantity of hydrogen present. In the spiral arms of anemic galaxies, the concentration of neutral hydrogen is significantly lower than average. As neutral hydrogen is the primary material required for star formation, this leads to a reduced number of H II regions and virtually no star formation, resulting in a whitish hue (whereas blue is attributed to newly-formed young stars).
Most anemic galaxies are observed within highly concentrated clusters, suggesting a correlation between these two factors. It is theorized that anemic galaxies were once ordinary spiral galaxies that lost their neutral hydrogen due to interactions with other galaxies in the cluster and the intergalactic medium (through a phenomenon known as ram pressure).
Image Credit: NASA, ESA, Hubble.
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