Anemic galaxy

An anemic galaxy is a spiral galaxy characterized by a low star formation rate compared to other spiral galaxies of similar size and luminosity. This results in a "washed-out" or "faded" appearance, primarily due to a reduced presence of bright, young, blue stars that are typically associated with active star formation.

Key characteristics of anemic galaxies include:

• Weak Spiral Arms: The spiral arms are often poorly defined and fragmented compared to the prominent, well-defined arms seen in normal spiral galaxies.

• Low Gas Content: Anemic galaxies typically have a lower abundance of neutral hydrogen gas (HI), the raw material for star formation, compared to typical spiral galaxies. This depletion of gas is a major factor in the reduced star formation rate.

• Redder Colors: Due to the scarcity of young, blue stars, anemic galaxies appear redder in color, reflecting the dominance of older, redder stellar populations.

• Location in Clusters: Anemic galaxies are frequently found in galaxy clusters, suggesting that environmental factors within the cluster, such as ram-pressure stripping (the removal of gas by the intergalactic medium) or tidal interactions with other galaxies, play a role in suppressing their star formation.

• Lower Surface Brightness: Often, anemic galaxies exhibit a lower surface brightness compared to normal spirals, contributing to their fainter and more diffuse appearance.

The classification of a galaxy as "anemic" is often subjective and based on visual inspection and comparison with other spiral galaxies. Quantitative measurements of gas content, star formation rate, and color can be used to support the classification. The study of anemic galaxies provides valuable insights into the processes that can quench star formation in galaxies and influence their evolution, particularly in dense environments like galaxy clusters.